Section 6.0 Nuclear Materials

Nuclear Weapons Frequently Asked Questions

Version 2.18: 20 February 1999


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6.0 Nuclear Materials

This section describes the basic facts and properties about the materials that are uniquely important to applications involving the large scale release of nuclear power.


6.1 Production of Isotopes

The most critical materials required for nuclear weapons are special isotopes of particular elements. Some of these isotopes exist in nature, but are highly diluted by other isotopes of the same element (e.g. deuterium, Li-6, U-235). These isotopes must be enriched (concentrated) to be useful. Other isotopes naturally exist only in minuscule quantities, if at all, and must be manufactured through nuclear reactions (e.g. tritium and Pu-239). An overview of these two classes of techniques are given below.

6.1.1 Isotopic Enrichment

All isotopes of the same element have nearly identical chemical properties, consequently separating these isotopes must rely on subtle effects created by the differences in mass of the atoms. Some of these techniques are physical - they rely on physical processes that have mass dependent effects. Others are chemical - isotopic mass differences can affect the rate of chemical reactions.

Since small effects are being used to achieve separation, the achievable concentration increase for any enrichment process tends to be small. This can be overcome by applying the enrichment process many, many times in a series of enrichment steps or "stages". Each step takes the enriched product of the previous step and enriches it further. This creates a sequential enriching system called a cascade.

There are two important factors that affect the performance of a cascade. First is the separation factor, which is a number greater than one, that measures the inherent enrichment capability of a single stage. The lower the separation factor, the more stages required to achieve the desired degree of concentration. The second factor is the allowable loss of the desired isotope in the waste stream.

To understand this second factor, note that at each stage the material flow feeding the process is split into two parts: the enriched product, and the depleted waste. Since the enrichment in the product is small, so is the depletion in the waste. The amount of desirable isotope in the waste stream can easily exceed the amount in the enriched product. If the isotope being concentrated is rare, this is very undesirable. Also if the feed stock is already significantly enriched, then the waste stream is also enriched and too valuable to simply throw away.

These considerations demand that the cascade be constructed so that the waste from downstream (later) enrichment stages be fed back to earlier stages. Each stage then receives feed stock consisting of the enriched product from the next upstream stage, and the waste from the next downstream cascade.

The starting material usually does not enter at the beginning of the cascade (that is, the stage that is farthest upstream), it enters farther down. The stages that are upstream from the starting feed (called the tail of the cascade) strip the waste stream of the desired isotope so that the waste that actually leaves the cascade is greatly depleted. The stages making up the head of the cascade produce progressively more enriched product until the most highly enriched product exists the last stage.

For any particular cascade the actual amount of enriched product produced depends on the degree of enrichment desired AND on the degree of depletion in the waste stream. If it is okay to reject as waste most of desired isotope that enters the cascade, then the output of the cascade can be increased by producing a large waste stream of slightly depleted material (this can be done if the starting material is cheap). If the starting material is precious, then a high percentage of the desired isotope can be stripped - which produces highly depleted waste. Examples of these two extremes are deuterium enrichment where the starting material is water and the feedstock cost is essentially zero, and uranium enrichment where the feedstock cost is substantial.

There is quite an assortment of isotope enrichment processes that have been used (or proposed) to produce substantial amounts of enriched isotopes. A sampling of important ones are:

The effectiveness of any technique varies from element to element. Some of the techniques listed above can be applied to almost any element (e.g. electromagnetic separation), others are useful for only a small number of isotopes (e.g. electrolysis).

The separation factor for many of these processes (diffusion, chemical exchange, distillation) depends on the mass ratio between the isotopes. These techniques work best for light elements since each unit of atomic weight difference has a bigger effect. The largest isotope mass ratio (for naturally occurring isotopes) is 2, for deuterium and hydrogen-1.

Other processes depend on the absolute mass difference, or are otherwise insensitive to mass ratios (electromagnetic, centrifuge, aerodynamic, and AVLIS separation techniques). These work well for heavy elements, and are the methods of choice for uranium enrichment.

The cost of an enrichment process is mostly dependent on two factors: the capital cost of the plant, and the energy consumption for operation. Low separation factors require more cascade stages and thus both larger capital and operating costs. The capital cost is roughly proportional to 1/(s - 1)^2 where s is the separation factor. Different separation techniques differ dramatically in the amount of energy required for a single stage, so a low energy cost technique can offset a larger number of stages, or a higher cost per stage.

Another factor of some interest is the cascade equilibrium time. Due to limitations in the output rate of each cascade unit, the amount of material that is passed back to earlier stages by each unit, and the fact that each unit holds a certain amount of material, it takes a significant amount of time when a cascade is started for the final product to emerge and reach a steady output rate. Some types of cascades can take a year to reach equilibrium.

6.1.1.1 Electromagnetic Separation

This technique uses magnetic fields to separate a stream of ionized atoms in a device called a calutron. This is basically a scaled up mass spectrometer. The different masses of the isotopes give each a different radius of curvature in the magnetic field, causing the stream to separate into two (or more). The streams are then captured by target cells. Electromagnetic separation is the most versatile technique and can be used with any combination of isotopes. It also has an extremely large separation factor. Typically two passes are sufficient to produce high purity isotopes (>80%) from low concentration starting material ( less than 1%), and will also efficiently strip the desired isotope from the waste stream. However it is an inefficient production process, since most of the feed is lost inside the calutron and must be recovered during periodic maintenance work. Other shortcomings of electromagnetic separation are the enormous energy consumption, the difficult and costly equipment maintenance, and the limited production rate given the capital cost. It is mainly used for producing small quantities of pure isotopes for research.

6.1.1.2 Gaseous Diffusion

This technique is practical for isotopes that have a substantial mass difference, and can be converted to gaseous form (and important caveat, the mass difference of the gas molecules should be due only to the mass difference of the isotopes to be separated). The technique works by allowing the gas to diffuse through a porous barrier. Lighter, faster molecules diffuse slightly quicker. The quality of the barrier material is critical. It must have even pore size, have no leaks at all, and be able to withstand substantial pressure differences, and chemical attack by the gaseous isotope. Aside from the barrier material problem, gaseous diffusion requires rather ordinary chemical plant technology (by today's standards). For some light elements the separation factor can be fairly large, but for most it is small (1.00429 for uranium). A large number of cascade stages can still achieve high purity, although the plant size must be large, and the cost of construction and operation is high. This technique has been the dominant enrichment process for uranium since the first plant was built in 1945. Although gas diffusion has been displaced by gas centrifuges for new plants since the late 70s, the large previously installed base makes this still the largest enrichment process worldwide.

6.1.1.3 Thermal Liquid Diffusion

This method makes use of the fact that different molecular velocities cause lighter molecules to concentrate in hotter regions when a temperature gradient exists. The separation factor is determined by the ratio of the mass difference and the total masses, so it is larger for light elements. In general the separation factor is small (around 1.01), so many cascade stages are needed. Although the equipment required is simple, the power requirement (for heat) is extremely large, although this can be reduced if waste heat is available from other industrial processes. This prevents it from being practical on a large scale.

6.1.1.4 Gas Centrifuge Separation

Although Germany developed this technology during W.W.II, it did not see application until the fifties and sixties. This technique works by passing gaseous isotope mixture through high speed centrifuges. Centrifugal force causes the isotopes to concentrate in lighter and heavier gas layers which are then bled off. The major advantage of gas centrifuges is that the separation factor is due to absolute mass differences, not to mass ratios. It works as well with heavy elements as light ones. Separation factors are proportional to the square of the ratio between the rotational speed and the molecular velocity in the gas. The highest speeds possible are thus very desirable. Rotational speeds above the speed of sound (>330 m/sec) are typical, advanced designs using carbon fiber rotors can exceed 600 m/sec (in principal Kevlar rotors could reach speeds as high as 1100 m/sec). Typical separation factors can range from 1.01 to over 1.10. Separation factors depend on the absolute mass difference between isotopes (not the mass ratio) and the square of the peripheral speed. Thus compared with gaseous diffusion, each centrifuge produces a much larger separation factor for heavy elements. A series of separation stages are still needed, but there can be many fewer. Gas centrifuges consume much less energy than gaseous diffusion, and it is much easier to adjust the scale of production plants. Gas centrifuges are the dominant uranium enrichment process for new plants in the world today. A high level of technical proficiency is required to build efficient centrifuges, which delayed their commercial introduction (along with competition from government-subsidized gaseous diffusion plants).

6.1.1.5 Aerodynamic Separation

This can be thought of as a variant on the gas centrifuge approach, except that instead of spinning a chamber through which a gas is passed, the gas is spun inside a nozzle or vortex tube by injecting it under high pressure. It is considerably more energy intensive than centrifuge separation and appears at least as intensive as gaseous diffusion. This technology has been developed by South Africa and Germany for uranium enrichment.

6.1.1.6 AVLIS (atomic vapor laser isotope separation)

This technology makes use of the fact that isotopes of different masses absorb slightly different wavelengths of light (an indirect consequence of the nucleus mass difference). Precisely tuned lasers would excite only the isotope atoms desired in a stream of atomic vapor. The ionized atoms would then be separated from the neutral ones electromagnetically or by chemical reaction. AVLIS has not been used on an industrial scale yet. This technique promises to allow high efficiency production of high-purity U-235 and Pu-239, although its true useful is difficult to judge without an operating plant to observe. AVLIS technology, if available, could make it possible for a country to produce substantial batches of weapon-grade uranium or plutonium (from commercial reactor waste) without being detected. The energy required for separation itself is very low, only enough to break the molecular bond or ionize the atom. Energy consumption is mostly determined by the efficiency of the laser used, which is generally on the order of 0.1%.

6.1.1.7 Chemical Exchange

This technique makes use of different chemical reaction rates between isotopes. It works best for light elements where the reaction rate differences are large. Practical plants use reactions that allow the two reactants to be in different phases (gas/liquid, solid/liquid, immiscible liquids). This permits convenient separation of the enriched and depleted materials, and allows continuous countercurrent operation. By also including temperature differences between the phases, the separation factor can be considerably enhanced. This is the most important process today for producing heavy water, for which it is by far the most energy-efficient method. Chemical exchange techniques for uranium have also been developed by Japan and France, but have never been used for production work. Chemical exchange is also used for lithium-6 enrichment.

6.1.1.8 Distillation

This is another thermal concentration technique, light isotopes evaporate more rapidly than heavier ones. Again, it works best for light elements with large mass ratios. It also helps if the heat of vaporization of the material being distilled is low. This has been applied successfully to heavy water production.

6.1.1.9 Electrolysis

This technique is unique to the production of heavy water. For reasons that are not entirely understood, when water is electrolyzed deuterium is concentrated with the remarkable separation factor of 7 or even more. However, electrolysis is so energy intensive it is only practical for processing enriched water in the later stages of heavy water production.


Representative Separation Factors

                        H/D    C-12/13    U-235/238
Chemical Exchange     1.2-3     1.02       1.0015
Distillation         1.05-1.6   1.01       nil
Gaseous Diffusion       1.2     1.03       1.00429
Centrifuge (250 m/sec)  1.01    1.01       1.026
Centrifuge (600 m/sec)                     1.233
Electrolysis            7

The total number of stages in a cascade is given by:

Ln[R[N_p]/R[N_w]]/Ln[s] - 1

where N_p and N_w are the isotope concentrations in the feed and waste,
s is the separation factor, and
R[N] = N/(1 - N).

6.1.2 Transmutation

The transmutation of one isotope or element into another requires an extraordinary type of reagent: sub-atomic particles. Large nuclear weapon programs consume some of these artificial isotopes in ton quantities. Only one sub-atomic particle is available in the necessary kilogram amounts: neutrons. A 1 gigawatt nuclear reactor will produce about 3.75 kg of neutrons during the course of a year (assuming constant operation). The fact that neutrons have no electrical charge is very important since it allows them to easily penetrate the charged electron shells and nucleus of an atom.

Nuclear reactors produce copious quantities of neutrons by their very nature. Each fission produces roughly 2.5 neutrons, one of these is required to keep the reaction going, the others are available for transmuting other materials although some portion of these are unavoidably captured by materials making up the reactor structure. Reactors were the first source, and so far remain the only source, of large amounts of neutrons. They are not the only conceivable source though. At least two others have been proposed.

The first of these was simply to use a nuclear bomb to provide large amounts of neutrons. By incorporating the starting material into the bomb structure, or packing it around the bomb, substantial quantities of material could be transmuted in an instant. The shortcomings of this scheme are collecting the product, and avoiding undesirable side effects. Exploding the bomb underground could overcome these problems, the product would be removed from the rubble filled cavern left behind by mining. The difficulties with this process are formidable however, and nuclear explosions are themselves very unpopular no matter how well contained so this approach will probably never be used.

An alternative seriously proposed recently is to use particle accelerators to produce neutrons without reactors. Recent advances in accelerator technology make it possible to produce very intense beams of protons relatively efficiently. The protons, upon striking a target material of moderate atomic number with energies of a few hundred MeV, would knock loose some 50 neutrons each. Considerable time and money would be required to make this practical of course.

The principal isotopes produced by neutron transmutation are plutonium (produced from U-238) and tritium (from lithium-6). U-233 from thorium is also of considerable interest. Plutonium is produced as a byproduct by most nuclear reactors. Practical tritium production on the other hand requires special isotope production reactors. This is because most reactors use low-enriched uranium as fuel, which is mostly U-238 (96-97%). U-238 is thus normally present in the reactor in large amounts, and it absorbs most of the excess neutrons leaving few available for other purposes. Any natural or low-enriched uranium reactor could be considered a plutonium breeder, although the quality of the plutonium produced varies considerably depending on how the fuel and reactor are managed (this is discussed in greater detail in the section below on plutonium). Normally the term "plutonium breeder" in reserved for reactors specially designed and operated to produce high quality bomb-grade Pu.

Isotope production reactors use more highly enriched fuel, and heavy water as the moderator to minimize neutron losses. These reactors produce substantial excesses of neutrons that are absorbed by breeding materials inserted into the reactor. These reactors generally produce no power, but they are flexible for producing different isotopes. Lithium, depleted or natural uranium, or thorium can all be used. Since producing one atom of any isotope requires one neutron, the production rate for plutonium or U-233 in an isotope production reactor is 80 times higher than that of tritium.

The production in a breeder reactor is determined by the reactor operating power. Typically these reactors produce somewhat less than one atom of product for every atom consumed by fission. The breeding ratio (number of product atoms/number of fuel atoms) is usually 0.8-0.9. U.S. isotope reactors at Savannah, GA have a ratio of 0.86. A reactor consumes about one gram of fuel for every megawatt-day of operation,. A 100 megawatt reactor can thus produce about 85 g of Pu a day, or 1 g of tritium. Natural uranium fueled breeder reactors may have a higher breeding ratio than this for plutonium, but they have limited capacity for producing other isotopes.

The rate of isotope production can also be thought of as the product of the neutron flux intensity, the concentration of raw material in the reactor core, and the volume of the core. This view point helps explain another important factor, the build up of contaminating isotopes, which is a problem with plutonium and U-233 production.

Taking Pu-239 as an example, we note that initially there is no Pu-239 present in a freshly loaded reactor. Its concentration builds up at a rate proportional to: (neutron flux)*(U-238 concentration). This Pu-239 is essentially pure. The plutonium is exposed to the same flux as the U-238, so it also participates in neutron reactions. The rate of these reactions is proportional to the Pu concentration, so the rates are initially very small. Mostly these reactions are simply fission, partially offsetting the Pu production but also releasing more neutrons for breeding.

Pu-239 has a larger fission cross section than U-235, but it also has a much larger neutron absorption (non-fission capture) cross section. Absorption results in the following reaction:

Pu-239 + n -> Pu-240
As the concentration of Pu-239 grows, so too does the rate of Pu-240 production. Since the ratio of these two isotopes in the plutonium present in the reactor ultimately depends on the ratio of their respective production rates, the percentage of the plutonium consisting of Pu-240 grows steadily as more Pu accumulates.

Pu-240 in turn breeds Pu-241, which leads to Pu-242. The longer the irradiation of the U-238 continues (either in fuel rods, or in a Pu production blanket), the lower the percentage of the most desirable isotope (Pu-239) in the plutonium. Other plutonium isotopes are also produced (see below under Plutonium). Similar processes lead to contamination of reactor-produced U-233 as well.


6.2 Fissionable Materials

There are three isotopes known which are practical for use as fission explosives. These are U-235, Pu-239, and U-233. Of these only U-235 occurs in nature. Pu-239 and U-233 must be produced by bombarding other isotopes with neutrons. A third element, thorium (Th-232), can only undergo fast fission, but can also be used for breeding U-233. There are other elements that are also fissile but they have no practical significance for a variety of reasons. These elements are summarized in subsection 6.2.4.

6.2.1 Uranium (U)

Uranium, element 92, named after the planet Uranus, is usually given credit for being the last element on the periodic table to have survived from the time that the Earth formed (but see section 6.2.2 Plutonium below).

Its properties have been exploited at least since AD 79, since it has been found in yellow ceramic glazes (containing more than 1% uranium oxide) recovered from the ruins of Pompeii and Herculaneum, near Naples, Italy.

Uranium was discovered in 1789 in pitchblende by the German chemist Martin Heinrich Klaproth, who named it after the planet Uranus, which had been discovered in 1781. It was was first isolated as a metal in 1841 by French chemist Eugene-Melchior Peligot, who reduced the anhydrous uranium tetrachloride with potassium metal. A key milestone in the history of science was reached in 1896 when the French physicist Antoine Henri Becquerel discovered the existence of radioactivity by accidentally exposing a photographic plate by the ionizing radiations of potassium uranyl sulfate.

Uranium is a very heavy, silver-white lustrous metal. It is the eighth densest element. When pure it is a little softer than steel; is malleable, ductile, and slightly paramagnetic. Uranium has three allotropic forms, the alpha (orthorhombic, stable up to 667.7 C), the beta (tetragonal, stable from 667.7 C to 774.8 C), and the gamma (body-centered cubic, from 774.8 C to melting point) which is the most malleable and ductile. The alpha phase is very unusual type of orthorhombic structure, consisting of corrugated sheets of atoms in a highly asymmetrical orthorhombic structure. This anisotropic, or distorted, structure makes it difficult for the atoms of alloying metals to substitute for uranium atoms or to occupy spaces between uranium atoms in the crystal lattice. Only molybdenum and niobium have been observed to form solid-solution alloys with uranium. Uranium metal does however react with a wide variety of alloying metals to form intermetallic compounds.

The basic properties of pure uranium are:

melting point 1132.2 degrees C (+/- 0.8); similar to that of copper
boiling point 3818 degrees C
density 18.95 (alpha form, stable to 667.8 C)
specific heat 6.65 cal/mole/C (25 C)
tensile strength 450 MPa

It is chemically very reactive. It oxidizes readily, a piece of metal progressively turns different colors as a thin oxide layer builds up (the same optical phenomenon that makes oil films colorful). Powdered uranium is pyrophoric, spontaneously bursting into flame in air. It ignites in air at 150-175 C forming U3O8. At 1000° C (1832° F), uranium combines with nitrogen to form a yellow nitride. Water attacks the metal, slowly when cold and faster when boiling. Uranium is soluble in hydrochloric, nitric and other acids forming tetravalent salts. It is not attacked by alkalies. Uranium displaces hydrogen from mineral acids and from the salt solutions of such metals as mercury, silver, copper, tin, platinum, and gold. On vigorous shaking metallic particles exhibit luminescence.

Uranium has four oxidation states, III-VI. The hexavalent compounds include uranyl trioxide, UO3, and uranyl chloride, UO2Cl2. Uranium tetrachloride, UCl4, and uranium dioxide, UO2, are examples of the tetravalent, or uranous, compounds. Uranous compounds are usually unstable; they revert to the hexavalent form when excessively exposed to air. Uranyl salts, such as uranyl chloride, may decompose in the presence of strong light and organic matter.

Uranium has fourteen isotopes, all radioactive. In nature only three are found. The approximate isotopic composition of natural uranium, with half-lifes is:


          Mass %   Atomic %     Half Life
 U-234   0.0054%   0.0055%  247 thousand years (alpha emitter)
 U-235   0.7110%   0.7202%  710 million years  (alpha emitter)
 U-238  99.2836%  99.2742%  4.51 billion years (alpha emitter)

Although the U-235 content of uranium is in general constant, occasional variations do occur on the order of 0.1% of the U-235 content (that is in the range of 0.7195-0.7209% as atomic %) due to depletion from ancient fission reactions that took place when U-235 existed in much higher concentrations than today. The 1.9 billion year old reactor centers discovered in 1972 at the Oklo mine in Gabon are the most famous. When the Oklo reactors were active natural uranium was about 3% U-235, the same as commercial power reactor fuel today. In the most highly depleted core at Oklo the U-235 content was only 0.44%. The Oklo reactor cores, and a few others found at another Gabon site near Oklo, are the only intact ancient reactors discovered so far.

The U-234 content can also vary significantly. Unlike U-235 and U-238 the isotope U-234 is not a ancient survival from stellar nucleosynthesis, it is an indirect decay product of U-238 and is produced by the decay chain:

U-238 -> (4.51 billion years, alpha) -> Th-234
Th-234 -> (24.1 day, beta) -> Pa-234
Pa-234 -> (6.75 hr, beta) -> U-234

Normally the U-234 exists in equilibrium with U-238, with the U-234 decaying away as fast as it is formed. Since the decaying atoms exist briefly as thorium and protactinium isotopes prior to the formation of U-234, they can become separated from the uranium bearing ore through chemical or physical processes. Ground water can preferentially leach one or both isotopes from uranium bearing soil, giving rise to ground water uranium with an isotopic composition enriched as much as 13 fold in U-234. Due to the short half-life, essentially all of the U-234 present in uranium has been formed in the last few million years. In equilibrium it accounts for about half of natural uranium's radioactivity.

U-236, with a half-life of 23.9 million years, is not found in nature in significant quantities. It accumulates when uranium is exposed to neutrons in reactors and can thus be used as a tracer for identifying uranium containing reprocessed fuel.

During the Manhattan Project natural uranium was code named "tuballoy" (abbreviated "Tu"), after the code named Tube Alloy Division of the project. This name is still encountered occasionally to refer to natural or depleted uranium. The code name for highly enriched uranium (specifically weapon-grade uranium) was "oralloy" (abbreviated "Oy"), which is also still encountered. The names "Q-metal", "depletalloy", and "D-38", once applied to depleted uranium, have fallen into disuse.

The specific activity of natural uranium is 0.67 microCi/g (split almost 50/50 between the rare U-234 isotope, and U-238; U-235 is a minor contributor). Natural uranium is sufficently radioative to expose a photographic plate in an hour or so.

A particularly important uranium compound is uranium hexafluoride (UF6). This is the only uranium compound that is stable and volatile near room temperature, and is used for gas phase uranium enrichment processes (diffusion and centrifugation). In this application it has the additional advantages that fluorine has only a single isotope (thus adding no complicating mass differences), and that UF6 is a stoichiometric compound (consisting of exactly 6 atoms of fluorine to 1 of uranium). It forms colorless crystals at room temperature, but sublimes (evaporates without melting) at 56 C. Its melting point is 64 C. It has a density of 4.87 (solid) and 3.86 (liquid). It is a powerful fluorinating agent, attacking most metals and oxides. It does not attack aluminum oxide (alumina). Aluminum metal resists attack (due to the thin alumina coating the metal possesses) as does nickel (due to the formation of a nickel fluoride layer). Most equipment for handling UF6 are made of aluminum or nickel alloys, or are nickel plated. Fluorinated hydrocarbons (e.g. Teflon) are excellent. It reacts instantly with water. Reactions with trace contaminants produce non-volatile lower fluorides that clog equipment.

As a footnote to uranium's role in weapons research, the compound uranium hydride, UH3, deserves mention. It was extensively studied at Los Alamos during the Manhattan Project as a possible bomb material. The theory was that the presence of hydrogen would moderate the neutrons, slowing the down to a region where U-235 has a much higher fission cross section. Although this would make the bomb less efficient, it was hoped that much smaller amounts of fissionable material might be needed. Further research showed that the unexpectedly low density (8), and the actual capture cross section of U-235 made this scheme unworkable. Implosion tests in the 1950s with (U-235)H3 cores verified this by producing negligible yields.

Before World War II uranium was thought to be a rare metal, it is now known to be more common than mercury or silver, and about as abundant as arsenic and molybdenum as a mineable ore. It has an average concentration in the crust of the earth of about 2 parts per 1 million, and, among the elements, ranks about 48th in natural abundance in crustal rocks. The discovery of large supplies has driven the price down to several dollars a pound. In the lithosphere (earth's crust) it is actually more common than such inexpensive elements as boron and zinc, existing in average quantities of 4 g/tonne. It is abundant enough in granitic rock to make radon gas (a decay product) a serious health hazard where outcrops are common. It is found in seawater at a concentration of 150 micrograms/tonne (cubic meter), the oceans are estimated to contain 4.5 x 109 tons of the element.

Uranium occurs as a significant constituent in more than 150 different minerals and as a minor component of another 50 minerals. Primary uranium minerals, found in magmatic hydrothermal veins and in pegmatites, include uraninite and pitchblende (the latter a variety of uraninite). The uranium in these two ores occurs in the form of uranium dioxide, which--owing to oxidation--can vary in exact chemical composition from UO2 to UO2.67. Other uranium ores of economic importance are autunite, a hydrated calcium uranyl phosphate; tobernite, a hydrated copper uranyl phosphate; coffinite, a black hydrated uranium silicate; and carnotite, a yellow hydrated potassium uranyl vanadate.

Uranium ores are widely distributed throughout the world. Reserves and commercial uranium transactions are expressed in U3O8 equivalent masses. The largest estimated reserves (May 1995, excluding Russia and Uzbekistan) are: Australia (27%), Kazakhstan (18%), Canada (14%), South Africa(9%), Namibia (9%), Brazil (8%), and the U.S. (4%). Russia and Uzbekistan also have large supplies, and commercial supplies have also been found in Gabon, Congo (or the Democratic Republic of the Congo, formerly Zaire), Kyrgyzstan, Czech Republic, Ukraine, and the UK (Cornwall). Worldwide reserves of inexpensive uranium ore (defined as uranium recoverable at a cost of U.S.$80/kg or less) is estimated at 3386 million kg U3O8 (again excluding Russia and Uzbekistan).

Deposits of pitchblende, the richest uranium ore, are found chiefly in Canada, Congo, and the United States. Most of the uranium mined in the United States is obtained from carnotite occurring in Colorado, Utah, New Mexico, Arizona, and Wyoming. A mineral called coffinite, discovered in 1955 in Colorado, is a high-grade ore containing nearly 61 percent uranium. Coffinite deposits were found subsequently in Wyoming and Arizona and in several foreign countries. In 1990, U.S. production of pure uranium concentrate was about 3417 metric tons. As of the end of 1994, known U.S. uranium reserves U3O8 were 133 million kg U3O8, and 432 million kg U3O8 at U.S.$110/kg. Approximately 73 percent of these reserves are located in deposits in New Mexico, Texas, and Wyoming. The average 1994 spot price for natural uranium in the United States in was $20.50/kg, a record low. Between 1985 and 1994 U.S. uranium prices averaged U.S.$69.51/kg U3O8 (U.S. suppliers) and U.S.$42.27/kg (foreign suppliers). World unrestricted prices were at U.S.$18.16 in October 1994, and are now (June 1996) at U.S.$38.90.

Uranium ores typically contain only a small amount of uranium-bearing minerals, so initial extraction and concentration steps are required. Physical separation steps (gravity, flotation, electrostatics, etc.) are not useful for uranium, so hydrometallurgical extraction (e.g. leaching) is the usual first step in ore processing.

In the classical acid leaching procedure for extracting uranium, the pitchblende ore is first broken up and roasted to dehydrate it, remove carbonaceous materials, oxidize sulfur compounds to sulfates, and oxidizes any other reductants that may interfere in subsequent leaching operations. It is then mixed with sulfuric and nitric acids. Uranium dissolves to form uranyl sulfate, UO2SO4; radium and other metals in the pitchblende ore are precipitated as sulfates. With the addition of sodium hydroxide, uranium is precipitated as sodium diuranate, Na2U2O7.6H2O, known as "yellow cake".

To obtain uranium from carnotite, the ore is finely ground and treated with a hot solution of caustic soda and potash to dissolve out uranium, radium, and vanadium. After the worthless sandy matrix is washed away, the solution is treated with sulfuric acid and barium chloride. A caustic alkali solution added to the remaining clear liquid precipitates the uranium and radium in concentrated form.

These classical methods of extracting uranium from its ores are now supplemented in current practice by such procedures as solvent extraction, ion exchange, and volatility methods.

In solvent extraction processes the uranium ore is removed from acidic ore leach-liquors by extracting with a solvent mixture such as tributylphosphate in kerosene. In modern industrial ore extraction methods, alkyl phosphoric acids--e.g., di(2-ethylhexyl) phosphoric acid--and secondary and tertiary alkyl amines are the usual solvents.

As a general rule, solvent extraction is preferred over ion-exchange methods for acidic leachates containing more than one gram of uranium per litre. Solvent extraction is not useful for recovery of uranium from carbonate leach liquors, however.

Uranium meeting nuclear-grade specifications is usually obtained from yellow cake through an additional solvent extraction step using the tributyl phosphate solvent-extraction process. First, the yellow cake is dissolved in nitric acid to prepare a feed solution. Uranium is then selectively extracted from this acid feed by tributyl phosphate diluted with kerosene or some other suitable hydrocarbon mixture. Finally, uranium is stripped from the tributyl phosphate extract into acidified water to yield a highly purified uranyl nitrate.

The nitrate is calcined to form UO3, which is reduced in furnaces under a hydrogen atmosphere to UO2. The UO2 is converted to UF4 with anhydrous HF.

The metal is usually prepared by reducing uranium halides (commonly the tetrafluoride) with magnesium by an exothermic reaction in a "bomb" (a refractory-lined, sealed container, usually steel), a general technique known as the Thermite process. The thermite reaction proceeds at temperatures exceeding 1,300 C. The production of uranium metal by magnesium reduction of the tetrafluoride specifically is also called the Ames process, after Iowa State University, Ames, where the American chemist F.H. Spedding and his colleagues developed it to industrial status in 1942. The bomb is necessary because the vapor pressure of magnesium metal is very high at 1300 C. The bombs are charged with granular UF4 and finely divided Mg (the latter in excess) and are heated to 500 to 700 C, at which point an exothermic thermite reaction begins. The heat of reaction is sufficient to liquefy the conversion contents of the bomb, which are essentially metallic uranium and a slag of magnesium fluoride (MgF2). The slag separates and floats to the top. When the bomb is cooled to ambient temperature, a uranium metal ingot or "button" is obtained which, despite its hydrogen content, is the best-quality uranium metal available commercially and is well suited for rolling into fuel shapes for nuclear reactors.

The metal can also be prepared by reducing uranium oxides with calcium, aluminum, or carbon at high temperatures; or by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.

It has been observed that the energy contained in coal in the form of trace uranium exceeds the energy content derived by combustion (assuming the U-238 is fissioned as well).

6.2.1.1 U-235

This relatively scarce isotope is the only naturally occurring material suitable for the production of energy through fission. For use in a fission weapon, or for convenient power production, it needs to be concentrated to levels higher than that found in nature. Although civilian power plants require uranium with a concentration of 2-4.5% U-235, weapons require a minimum of 80% U-235, and preferably more than 90%. U.S. weapon grade uranium is about 93.5% U-235, U.S. enrichment plants are capable of producing a 97.65% "top product" (this is used in naval reactors). Uranium enriched to 80% or more is known generically as HEU, "highly enriched uranium" (a cut-off of 20% is also used). U.S. weapons grade uranium is called "oralloy", a wartime code-name derived from "Oak Ridge Alloy" that has remained in use. In 1998 ORNL Isotopes Division was offering weapon grade (93% U-235) for sale at $53/gram. Uranium with enrichments ranging from 40% to 80% U-235 has been used in large amounts in U.S. thermonuclear weapons as a yield-boosting jacketing material for the secondary fusion stage.

Since it is lighter still, U-234 is proportionately enriched even more than U-235 by mass-based enrichment processes. Highly enriched U-235 typically contains 1.5-2.0% U-234.

U-235 has a spontaneous fission rate of 0.16 fissions/sec-kg. A pure mass of U-235 weighing 60 kg would thus emit only 9.6 fissions/sec, making gun assembly quite easy. U-238 produces 35 times as many neutrons per kg, so even a small percentage percent of U-238 contaminant multiples this rate several fold. The U-234 contaminant with a fission rate 22 times higher has a similar effect. The specific activity for U-235 is 2.1 microCi/g (for the pure isotope); 0.8% of U-234 would increase this to 51 microCi/g.

6.2.1.1.1 U-235 Isotope Enrichment

Five technologies have been used for the large scale concentration of U-235:

In addition serious attention continues to be paid to the following techniques:

The first three technologies listed (electromagnetic, gaseous diffusion, and thermal liquid diffusion) were used during World War II by the U.S. to separate significant amounts of U-235.

The separative capacity of a plant (or a single stage) is measured in mass separative work units (SWU) per unit time, for example kg-SWU/yr of ton-SWU/yr. The output of enriched product from a plant of a given capacity is determined also by the concentration of isotope in the product, feed, and waste. The concentration in the feed is usually fixed by nature, but the feed and waste concentrations can be varied. Allowing a higher concentration in the waste (or "tails") boosts product output, but at the expense of more feed per unit of product. The relationship is:

P = U/[V(N_p) + [(N_p-N_f)/(N_f-N_w)]V(N_w) - (1+[(N_p-N_f)/(N_f-N_w)])V(N_f)]

where P is the product output rate, U is the separative capacity; N_p, N_f, N_w are the molar concentrations of isotope in the product, feed, and waste (measured in fractions, not percent); and V(N_p), V(N_w), V(N_f) are the separative potential functions for each concentration. The separative potential function is defined as:

V(N) = (2N-1)Ln(N/1-N) where Ln is the natural logarithm

Assuming tails of 0.25%, a plant with a capacity of 3100 kg-SWU/yr will produce 15 kg of 90% U-235 annually (enough for one implosion-type bomb) from natural uranium. It should be observed that if 3% U-235 is used as feed (commercial pressurized water reactor fuel), and natural concentration uranium is the waste, then only 886 kg-SWU/yr are required for this same output rate. Even low-enriched uranium could greatly assist a weapon production program if it was diverted.

Worldwide enrichment capacity is currently at 49 million separative work units (SWU). An annual demand of 26 to 38 million SWU is projected over the next 20 years.

Electromagnetic Separation
This was the first technique capable of producing bomb grade uranium to be perfected, and is probably the easiest for a low technology country to acquire. It was used in the Y-12 plant built at Oak Ridge by the U.S. during W.W.II. Y-12 used two separation stages to produce material of 80-90% purity. Low enrichment uranium tetrachloride feed produced by the other two processes was used to boost output compared to using natural uranium. All of the U-235 in the Hiroshima bomb was purified by this technique. This method was abandoned in 1946 due to its cost. Apparently the only country to pursue this method seriously since has been Iraq. Gaseous Diffusion
This was the first practical industrial scale uranium isotope separation technique. This method uses the different molecular speeds of molecules in uranium hexafluoride vapor to achieve separation. Due to the fact that thousands of stages are needed for high enrichment, a diffusion plant producing weapon-grade U-235 must be enormous and to be practical have a large production capacity as well. Developing suitable diffusion barriers that can resist UF6 corrosion is difficult. Barriers are of two general types, thin film tubes and aggregated particle tubes. Thin films use a thin barrier with diffusion pores formed by etching. Nitric acid leaching of 40/60 Au/Ag alloy or Ag/Zn alloy have been studied. Electrolytic etching of aluminum foil forms a brittle porous alumina barrier. Teflon films have also been studied. Aggregate barriers are made of small particles that are sintered or compacted to form relatively thick porous walls. The barrier material suitable for large scale production appears to have been sintered nickel powder. Diffusion barrier technology has been kept classified by all countries that have developed it.

A 3024 stage plant called K-25 was built at Oak Ridge by the U.S. during W.W.II, and continued operating until the 1970s. Developing suitable barrier materials was difficult and held up completion of the plant until after the end of the war, although the partially completed plant contributed to the production of Little Boy. The barriers in use at this time were tubes made from sintered nickel powder, efforts to get a promising film barrier (probably electrolytically etched aluminum) into production in time failed. K-25 originally had 162,000 m^2 of barrier surface. This plant, with expansions, produced the majority of the fissionable material for U.S. weapons into the sixties. Improvements in barrier technology allowed performance enhancement by a factor of 23 by 1974.

Although basic plant construction is within the capabilities of standard chemical engineering technology today, developing barriers is a problem that must be overcome. Diffusion plants consume much less energy than electromagnetic plants, but the consumption is still quite large. Capital costs still dominate the cost of operation however. After an efficiency upgrade program, U.S. plants in 1981 had a specific power consumption of 2370 KWhr/kgSWU. Although low enriched uranium is valuable as feed to a plant intended to produce highly enriched uranium (HEU), a gaseous diffusion plant built specifically for low enriched uranium production cannot be easily diverted to produce HEU. High enrichment calls for diffusion stages much smaller than the large units used for low enriched material, both because the feed rate is dramatically lower, and because of criticality problems if a stage hold too much material. A commercial gaseous diffusion plant typically takes 1-2 months to reach equilibrium after start-up (an HEU plant would take longer, due to the greater number of successive stages).

A number of countries have used this technology, including Argentina which successfully developed gaseous diffusion technology and operated a plant in a secret weapon program (now terminated). In 1979 more than 98% of all uranium separation capacity in the world used this process. By the mid 80s, this had dropped to 95% as new gas centrifuge plants came on line.

Thermal Liquid Diffusion
The first technology to produce substantial amounts of (slightly) enriched uranium. This technology was used by the U.S. during W.W.II to boost the output of the Y-12 plant by providing slightly enriched feedstock. The thermal diffusion technology is the simplest of all these techniques, but is capable of enriching U-235 only to around 1% (the S-50 plant at Oak Ridge produced 0.85-0.89% U-235). Immense amounts of heat are required.

Gas Centrifuge Separation
Although Germany developed this technology during W.W.II, it did not see application until the sixties. It is the dominant U-235 separation technique for new plant construction today, although existing capacity is still mostly gaseous diffusion. By passing uranium hexafluoride vapor through high speed centrifuges, the isotopes concentrate in lighter and heavier gas layers which are bled off. Each centrifuge produces a much larger separation factor than a gaseous diffusion barrier. Fewer separation stages are needed, on the order of a thousand, but the cost of each stage is much higher than for a gaseous diffusion plant. Gas centrifuge separation requires less than 10% of the power required by gaseous diffusion, and it is easier to adjust the scale of production plants. A modern gas centrifuge provides at least 6 kg-SWU/yr of separative capacity. The specific power consumption is 100-250 kWhr/kgSWU. A high level of technical proficiency is required to build efficient centrifuges. This technology was successfully acquired by Pakistan and India, and was avidly pursued by Iraq and Brazil.

Aerodynamic Separation
This technology was developed by South Africa (UCOR process, using vortex tubes operating at a pressure of 6 bar) and Germany (using curved nozzles operating below atmospheric pressure, 0.25-0.5 bar). South Africa has been the only country to use it on a large scale. It produced about 400 kg of bomb-grade uranium at a plant at Valindaba, which was shut down in the late eighties. Separation factors of at least 1.015 have been achieved. It requires as much or more energy as gaseous diffusion, a typical figure is 3300 kWhr/kgSWU. AVLIS (atomic vapor laser isotope separation)
Never used in practice, this technology was being developed in the U.S. during the 70s and 80s until it was killed due to a world-wide glut of isotope separation capacity and reductions in the strategic arsenal. A billion dollar AVLIS plant was planned for Idaho Falls, Idaho during the eighties but was never constructed. Chemical Separation
Chemical exchange techniques for uranium have also been developed by Japan and France, but have never been used for a production plant. The French Chemex method involves a countercurrent of two immiscible liquids, each containing dissolved uranium, in a tall column. The Japanese Asahi process uses an exchange reaction between an aqueous solution and a finely powdered resin through which the solution percolates slowly. Both require special catalysts to accelerate the concentration process. The Chemex process has demonstrated a power consumption of 600 kWhr/kgSWU. Iraq pursued this technology (in the form of a combined Chemex/Asahi plant) as a means to partially enrich U-235 to 6-8% for use as a calutron feedstock. By cutting the amount of feed to be processed, output would have been boosted 8-11 times.

Approximate energy efficiency ranking of uranium enrichment techniques is given below, with typical relative power consumptions (gaseous diffusion = 1):


less than 0.01? AVLIS (if developed to production status)
 0.10-0.04      Gas Centrifuge
 0.30           Chemical Separation
 1.00           Gaseous Diffusion
 1.50           Aerodynamic Enrichment
 high           Electromagnetic Separation
 high           Thermal Liquid Diffusion

6.2.1.2 U-238

Although it can't be used as a primary fissile material due to the high threshold energy required for neutrons to cause fission, U-238 is still a very important nuclear material.

Due to its high density and atomic weight it is useful in bomb construction as a tamper/reflector for both fission and fusion assemblies. The fact that it *is* fissioned by fast neutrons means that it can boost weapon yield indirectly by multiplying reflected neutrons, or directly by the fast fission of the tamper. About 40% of fission neutrons, and all fusion neutrons, are energetic enough to fission U-238.

U-238 has a spontaneous fission rate some 35 times that of U-235, 5.51 fissions/sec-kg. This prevents its use as a tamper/reflector in bombs using gun assembly since a reasonable amount (200-300 kg)would emit too many neutrons. Pure U-238 has a specific activity of 0.333 microCi/g.

It is also the raw material for breeding plutonium-239, a very valuable primary fissile isotope. Any reactor that uses natural or partially enriched uranium (i.e. nearly every reactor in the world) contains U-238 and thus breeds plutonium.

6.2.1.3 U-233

This fissile uranium isotope (half-life 162,000 years) is not found in nature. It is instead bred from thorium-232 in a manner similar to the production of Pu-239:


Th-232 + n -> Th-233
Th-233 -> (22.2 min, beta) -> Pa-233
Pa-233 -> (27.0 day, beta) -> U-233

A two-step side reaction chain also occurs during breeding leading to the production of U-232:

Th-232 + n -> Th-231 + 2n
Th-231 -> (25.5 hr, beta) -> Pa-231
Pa-231 + n -> Pa-232
Pa-232 -> (1.31 day, beta) -> U-232

The production of U-232 through this process depends on the presence of significant amounts of un-thermalized neutrons since the cross section of the initial n,2n reaction is small at thermal energies.

If significant amounts of the isotope Th-230 are present then U-232 production is augmented by the reaction:

Th-230 + n -> Th-231
which continues as before.

The presence of U-232 is important because of its decay chain:

U-232 -> (76 yr, alpha) -> Th-228
Th-228 -> (1.913 yr, alpha) -> Ra-224
Ra-224 -> (3.64 day, alpha & gamma) -> Rn-220
Rn-220 -> (55.6 sec, alpha) -> Po-216
Po-216 -> (0.155 sec, alpha) -> Pb-212
Pb-212 -> (10.64 hr, beta & gamma) -> Bi-212
Bi-212 -> (60.6 min, beta & gamma) -> Po-212
                     alpha & gamma) -> Tl-208
Po-212 -> (3x10^-7 sec, alpha) -> Pb-208 (stable)
Tl-208 -> (3.06 min, beta & gamma) -> Pb-208

The rapid decay sequence beginning with Ra-224 produces a large amount of energetic gamma rays. About 85% of this total gamma energy output is due to the last isotope in the sequence, thallium-208 which produces the most energetic gamma rays (up to 2.6 MeV). The amount of gamma radiation emitted is proportional to the amount of Th-228 present.

The buildup of U-232 as a contaminant is unavoidable during the production of U-233. This is similar to the plutonium isotope contamination problem discussed below in plutonium production, but occurs to a much smaller extent rate. The first (n,2n) reaction only occurs when neutrons with energies in excess of 6 MeV are encountered. Only a small percentage of fission neutrons are this energetic, and if the thorium breeding blanket is kept in a reactor region where it is only exposed to a well moderated neutron flux (i.e essentially no neutrons above the Th-232 fission threshold of 500 KeV) this reaction can be nearly eliminated. The second reaction proceeds very efficiently with thermalized neutrons however, and minimizing U-232 from this source requires choosing thorium that naturally has a low Th-230 concentration.

If the above precautions are followed weapons-grade U-233 can be produced with U-232 levels of around 5 parts per million (0.0005%). Above 50 ppm (0.005%) of U-232 is considered low grade.

In a commercial fuel cycle the build-up of U-232 is not really a disadvantage, and may even be desirable since it reduces the proliferation potential of the uranium. In a fuel economy where the fuel is reprocessed and recycled the U-232 level could build up to 1000 - 2000 ppm (0.1 - 0.2%). In a system that is specifically engineered to accumulate U-232 levels of 0.5-1.0% can be reached.

Over the first couple years after U-233 containing U-232 is processed, Th-228 builds up to a nearly constant level, balanced by its own decay. During this time the gamma emissions build up and then stabilize. Thus over a few years a fabricated mass of U-233 can build up significant gamma emissions. A 10 kg sphere of weapons grade U-233 (5 ppm U-232) could be expected to reach 11 millirem/hr at 1 meter after 1 month, 0.11 rem/hr after 1 year, and 0.20 rem/hr after 2 years. Glove-box handling of such components, as is typical of weapons assembly and disassembly work, would quickly create worker safety problems. An annual 5 rem exposure limit would be exceeded with less than 25 hours of assembly work if 2-year old U-233 were used. Even 1 month old material would require limiting assembly duties to less than 10 hours per week.

In a fully assembled weapon exposures would be reduced by absorption by the tamper, case, and other materials. In a modern light weight design this absorption would be unlikely to achieve more than a factor of 10 attenuation, making exposure to weapons assembled two years previously an occupational safety problem. The beryllium reflectors used in light weight weapons would also add to the background neutron level due to the Be-9 + gamma -> Be-8 + neutron reaction. The U-232 gammas also provide a distinctive signature that can be used to detect and track the weapons from a distance. The heavy tampers used in less sophisticated weapon designs can provide much high levels of attenuation - a factor of 100 or even 1000.

With deliberately denatured grades of U-233 produced by a thorium fuel cycle (0.5 - 1.0% U-232), very high gamma exposures would result. A 10 kg sphere of this material could be expected to reach 11 rem/hr at 1 meter after 1 month, 110 rem/hr after 1 year, and 200 rem/hr after 2 years. Handling and fabrication of such material would have to done remotely (this also true of fuel element fabrication) In an assembled weapon, even if a factor of 1000 attenuation is assumed, close contact of no more than 25 hours/year with such a weapon would be possible and remain within safety standards. This makes the diversion of such material for weapons use extremely undesirable.

The short half-life of U-232 also gives it very high alpha activity. Denatured U-233 containing 1% U-232 content has three times the alpha activity of weapon-grade plutonium, and a correspondingly higher radiotoxicity. This high alpha activity also gives rise to an even more serious neutron emission problem than the gamma/beryllium reaction mentioned above. Alpha particles interact with light element contaminants in the fissile material to produce neutrons. This process is a much less prolific generator of neutrons in uranium metal than the spontaneous fission of the Pu-240 contaminant in plutonium though.

To minimize this problem the presence of light elements (especially, beryllium, boron, fluorine, and lithium) must be kept low. This is not really a problem for U-233 used in implosion systems since the neutron background problem is smaller than that of plutonium. For gun-type bombs the required purity level for these elements is on the order of 1 part per million. Although achieving such purity is not a trivial task, it is certainly achievable with standard chemical purification techniques. The ability of the semiconductor industry to prepare silicon in bulk with a purity of better than one part per billion raises the possibility of virtually eliminating neutron emissions by sufficient purification.

U-233 has a spontaneous fission rate of 0.47 fissions/sec-kg. U-232 has a spontaneous fission rate of 720 fissions-sec/kg.

Despite the gamma and neutron emission drawbacks, U-233 is otherwise an excellent primary fissile material. It has a much smaller critical mass than U-235, and its nuclear characteristics are similar to plutonium. The U.S. conducted its first test of a U-233 bomb core in Teapot MET in 1957 and has conducted quite a number of bomb tests using this isotope, although the purpose of these tests is not clear. India is believed to have produced U-233 as part of its weapons research and development, and officially includes U-233 breeding as part of its nuclear power program.

Its specific activity (not counting U-232 contamination) is 9.636 milliCi/g, giving it an alpha activity (and radiotoxicity) about 15% of plutonium. A 1% U-232 content would raise this to 212 milliCi/g.

6.2.1.4 Depleted Uranium

When U-235 has been extracted from natural uranium, the remaining uranium is called "depleted uranium" because it has been depleted of U-235. Typically it still contains 0.25-0.4% U-235 since it is not economical to strip U-235 in concentrations this low (it is cheaper just to buy more natural uranium with higher concentrations). The U.S. tails concentration was 0.2531% in 1963, rising to 0.30% in the 70s. Due to the recent low cost of uranium, the trend has been toward higher tails concentrations in recent years.

The U.S. has some 560,000 tonnes of depleted uranium hexafluoride (UF6) currently stored in cylinders at DOE's three gaseous diffusion plant sites: Paducah, Kentucky; Portsmouth, Ohio; and Oak Ridge, Tennessee. (This is probably not the complete U.S. DU inventory.)

Depleted uranium is about half as radioactive as natural uranium, almost entirely due to the removal of U-234, but its bulk properties are otherwise identical. Because the major use of uranium is for power production, depleted uranium is a waste product with little economic value. Finding uses for depleted uranium stockpiles has been a problem for enrichment plants.

The uses it has found have generally been related to its great density and comparatively low cost. It two most important uses are as a radiation shielding material, and as balance weights in aerospace applications such as aircraft control surfaces, each Boeing 747 contains 1500 kg of DU for this purpose. It is used extensively in oil-well drilling in the form of sinker bars, weights used to lower instruments into mud-filled well holes. It has also been used in high-performance gyroscope rotors, large inertial energy-storage flywheels, and as ballast in ballastic missile reentry vehicles and racing yachts.

Most famously, it is used in armor-piercing ammunition by the U.S.. When suitably alloyed and heat treated (alloyed with 2% Mo or 0.75% Ti; rapid quenching for 850 C in oil or water, followed by aging at 450 deg. for 5 hrs) uranium is as hard and strong as hardened tool steel (tensile strength >1600 MPa). Combined with its great density this makes it very effective at punching through armor, consistently superior to the much more expensive monocrystalline tungsten, it nearest competitor. The penetration process pulverizes much of the uranium, which explodes into burning fragments when it hits the air on the other side, adding to the destruction. Some 300 tonnes of depleted uranium were fired on the battlefield during Desert Storm (mostly from the GAU-8 30-mm guns of A-10 attack planes, each round contains 272 grams of DU alloy). It is also used in advanced armor, like that used on the M-1 Abrams tank.

6.2.2 Plutonium (Pu) Plutonium, element 94, named after the planet Pluto, was discovered by Glenn Seaborg, Edwin McMillan, Kennedy, and Arthur Wahl in 1940 at Berkeley by bombarding uranium with deuterons from the 60 inch cyclotron.The fissile properties of Pu-239 had been predicted in advance of the element's discovery by Louis Turner in May 1940. The isotope Pu-239 exists naturally in trace amounts in uranium ore (several parts per quadrillion) produced by the capture of spontaneous fission neutrons by U-238. Extremely small amounts of Pu-244 (the longest lived plutonium isotope, with a half-life of 80 million years) have been reportedly detected in cerium ore, apparently surviving remnants of plutonium present at the formation of the earth.

Plutonium is produced by bombarding U-238 with slow neutrons, in a nuclear reactor for large amounts (a process called breeding). If a slow neutron is captured, then U-239 is produced which quickly decays into neptunium-239 and then plutonium:


  U-238 + n -> U-239
  U-239  -> (23.5 min., beta) -> Np-239
  Np-239 -> (2.35 days, beta) -> Pu-239

Fifteen isotopes of plutonium are known, all are radioactive. The principle ones of interest to the design of nuclear weapons, and their half-lifes, are:

Pu-238 -> (86 years, alpha) ->     U-234
Pu-239 -> (24360 years, alpha) ->  U-235
Pu-240 -> (6580 years, alpha) ->   U-236
Pu-241 -> (14.0 years, beta) ->    Am-241
Pu-242 -> (370000 years, alpha) -> U-238

Plutonium is a very heavy silvery metal, bright like nickel when freshly cleaned. It is a highly electronegative, chemically reactive element, considerably more so than uranium. It quickly tarnishes, forming a succession of interference colors (like an oil film), initially a pale yellow, and eventually becoming dark purple-black. If corrosion is sufficiently severe, a loose olive-green powder (PuO2) forms on the surface.

Plutonium oxidizes readily, and corrodes very quickly in the presence of even traces of moisture. Oddly, it corrodes much faster in an inert atmosphere if moisture is present, than it does in ordianry air or pure oxygen. The reason is that direct attack by oxygen forms an adherent plutonium oxide layer that retards further corrosion, attack by moisture produces a loose oxide and hydride mixture. efficient desiccator is necessary to preve and oxidizes and corrodes readily, and is attacked by moisture.

It has four valences, III-VI. It has significant solubility only in very acidic solutions, such as nitric and hydrochloric acid (as the nitrate and chloride), as well as the hydroiodic and perchloric acids. Plutonium salts hydrolyze readily in contact with neutral of basic solution, forming insoluble plutonium hydroxide. Concentrated plutonium solutions are unstable due to radiolytic decomposition, resulting in precipitation.

Due to radioactive heating, a substantial piece of plutonium is warm to the touch. A large piece of plutonium that is thermally insulated can exceed the boiling point of water.

The basic properties of plutonium are:

melting point 641 degrees C; similar to that of aluminum
boiling point 3232 degrees C
density 19.84 (alpha phase)

Plutonium has a large number of very peculiar properties. It has the lowest thermal conductivity of any metal, its electrical conductivity is lower than any metal but manganese (other reports claim flatly that it is the lowest). It is the most viscous liquid metal known. And it undergoes the most extreme and bizarre density changes with temperature of any element.

Plutonium has six distinct phases (crystal structures) in solid form, more than any other element (actually there is a seventh that exists under certain conditions). Some of the phase changes involve dramatic changes in volume. In two of these phases (delta and delta prime) plutonium has the unique property of shrinking with increasing temperature, in the others it has an extremely large thermal expansion coefficient. Plutonium also contracts upon melting, allowing unmelted plutonium to float. In its densest form, the alpha phase, plutonium is the sixth densest element (only osmium, iridium, platinum, rhenium, and neptunium are denser).

It is brittle in the alpha phase, which exists at room temperature in pure plutonium, but ductile alloys exist.

The densities and temperature ranges of these phases are:


Phase            Density       Stability Range (degrees C)

alpha            19.84  (20 C)   stable below 122
beta             17.8  (122 C)   122 - 206
gamma            17.2  (206 C)   224 - 300
delta/
delta prime      15.9  (319 C)   319 - 476
epsilon          17.0  (476 C)   476 - 641 (melting point)
liquid           16.65 (641 C)   641 - to boiling point

By the end of 1995 about 1270 tonnes of plutonium had been produced world wide, 257 tonnes for weapon use, the rest as a by-product of commercial power production. Due to the rapid recent growth of the world wide nuclear power industry, reactor grade uranium is currently being generated by commerical operations at a rate of 75 tonnes/yr. About 210 tonnes of the commercial plutonium has been separated from nuclear fuel (and all of the military plutonium, of course) so far. Reprocessing is producing about 18 tonnes a year right now (only the UK, France, Russia, Japan, and India engage in reprocessing).

Besides its use in nuclear weapons, plutonium has potential utility as a source of commercial power. Japan currently has the only major program for using plutonium for power. It appears that plutonium will not be economically competitive with uranium for decades to come for several reasons. The cost of reprocessing reactor fuel to extract plutonium is more expensive than the current commercial cost of low enriched uranium. Most current fuel fabrication plants are not equipped to handle the more hazardous plutonium oxide. The costs of safeguarding plutonium to prevent its theft or diversion for weapons use is quite substantial. Existing power reactors can only use fuel containing fairly small amounts of plutonium, which makes its use of little value, and the costs of designing and building new reactors for this purpose also adversely affects its cost. The current abundant supplies of uranium, excess enrichment capacity, and the large stockpiles of U.S. and Russian weapons grade uranium now being mixed down for commercial use guarantee rock bottom uranium prices for the next 20-30 years.

Plutonium has few other uses. The most widespread is its use as a smoke detector radioisotope in Europe (U.S. smoke detectors use americium due to its shorter half-life). Plutonium-beryllium alloy is used as a laboratory neutron source. The isotope Pu-238 is useful for radioisotope-powered thermoelectric generators (RTGs) to power long duration deep space probes due to its high heat output and long lifetime.

In Sept. 1998 the prices charged by the ORNL Isotopes Division for different isotopes of plutonium was $8.25/mg of Plutonium-238 (97% purity); $4.65/mg of Plutonium-239 (>99.99%); $5.45/mg of Plutonium-240 (>95%); $14.70/mg of Plutonium-241 (>93%); and $19.75/mg of Plutonium-242.

Although extremely scarce naturally, about 5000 kg of plutonium has been released into the atmosphere by nuclear weapons tests. The soil of the U.S. contains an average of about 2 millicuries (28 milligrams) per km^2 from fallout.

6.2.2.1 Plutonium Metallurgy

At room temperature pure plutonium exhibits a crystal structure that is termed the "alpha phase". In this form plutonium is at its maximum density, some 19.84 at 20 C. The atoms in the alpha phase are essentially covalently bonded (as opposed to metallically bonded), giving it physical properties more nearly like a mineral than a metal. It is hard, strong, brittle, and fractures in a highly directional manner. The alpha phase is unworkable using normal metal fabrication techniques.

In its lowest density phase (density of about 15.9), the delta phase, it is quite malleable. Plutonium is also quite malleable in the gamma phase.

Delta phase plutonium has normal metallic properties, including excellent ductility. The delta phase has strength and malleability similar to aluminum, which makes forming and machining straightforward. Although the delta phase exhibits the anomalous property of shrinking when heated, this negative coefficient of expansion is not large.

The delta phase is only marginally stable. It tends to collapse into the dense alpha phase under very low pressures (as these things go), causing a 25% density increase. The delta phase does not exist in pure plutonium above a pressure of about 1 kilobar. For comparison, a density increase of 25% in uranium (or alpha phase plutonium) requires pressures of 450 kilobars. Above 30 kilobars only the alpha and beta phases of plutonium exist.

Plutonium can be stabilized in the delta phase at room temperature by alloying it with certain trivalent atoms such as gallium, aluminum, cerium, indium, scandium, and americium at concentrations of a few molar% (% of atoms that are the alloying agent).

Even when stabilized, the delta phase still collapses easily, a pressure of a few kilobars is sufficient to make it revert to the alpha phase. Of special interest is the fact that in gallium stabilized plutonium, the delta phase is actually metastable when the gallium content is below 4.0 molar%. This means that the pressure induced phase change to the alpha form is irreversible.

For use in weapons plutonium is normally stabilized in the delta phase by alloying it with 3-3.5 molar% gallium (0.9-1.0% by weight). This alloy is stable from at least -75 to 475 degrees C. The stabilization prevents low temperature phase changes from occurring after fabrication that would ruin the precisely made components of a weapon. It has an almost zero coefficient of expansion. It also makes casting easier since only the epsilon -> delta phase change occurs during cooling. Finally, the gallium reduces the susceptibility to corrosion of plutonium.

The 3% gallium alloy was used in the Gadget and Fat Man bombs. Aside from this alloying agent the plutonium was otherwise highly pure.

Alpha phase plutonium is known to have been used as weapon components however.

Aluminum is a good alloying agent, but early in the U.S. weapons program it was not used in weapons due to the existence of neutron producing alpha->n reactions (see below). Cerium is not used since (among other reasons) it does not confer corrosion resistance.

Plutonium pits are plated with metal (usually nickel) to protect them from corrosion, as well as reducing the radiological hazard. The hemispheres for Gadget were electroplated with nickel (also reported - apparently incorrectly - as being silver), a process that was not very successful and led to blistering. A process was developed for chemically plating the surface by exposing the plutonium pit to a nickel carbonyl atmosphere. The pits for the Fat Man bomb as well as the Operation Crossroads devices were plated with nickel, as was the pit for Joe 1/RDS-1. Evaporation coating with aluminum and electroplating with zinc do not work.

A potentially serious problem with using plutonium in weapons is the existence of a high spontaneous neutron emission rate. The presence of neutrons during the assembly of a supercritical mass leads to a premature nuclear reaction, an inefficient release of energy, even a near total weapon failure in some cases. There are two sources for this neutron background. The most important is the presence of the isotope Pu-240, which spontaneously fissions fast enough to release some 10^6 n/sec/kg. This isotope inevitably forms during Pu-239 production. The second source is from the interaction of the strong alpha emissions with light element contaminants in the plutonium. Although this problem aroused great concern on the Manhattan Project when the use gun assembly was originally planned, the discovery of the Pu-240 problem rendered it moot. To minimize (but not eliminate) this problem the presence of light elements (especially beryllium, fluorine, and boron) must be kept to parts-per-million levels, a task of substantial difficulty. Aluminum also undergoes the alpha -> n reaction to some extent making it less desirable as an alloying agent in weapons, although with modern weapons grade plutonium this contribution to neutron emissions is insignificant.

Ultimately the satisfactory performance of gallium alloys, the established base of experience with them, and the relative unimportance of such issues as the cost of the alloying agent preclude the consideration of agents like aluminum.

The original techniques for preparing plutonium metal involved pyrochemical reduction of plutonium halides with alkali metals. Typically PuF4 was reduced with calcium and iodine, this was the standard method in the U.S. at least into the 1970s. Higher purity can be achieved by electrorefining of the pyrochemically produced metal (a step not necessary for weapons use). This makes use of an electrolytic cell at 700 C, with a sodium, potassium, and plutonium chloride electrolyte, and a tungsten or tantalum cathode, and produces 99.99% pure plutonium. More recent techniques are based on direct pyrochemical reduction and electrorefining of plutonium oxides. Among other advantages, these processes produce fewer waste products that must be disposed of.

Handling of molten plutonium, and plutonium casting, is performed today using equipment made of slightly oxidized tantalum. Casting molds can also be made of machined graphite, mild steel, or cast iron if they are lined with calcium fluoride or the oxides of zirconium or yttrium. It has also been discovered that pure plutonium can be successfully cast in chilled aluminum molds. The cooling is so fast that the intermediate phase transformations, which occur relatively slowly, are almost entirely bypassed.

6.2.2.2 Plutonium Toxicity
Although plutonium presumably exhibits chemical toxicity like other heavy metals, this effect is insignificant (in fact, unobservable) compared to its radiotoxicity. Plutonium's toxic properties are due to the fact that it is an active alpha emitter. Alpha particles are hazardous only if they are emitted inside the body (i.e. the plutonium has been ingested). Although plutonium emits gammas and neutrons that penetrate the body from outside, the emission rate is too small to be a significant hazard. Since the toxicity of plutonium is determined by its alpha activity, the isotopic composition is a significant influence. Weapon grade plutonium, free of americium, is assumed below (activity approx. 0.071 Ci/g).

Alpha particles affect only tissues that contain plutonium or are in direct contact with it. Two types of effects are significant: acute and chronic toxicity. If the exposure rate is high enough, tissues can suffer acute radiation poisioning with toxic effects appearing quickly. If the rate is low, then the cumulative carcinogenic effect occurs.

It is very poorly absorbed through the GI tract, even when ingested as a soluble salt since it tends to bind with the contents of the stomache and intestines. Given its tendency to precipitate from aqueous solution, and to form insoluble complexes with other materials, plutonium contamination of water tends to be a self-limiting phenomenon.

Swallowing 500 mg (7 curies) of plutonium as a finely divided or soluble material can cause death from the acute exposure of the GI tract in several days to a few weeks. Inhalation of 100 mg (1.4 Ci) of plutonium as particles of optimal size for lung retention can cause death from lung edema in 1 to 10 days. An inhaled dose of 20 mg (0.28 Ci) will cause death by fibrosis in about 1 month. In doses much below these values, the chronic carcinogenic effects become the important ones.

To exert chronic effects, the plutonium must be continuosly present in the body. Inhaled insoluble particles of the appropriate size range for lung retention (1-3 microns) will most likely be permanently deposited in the lungs (a high explosive detonation, like a non-nuclear weapons accident, can convert 20-50% of the plutonium present to this form). The most likely chemical form to which a person might be exposed is plutonium oxide. The oxide is used in reactor fuel, and metallic plutonium particles are rapidly oxidized. The oxide is nearly insoluble in water.

The lifetime risk of lung cancer from deposited plutonium particles for an adult is roughly proportional to the amount ingested. The ingestion of 1 microgram of Pu (0.07 microCi) represents a risk of 1% of developing cancer (the normal rate of cancer incidence is 20%). Thus ingesting 10 micrograms boosts the lifetime risk of cancer from 20% to 30%. Ingesting 100 micrograms (7.1 microCi) or more virtually guarantees eventual development of lung cancer (usually after several decades)although evidence of lung damage may surface within several months.

Plutonium normally exists in biological systems in the +4 oxidation state which chemically resembles Fe 3+. If it is absorbed into the circulatory system it thus a high probability of being concentrated in tissues that contain iron: bone marrow (which is highly sensitive to radiation), liver, and spleen. Plutonium has a biological half-life of 80-100 years when deposited in bone tissue, essentially permanent. Its biological half-life in the liver is 40 years. Chelating agents may help accelerate plutonium removal. If 1.4 micrograms (0.1 microCi) is deposited in an adult's bones, immune system impairment will result, and bone cancer is likely to develop within several years.

The International Commission on Radiological Protection (ICRP) specifies an Annual Limit on Intake (ALI) of 20 nanoCi/yr (280 nanograms). This translates into an air concentration of 7 picroCi/M^3 for occupational exposure. The maximum allowable body burden of Pu-239 (occupational exposure) is 40 nanoCi (0.56 micrograms) and 16 nanoCi for lung burden (0.23 micrograms). Due to the problems of contamination, skin contact with plutonium is strictly forbidden in U.S. laboratories.

6.2.2.3 Plutonium Production
Plutonium-239 is the preferred isotope for weapons use. As I have discussed above in the section on transmutation, it is produced in nuclear reactors where U-238 is exposed to a flux of slow neutrons. This occurs automatically in the vast majority of the world's reactors since they use low-enriched or natural uranium as fuel, which consists mostly of U-238. It can also be produced in special reactors that use highly enriched uranium or plutonium as fuel, but include a blanket of natural or depleted uranium for plutonium breeding.

Non-fission capture of neutrons by Pu-239 causes Pu-240 to form as irradiation proceeds. In turn, Pu-241 and Pu-242 also accumulate though in diminishing quantities.


     Pu-239 + n -> Pu-240
     Pu-240 + n -> Pu-241
     Pu-241 + n -> Pu-242

A side reaction chain also produces Pu-238:

     U-238 + n -> U-237 + 2n
     U-237 -> (6.75 days, beta) -> Np-237
     Np-237 + n -> Np-238
     Np-238 -> (2.1 days, beta) -> Pu-238

The total irradiation that a fuel element or blanket element receives is measured in megawatt-days/tonne. When fuel elements are being discussed, this is referred to as the fuel's "burn-up". Better quality plutonium (for weapons) comes from elements (or "targets") with low MWD/tonne exposures which result in lower concentrations of the other undesired isotopes. The fuel elements in a modern enriched uranium light water reactor can reach 33000 MWD/tonne. Typical exposures in weapon breeder reactors are around 1000 MWD/tonne. Weapons grade plutonium in the Hanford graphite piles was produced with exposures of some 600 MWD/tonne, the Savannah River heavy water piles produced equivalent compositions with 1000 MWD/tonne (presumably because part of the neutron flux is being absorbed by tritium production). During the Manhattan Project, the natural uranium fuel in the Hanford production reactors received only 100 MWD/tonne exposures, due to the urgency of wartime production, and produced super-grade weapons plutonium (0.9-1.0% Pu-240, and negligible amounts of other isotopes).

6.2.2.4 Pu-238
This isotope has a spontaneous fission rate, 1.1x10^6 fission/sec-kg (2.6 times that of Pu-240) and a very high heat output (567 W/kg!). Its very high alpha activity (283 times higher than Pu-239) makes it a much more serious source of neutron emission from the alpha -> n reaction. In high-burnup commerical reator fuels it makes up no more tha one or two percent of plutonium composition in extracted plutonium, but even so the neutron production and heating can make it very troublesome. It is used in radioisotope thermal generators (RTGs) which produce electricity for applications such as long duration space missions such as Cassini and deep sea intelligence gathering systems. For these purposes it is produced by bombarding pure neptunium-237 targets in breeding reactors. Its specific activity is 17.5 Ci/g.

6.2.2.5 Pu-239
Pu-239 is the only desired isotope for weapons use, other isotopes are important through their adverse effects. Pu-239 has higher fission and scattering cross sections than U-235, and a larger number of neutrons produced per fission, and consequently a smaller critical mass.

Pure Pu-239 has a moderate rate of neutron emission from spontaneous fission, about 10 fission/sec-kg (some 30 neutrons/sec-kg). Considering the small critical mass required, 6 kg or less, gun assembly could be used if pure Pu-239 were available (although, due to its high alpha activity, light element impurities would have to be kept to a few ppm to avoid alpha -> n reactions).

The relatively short half-life of Pu-239 (compared with U-235) means that a significant amount of energy is emitted through radioactive decay. In fact Pu-239 produces 1.92 watt/kg. This is higher than the average metabolic rate of an adult human by weight, and the output is concentrated in one-twentieth the volume. Pieces of Pu-239 are consequently quite warm. If a piece were thoroughly insulated, its temperature would rise from room temperature to the boiling point of water in less than two hours, and to the alpha-beta transition point soon after. This presents a problem in weapon design since elevated temperatures can be reached from self-heating, even if environmental heating is avoided. It is this concern about an alpha phase pit reaching the alpha->beta transition temperature that prevents this phase from being used in weapons. The specific activity of Pu-239 is 61.5 milliCi/g.

The plutonium used in the U.S. weapons program was produced at Hanford, Washington and Savannah River, Georgia. Soviet/Russian plutonium production is located at Kyshtym, near Chelyabinsk.

6.2.2.6 Pu-240
Pu-240 is the major contaminant of concern in plutonium intended for weapons use. The level of isotopic contamination by Pu-240 is important primarily for its high spontaneous fission rate. It has a spontaneous fission rate of 415,000 fission/sec-kg, but emits about 1,000,000 neutrons/sec-kg since each fission produces about 2.2 neutrons. This rate is over 30,000 times that of Pu-239. A contamination of only 1% produces so many neutrons that implosion systems are required to produce efficient bombs. Standard weapon grade plutonium requires a Pu-240 content of no more than 6.5%. Higher levels guarantee that pre-detonation (and reduced yields) will occur even with high performance implosion.

Pu-240 is actually fairly fissile, somewhat more so than U-235. Nonetheless, high concentrations of Pu-240 raise the required critical mass thus aggravating the neutron background problem. Due to its relatively short half-life (1/4 that of Pu-239) Pu-240 produces a correspondingly higher decay heat output (7.1 watts/kg) thus increasing cooling problems in bomb design. Its specific activity is 0.227 Ci/g.

6.2.2.7 Pu-241
This isotope is about as fissile as Pu-239, has a low neutron emission rate, and a moderate heat production rate, and thus does not adversely affect weapon usability of plutonium directly. It decays with a short 14.1 year half-life though, into americium-241 which is non-fissile and does produce a great deal of heat: 106 W/kg. If a weapon thus does include Pu-241 initially over several years or decades its reactivity will decline somewhat, which must be taken into account when the weapon is designed to avoid reduced yield, and its self-heating will increase. The reason that Pu-241 does not emit much heat (3.4 W/kg) despite its very short half-life is its weak beta emission decay mode. Its specific activity is 106 Ci/g.

6.2.2.8 Pu-242
This isotope has a high neutron emission rate, 8.4x10^5 fissions/sec-kg (twice that of Pu-240), and is non-fissile. A substantial concentration thus has the serious adverse effect of driving up the critical mass, while also adding a high neutron background. It has a long half-life, and has a relatively low neutron capture cross section, so it tends to accumulate in recycled reactor plutonium. Its specific activity is 4.0 milliCi/g.

6.2.2.9 Weapon Grade Plutonium
This term is used by the U.S. for plutonium with a Pu-240 content of less than 7%. Typical assays of weapon grade plutonium are given below. The first two are average assays of weapons grade plutonium produced at Hanford, and Savannah River in June 1968. The third is based on soil samples taken outside the Rocky Flats Plant in the 1970s, and is adjusted for the americium-241 also present (the decay product of Pu-241).


TYPICAL WEAPONS GRADE PLUTONIUM COMPOSITIONS

           Hanford           Savannah      Rocky Flats Soil
         (avg. 6/68)        (avg. 6/68)     (avg. 1970s)
Pu-238  less than 0.05%   less than 0.05%       trace
Pu-239           93.17%            92.99%       93.6%
Pu-240            6.28%             6.13%        5.8%
Pu-241            0.54%             0.86%        0.6%
Pu-242  less than 0.05%   less than 0.05%       trace

The U.S. has also produced supergrade plutonium with Pu-240 content of 3%, for use as an enricher for lower grade plutonium, and perhaps as an ingredient in special weapon designs. Some U.S. designs have required plutonium with a Pu-240 content as low as 1.5%.

An important question is what the designation as "weapons grade" actually means. The prevalent interpretation has been that this indicates that plutonium with a Pu-240 content less that 7% is actually required for successful weapon construction, or at least, there is a serious compromise of weapon performance above this level.

The Pu-240 content definitely does have weapon design consequences, since it determines the neutron background level and has secondary effects in increasing critical mass (slightly) and thermal output. The neutron background constrains the design by limiting the amount of plutonium included, and requiring implosion speeds above some specified threshold. As noted above, some U.S. weapons designs (presumably older ones) required low Pu-240 contents for these reasons.

However, it is now apparent that these issues are unimportant in advanced weapons designs used by the U.S. since at least the early 1960s. Recently declassified government documents (WASH-1037 Revised, An Introduction to Nuclear Weapons, June 1972) make it clear that the designation "weapons grade" is purely an economic one. The cost of plutonium goes down the higher the Pu-240 content. On the other hand, the critical mass goes up with higher Pu-240 contents. Around the 6-7% Pu-240 level, the total cost of the plutonium in the weapon is at a minimum.

This does not mean that plutonium with higher levels of Pu-240 can be used in EXISTING weapon designs. These have been optimized for the use of a specific material and would probably suffer in performance if a different plutonium composition were used.

Assuming an average composition of 93.4% Pu-239, 6.0% Pu-240, and 0.6% Pu-241 (with negligible amounts of other isotopes), the following properties of WG plutonium can be calculated. The initial heat output of freshly prepared WG-Pu would be 2.2 W/Kg, and the spontaneous fission rate would be 27,100 fissions/sec. This fission rate permits a weapon using 4-5 kg of plutonium to be assembled with a very small probability of predetonation by a good implosion system. Over the course of a couple of decades, most of the Pu-241 will decay into Am-241, eventually raising the heat output to 2.8 W/kg. Since Pu-241 is highly fissile, but Am-241 is not, this reduces the reactivity margin in the weapon slightly and must be taken into account in weapon design.

The neutron emission for a 5 kg of WG-Pu, 3x10^5/sec, represents an exposure of some 0.003 rad/hr at 1 m. This is reduced by the reflector and explosive surrounding it, a light-weight weapon might attenuate it by a factor of 5-10. The high RBE for neutrons on the other hand, enhances the risk. Constant close contact with a weapon during a normal work schedule would result in radiation exposures close to the annual occupational limit. Weapon plant employees who handle plutonium cores directly or in glove boxes have limited shielding and may need to be rotated to other tasks to keep exposures down.

Due to the small mass difference between Pu-239 and Pu-240, it has not been considered practical to strip Pu-240 by the common enrichment schemes used with uranium. This has been done for small quantities of plutonium using electromagnetic separation for research purposes. There is no major reason for a sophisticated nation to reduce the Pu-240 content below 6% since it still allows the construction of efficient, reliable fusion bomb triggers. Very low Pu-240 content would allow some additional flexibility in weapon design, which may be desirable for specialized or exotic designs.

6.2.2.10 Reactor Grade Plutonium
For reasons of economics, power production reactor fuel has very high fuel burn-ups. The plutonium from discharged power reactor fuel has a high level of contaminating isotopes. Compositions vary with reactor design, and the exact operating history, but typical ones are:


Isotope  Light Water Reactors  CANDU Reactor  MAGNOX Reactor
           Typical 33000 MWD     7500 MWD      3000 MWD

Pu-238       2%       1.5%          low           0.1%
Pu-239      61%      56.2%         66.6%         80.0%
Pu-240      24%      23.6%         26.6%         16.9%
Pu-241      10%      14.3%          5.3%          2.7%
Pu-242       3%       4.9%          1.5%          0.3%

The 33000 MWD burnup figure is based on the 3% enrichment commonly used in the 1970s and 80s. With the current cheapness of enriched uranium (based on a world wide post-arms race glut of enrichment capacity), higher enrichment fuel is now being used (4-4.5%), allowing burnups of 45000 MWD or even higher. This results in higher concentrations of Pu-238, 240, 241, and 242.

Using the light water average as the basis, we can calculate a heat output of 14.5 W/kg rising to 19.6 W/kg in 14 years as the Pu-241 decays, reaching an eventual maximum near 24 watts. The neutron emission rate is 350,000 neutrons/kg. The specific activity is 11.0 Ci/g (0.442 Ci/g alpha activity).

Taking into account the effect of isotopic dilution on critical mass (both Pu-239 and Pu-241 are fissile, the others are less so) a bomb fashioned from 8 kg of this material would put out 116 watts (a light bulb of similar size and power is too hot to hold), and 2.8 million neutrons/sec. Using this material in a bomb would be a challenge. Continual active cooling would be needed to prevent deterioration and damage to the core, explosives, and other components. The high rate of neutron emission means that predetonation is inevitable, even with a very efficient implosion system. However, even the relatively primitive Fat Man design would have produced a 0.5 kt or so yield with this material. With optimal implosion design yields in the range of at least several kilotons are possible. If fusion boosting is used, then the adverse effects properties of reactor grade plutonium can be completely overcome, allowing its use in efficient high-yield designs although the material would be less convenient to use.

While reactor grade plutonium would probably be of no interest to a nation with access to better grade material, it could be effectively used by a nation capable of good weapon design, but without access to better fissile material. Even a low technology nation could fashion powerful weapons from it, after all even a 1 kt device greatly exceeds the destruction of any conventional weapon.

Over long periods of time, several decades or centuries, the heat output of reactor grade fuel diminishes greatly from the decay of Pu-238 and Am-241. This has little effect on the neutron background however. Currently, used reactor fuel is commonly stored indefinitely in containment pools near the reactor site. It will remain a proliferation risk until otherwise disposed of, in fact as the fission product radiation levels and plutonium heat output declines, the proliferation risk increases.

Storage for 40 years allows 30% of the Pu-238 and 88% of the Pu-241 to decay. The plutonium composition would then be: 1.5% Pu-238, 67.3% Pu-239, 26.4% Pu-240, 1.3% Pu-241, and 3.3% Pu-242. This would decrease the heat output to 11.7 W/kg and largely stabilize it from post-fabrication increase (maxing out at 13.8 W/kg). Storage for 150 years would result in a composition of 0.66% Pu-238, 69.06% Pu-239, 26.86% Pu-240, 0.01% Pu-241, and 3.41% Pu-242; for a stable heat output of 7.5 W/Kg.

Using uranium enrichment technology to strip out undesirable isotopes is a real possibility. It is complicated by the presence of multiple isotopes, each separated by one atomic mass unit, compared to the 3 amu difference in uranium. In general this reduces the separation parameter (s - 1) by a factor of 3, and the separation capacity of a stage or plant by a factor of 9. Also a double enrichment process might be necessary. After separating Pu-240 and higher isotopes, a second enrichment might be needed to strip out Pu-238 (depending on the content of the starting material, and how objectionable the heating effect is). The toxicity, neutron emission, and self-heating of the feed, waste, and product would also complicate enrichment operations, compared to uranium.

On the other hand the amount of feed that must be processed to produce a bomb is more than two orders of magnitude smaller than natural uranium. This is due both to the high Pu-239 content (60-70% compared to 0.72%), and the smaller critical mass required (6 kg vs 15 kg). Even with the complications mentioned above, an enrichment plant for upgrading reactor plutonium would be much smaller than a natural uranium plant regardless of the technology used. This material must thus be considered a serious proliferation risk on the national level.

It should be fairly straightforward to produce weapon grade plutonium from reactor grade by electromagnetic separation. Due to the extremely high enrichment factor of this process only one stage of enrichment is needed, and plant output is proportional to the concentration of desired isotope in the feedstock. An electromagnetic separation facility of capable of producing 0.5 bombs a year (similar to the facility that Iraq was planning before the 1991 war) could produce over 100 bombs a year if reactor grade plutonium were available.

Gas diffusion and centrifuges are also viable candidates. The properties of plutonium hexafluoride are similar to uranium hexafluoride, and requires only small changes in diffuser or centrifuge design. If 60% Pu-239/25% Pu-240 material were used as feed, with a product of 94% Pu-239 and tails of 50% Pu-239 (product contains half of the feed Pu-239), then only 2 kg-SWU would be required to produce 1 kg of weapon grade plutonium. This is 1% of the effort required to produce 1 kg of 90% U-235 from natural uranium. Taking into account to the smaller number of SWUs, and the offsetting smaller critical mass of plutonium, a plutonium enrichment plant could produce 29 times as many bombs as an equivalent U-235 plant. A centrifuge plant might need only few dozen stages for a cascade.

The AVLIS technology offers the possibility of inexpensive separation and could have used commercial reactor grade fuel as a feed. This was perhaps the real motivation for pursuing AVLIS research during the eighties.

6.2.2.11 Denatured Plutonium
If plutonium is extracted from reactor fuel and reused in power reactors, its composition becomes progressively less desirable for weapons use. After several fueling cycles the buildup of Pu-238, Pu-240, and Pu-242 makes it difficult to use in weapons. It has been suggested that blending such recycled fuel is a useful means to "denature", or render proliferation-resistant, supplies of retired weapons-grade plutonium.

This is mainly a deterrent against the use in low technology designs. The elevated thermal output and radiation levels merely have nuisance value, and are not serious deterrents although they create significant design constraints and handling problems. Advanced designs and adequate handling facilities can completely overcome these obstacles. Even at low technology levels destructive devices could be constructed with this material.

6.2.3 Thorium (Th)

Named after Thor, the Norse god of thunder, element 90 was discovered in 1828 by Swedish chemist and minerologist Baron Jons Jakob Berzelius. Berzelius isolated the element from a black silicate mineral, subsequently named named thorite, from the island of Lövö near Brevig, Norway. Thorium’s radioactivity was discovered independently in 1898 by Madame Marie Curie and Gerhard Carl Schmidt.

Thorium is a silvery-white lustrous metal that resists oxidation when pure, but typically tarnishes slowly to black over time. Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. It is slightly soluble in sulfuric acid and nitric acids. Powdered thorium metal is often pyrophoric and should be carefully handled. When heated in air, thorium turnings or scraps ignite and burn with a brilliant white light. Pure thorium is soft, very ductile, malleable, and can be readily worked (cold rolled, swaged, etc.), but drawing is difficult because of thorium's low tensile strength. The oxide content strongly affects thorium's mechanical properties; even pure thorium samples typically contain a few tenths of a percent of thorium oxide. When strongly heated it reacts with halogens, sulfur, and nitrogen. Its electronic configuration is 2-8-18-32-18-10-2 or (Rn)6d2-7s2.

Its principal physical properties are: Density 11.72
Undergoes cubic->body centered cubic crystal change at 1400 C
Melting point 1750 C
Boiling Point 4790 C

Thorium oxide (known as thoria) has an extremely high melting point, 3300 C, the highest of all oxides and higher than the melting points of all but a few substances. This property once accounted for thorium's major commerical use as a refractory ceramic - principally in ceramic parts, investment molds, and crucibles. Its most commonly encountered use was in the Welsbach mantle for "camping" lanterns which produce a dazzling light when heated to very high temperatures.

Thorium is rather abundant. It is the 39th most abundant of the 78 common elements in the Earth’s crust, at 7.2 parts per million. It is about three times more abundant than uranium, about equal to lead. It is associated with uranium in igneous rock. As the primary thorium minerals are more resistant to geochemical and physical weathering, the thorium/uranium ratio in sedimentary rock is typically higher than in igneous source rock. Thorium occurs in several minerals, the most common being monazite (a rare-earth-thorium-phosphate mineral, containing from 3 to 9% ThO2) and thorite (thorium silicate). It also occurs as orangite, a variety of thorite; and in thorianite, a mineral composed of thorium oxide and uranium.

It is estimated that the potential energy represented by accessible thorium deposits exceeds that of uranium and fossil fuels combined by a wide margin. The major deposits of thorium are usually in the form of monazite sands. Large deposits of thorium are found in several parts of the world including the U.S. (New England and North Carolina) and India, which has the largest reserves.

Most thorium production is as a byproduct of rare earth extraction from monazite, which is mined and concentrated in turn as a byproduct of processing heavy-mineral sands for titanium, zirconium, or tin. Only a small portion of the thorium produced was consumed; most was discarded as radioactive waste, an operation that has become increasingly expensive as safety standards regarding radiation have been tightened. The major monazite-producing countries were India, Brazil, Malaysia, China, and Sri Lanka. In 1997 worldwide monazite concentrate production was 7000 tonnes, with a value of US$0.73/kg (calculated on the basis of rare earth content). In 1997 there was no domestic U.S. production. The market for thorium is very small. U.S. domestic consumption of refined thorium products in 1997, according to the U.S. Geological Survey (USGS), was 13.0 tonnes of thorium oxide equivalent valued at only $300,000.

The commercial use of thorium has been declining due to increasingly strict radiation regulations, causing business and industry to replace thorium with non-radioactive substitutes. Thorium is no longer used in the U.S. to make gas lantern mantles, and its use as a refractory ceramic is also declining.

Important uses remain in the production of thoriated tungsten welding electrodes, which are employed used to join stainless steels, nickel alloys, and other alloys requiring a continuous and stable arc to achieve precision welds.

Due to its ability to emit electrons at relatively low temperatures when heated in a vacuum thorium is used in a variety of electronic tube devices. It is used in the thoriated tungsten negative poles of microwave generating magnetron tubes. It is also useful in certain high speed, high power vacuum tube switches that may have seen use in nuclear weapon firing circuits.

Thorium has been widely used in alloys with magnesium by the aerospace industry. These alloys are lightweight and have high strength and excellent creep resistance at elevated temperatures. At least one U.S. missile nuclear warhead, the W40 (or else perhaps the Bomarc missile that carried the warhead), has used thorium-magnesium alloy for structural reasons. Thorium-free magnesium alloys with similar properties have been developed and are expected to replace most of the thorium-magnesium alloys presently used. Small quantities of thorium were used in dispersion-hardened alloys for high-strength, high-temperature applications.

Thorium is used in other applications such as chemical catalysts (paints, fuel cell elements), elements in special use light bulbs, thoria-containing high-refractivity glass, photo-conductive films, radiation detectors, and target materials for X-ray tubes.

Several methods are available for producing thorium metal; it can be obtained by reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal.

In 1997 thorium oxide prices were quoted at $65.55 per kilogram (base price), $82.50 for 99.9% purity and $107.25 per kilogram for 99.99% purity. Thorium alloy prices were no longer available due to lack of sales.

Government stocks of thorium nitrate in the National Defense Stockpile (NDS) were 3,217 tonnes (as thorium oxide equivalent) on 31 December 1997, 2,944 tonnes of which was considered surplus and marked for disposal. The NDS only shipped 1,724 kilograms (1.7 tonnes) for waste disposal research in 1997, no stocks of thorium nitrate were sold during the year.

Twelve isotopes of thorium are known, but the natural primordially existing element consists entirely of a single isotope - Th-232. This isotope has a half-life of 1.405 x 10^10 years and is an alpha emitter. Its specific activity is 0.109 microCi/g, about one-sixth of natural uranium. Thorium is radioactive enough to expose a photographic plate in a few hours. The thorium decay chain produces radon-220 gas ("thoron") which is an inhalation radiaton hazard.

Five other thorium isotopes occur naturally in small amounts as a result of production by the decay of U-238, U-235, and Th-232. These are Th-227 (half life 18.72 days), Th-228 (1.9116 years), Th-230 (75,380 years), Th-231 (1.063 days) and Th-234 (24.1 days). Since all of these isotopes but Th-228 are produced by the uranium decay chains, the concentration of these isotopes in natural thorium depends on the relative amount of uranium present and the possible effects of chemical leaching of the protactinium or actinium intermediates. For practical purposes, the only isotopes that are present at significant levels in purified thorium are Th-228 and Th-230 since the others have very short half-lifes, and Th-228 decays considerably after several years of storage.

Thorium is of interest for nuclear purposes due to its ability to breed fissile U-233 through the chain:

Th-232 + n -> Th-233
Th-233 -> (22.2 min, beta) -> Pa-233
Pa-233 -> (27.0 day, beta) -> U-233

There has been interest in the possibility of establishing a thorium/U-233 fuel cycle using breeder reactors, especially in uranium-poor but thorium-rich India. As a nuclear explosive U-233 is about as good as plutonium-239, and has been employed in several tests by the U.S. (e.g. Teapot MET).

A complication in the use of U-233 in weapons are side reactions that produce the potent gamma-ray emitting U-232. U-232 is produced by the reaction:

Th-231 -> (25.5 hr, beta) -> Pa-231
Pa-231 + n -> Pa-232
Pa-232 -> (1.31 day, beta) -> U-232

The Th-231 is produced in turn by two reactions:

Th-232 + n -> Th-231 + 2n

and

Th-230 + n -> Th-231

The first (n,2n) reaction only occurs to any significant extent when neutrons with energies in excess of 6 MeV are encountered. Only a small percentage of fission neutrons are this energetic, and if the thorium breeding blanket is kept in a reactor region where it is only exposed to a well moderated neutron flux (i.e essentially no neutrons above the Th-232 fission threshold of 500 KeV) this reaction can be nearly eliminated. The second reaction proceeds very efficiently with thermalized neutrons however, and minimizing U-232 from this source requires choosing thorium that naturally has a low Th-230 concentration.

Thorium-232 also undergoes fast fission and could thus be used in thermonuclear secondary stage tampers, but it has a markedly lower cross section for this than does U-238 (at 14 MeV 0.35 barns vs 1.14 barns, and for fission spectrum neutrons 0.078 barns vs 0.308 barns) and is thus not a serious contender for this use.

A major problem with using thorium breeding to obtain fissile material is that it is not naturally present in economical reactor fuels, unlike U-238. To use thorium breeding, a highly enriched fissile material (U-235, U-233, Pu-239) must be used as a fuel with thorium included in the reactor mostly for its breeding potential (i.e. it contributes little or nothing to power production itself, although burning of U-233 produced in situ may contribute to power production). On the other hand thermal breeder reactors are possible using a U-233/thorium breeding cycle, especially if heavy water is used as a moderator.

U-232 contamination is not a serious problem for civilian power use, although unshielded exposure to fuel material during fabrication and handling prior to irradiation would have to be limited. The ability of high levels of U-232 to make diversion difficult and undesirable has been promoted as an anti-proliferation feature of the thorium/U-233 fuel cycle.

Thorium is not currently being used in any foreign or domestic commercial reactors. The fuel is not expected to grow due to the current availability of low-cost uranium. India is interested in developing an indigenous thorium breeding fuel cycle due to its vast thorium deposits, and is accumulating plutonium as the start-up fuel for future thorium cycle breeders. It is believed that India has included U-233 breeding as part of its weapons related activities also.

6.2.4 Other Fissile Elements

U-233, U-235 and the various isotopes of plutonium are not the only known elements and isotopes with the necessary nuclear properties to be usable in fission bombs. Several other candidates are described here, of varying degrees of practicality. None of them have properties that are sufficiently attractive to merit serious consideration in favor of the other "big three", but all of the isotopes discussed below have attracted some attention for a variety of reasons.

Few of these isotopes have well measured critical mass values available in the open literature. Most of the critical mass data are estimates made from their measured or theoretically predicted nuclear properties. The literature is rather spotty in the availability and consistency of these estimates. Accordingly, I have suplemented the published figures with my own calculations for comparison purposes.

My critical mass estimates are for bare spheres at the densest STP phase. The calculations were performed using diffusion theory and a one-group representation of neutronic properties. The one-group parameters are fission spectrum averages calculated from the authoritative ENDF-6 evaluated nuclear data base. Where available I also list estimates from other sources which are usually based on more sophisticated numerical computations than what I have used.

The one-group calculation method consistently underestimates the true critical mass - primarily because it does not take into account the effects of inelastic scattering in softening the neutron spectrum. The one-group calculated critical mass estimates are thus lower bounds on the true value. Comparison between the one-group calculations and the actual values for the highly fissile isotopes for which good experimental data is available (U-233, U-235, Pu-239, and Pu-241) shows a consistent underestimate of 70-75% of the true value. For less fissile isotopes, where critical mass estimates have been offered by others (these are mostly calculated estimates also, but with more sophisticated models), the underestimates are more severe (at worst 22-29% of the 'true' value for Pu-242). This too is to be expected because the effects of inelastic scattering is relatively greater in less fissile materials. On the other hand, the estimates for extremely fissile transuranics like californium isotopes should be quite good.

The list below is not exhaustive. In particular it is likely that other fissile isotopes of transuranic elements exist (curium, berkelium, etc.).

6.2.4.1 Protactinium
Protactinium was identified by Kasimir Fajans and O.H. Gohring in 1913 from the short-lived Pa-234m isotope and was named brevium ("brief") due to its short lifetime (1.175 minutes). When the much longer-lived isotope Pa-231 was identified by Austrian-Swedish physicist Lise Meitner and the German physical chemist Otto Hahn in 1918 it was renamed protoactinium (i.e. predecessor of actinium). Pa-231 was independently rediscovered in 1918 by Frederick Soddy, John Cranston, and Sir Alexander Fleck; and by Fajans. The name protoactinium was shortened to protactinium in 1949.

The metal itself was not isolated until 1934 when Aristid V. Grosse developed two methods. One involved reduction of the pentoxide Pa2O5 with a stream of electrons in a vacuum and the second involved dissociating the iodide PaI5 by a heated filament under a high vacuum.

Protactinium is a malleable silvery metal with a bright metallic lustre which it retains for some time in air, tarnishing with an oxide coating. Its has a tetragonal crystal structure and a density is 15.37. Its melting point is 1570 C, its boiling point is 4227 C. Since its half-life is similar to Pu-239, it has similar alpha activity as well and must handled with similar safety precautions.

Protactinium has three valence states: III through V. Protactinium in most of its compounds exhibits an oxidation state of V (thus resembling tantalum). It is soluble in dilute HF. Its compounds readily hydrolyze in water, forming colloids, but dissolve by forming complex ions (as with the fluoride ion in hydrofluoric acid). It forms oxides, and halides, and reacts with H2 at 250-300 C to form PaH3. Its electronic configuration is 2-8-18-32-20-9-2 or (Rn)5f2-6d1-7s2.

Protactinium is one of the rarest and most expensive naturally occurring elements. Protactinium has 20 isotopes, its predominant and stablest isotope, protactinium-231, is a decay product of U-235 and exists in decay equilibrium with it in uranium ores. The creation and destruction is governed by the reaction chain:

U-235 -> (7.1 x 10^8 yr, alpha) -> Th-231
Th-231 -> (25.52 hr, beta) -> Pa-231
Pa-231 -> (32,500 yr, alpha) -> Ac-227

From the half-life ratio of U-235 and Pa-231 we can calculate that they exist in the ratio of (3.25 x 10^4)/(7.1 x 10^8) = 4.6 x 10^-5. Since 0.72% of natural uranium is U-235, the ratio for Pa-231 to all uranium atoms is 3.3 x 10-7 (3.2 x 10-7 by mass), discounting any chemical separation of the elements. Typical pitchblende ores contain about 0.1 ppm of Pa-231. Some ores from Congo (formerly Zaire) have up to 3 ppm.

Based on theoretical considerations Pa-231 in the past has been considered a potential weapon-usable material, the only such material with an atomic number lower than that of uranium. It was the only candidate fissile material that could in principle be obtained from natural sources through conventional chemical separation techniques alone. It was considered as a candidate fissile material during the early stages of atomic research in the 1940s.

This possibility has now been effectively excluded. State-of-the-art calculations by IRSN (L'Institut de radioprotection et de sûreté nucléaire) have shown that Pa-231 is probably not critical even in an infinite metal assembly (Report C4/TMR2001/200-1).

Pa-231 is extracted from uranium ores and can be concentrated from uranium ores as part of the normal process of ore refining. If Canada, the world's largest uranium producer, had done this with its 1994 uranium production (9700 tonnes) it would have obtained 3.1 kg of Pa-231. Accumulating an unreflected critical mass in this manner would take several decades.

Unless the "tails" piles left from uranium mining and extraction were stringently guarded (which would itself be suspicious) it would be impossible to conceal such activity since an examination of tails samples would immediately reveal if Pa-231 accumulation was being practiced.

Perhaps motivated by interest in its possible use in weapons, between 1959 and 1961 the Great Britain Atomic Energy Authority extracted by a 12-stage process 125 g of 99.9% protactinium, the world's only stock of the metal at the time. The extraction was made from 60 tons of waste material at a cost of about $500,000.

Protactinium can also be produced from natural thorium in a reactor by the following reactions:

Th-232 + n -> Th-231 + 2n
Th-231 -> (25.52 hr, beta) -> Pa-231

6.2.4.2 Neptunium
Neptunium was the first transuranium element discovered, and the first synthetic transuranium element to be prepared. Though traces of neptunium have subsequently been found in nature, it was discovered by Edwin M. McMillan and Philip H. Abelson in May 1940 at Berkeley, California, USA, who bombarded uranium with neutrons produced from a cyclotron, producing Np-239. Since it was the next element in the periodic table after uranium, by analogy it was named after the planet Neptune, which is the next planet out from the Sun after Uranus (this pattern was subsequently followed in the naming of plutonium).

As is also true of plutonium, trace quantities of the element are actually found in nature due to neutron-induced transmutation reactions in uranium ores produced by spontaneous fission neutrons.

Neptunium is a silvery metal, with a density of 20.45 (Np-237, 25 C), neptunium is the fifth densest element. It has at least three allotropic (crystalline) forms -- the alpha form which is the densest (stable below 280 C, orthorhombic, density 20.45 at 25 C), beta (above 280C, tetragonal, density 19.36 at 313 C), and gamma (above 577 C, cubic, density 18.0 at 600 C). Its melting point is 637 C, and its boiling point is estimated at 5235 C.

Neptunium is chemically reactive and similar to uranium with oxidation states from +3 (III) to +7 (VII). The metal is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200C. Neptunium of the five ionic oxidation states the pentavalent is the most stable in solution. Neptunium ions in aqueous solution possess characteristic colours: Np3+, pale purple; Np4+, pale yellow-green; NpO+2, green-blue; NpO2+2, varying from colourless to pink or yellow-green, depending on the anion present. The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides for the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2. Its electronic configuration is 2-8-18-32-22-9-2 or (Rn)5f4-6d1-7s2.

Fifteen isotopes of neptunium are now recognized. The isotope neptunium-237 (discovered in 1942 by Glenn T. Seaborg and Arthur C. Wahl) is the longest lived, with a half-life of 2.144 x 10^6 years, and is fissile. It is most easily produced by the following reactions:

U-235 + n -> U-236
U-236 + n -> U-237
U-237 -> (6.8 days, beta) -> Np-237

It is also produced by the decay of Am-241, but the 433 year half-life of this latter isotope makes the production rate very small.

Published estimates for the critica mass of neptunium-237 are around 90 kg (range of estimates is 75-105 kg), my one-group lower bound calculation is 19 kg. It has a very low spontaneous fission rate (< 5x10^-2 F/sec-kg). The high critical mass value (almost double that of HEU) and the high cost of manufacture makes it unattractive for weapons use. A certain amount Np-237 is produced normally by U-235 capture in reactors. A typical power reactor can discharge about 0.4 kg of Np-237 per ton of fuel. Fast reactors can discharge a significantly higher amount.

Np-237 is used in some types of neutron detection instruments

6.2.4.3 Americium
Americium (atomic number 95) was the fourth transuranic element to be synthesized (curium, atomic number 96, was discovered a few months previously). It was identified by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso in 1944 as the result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor to form Am-241. Americium was also prepared by Glenn Seaborg, et al by bombarding Pu-234 with alpha particles to produce Am-241.

It is a silvery metal, the luster of freshly prepared americium metal is whiter and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium. Americium tarnishes slowly in dry air at room temperature. It has two allotropic forms. The alpha form is stable at low temperatures and is a double close-packed hexagonal structure, density 13.67 (molar volume 17.6 cm^3/mole, 20 C). It transitions at 1074 C to the beta form with a face-centered cubit structure. The melting point is 1175 C.

Americium has valences III, IV, V, and VI, in acidic aqueous solution with the following ionic species: Am3+, pink; Am4+, rose (very unstable); AmO2+ , yellow; and ((AmO2)+2)2, light tan. Trivalent americium is most common in aqueous solution and in this state is very similar to the other actinide and lanthanide elements. There is some evidence that the ion Am2+ has been prepared in trace amounts, its existence suggesting that americium is similar to its lanthanide homologue, europium, which can be reduced to its divalent state. Am IV is known only in the solid state. Americium reacts with oxygen to form the dioxide AmO2 and with hydrogen to form the hydride AmH2. Americium dioxide is obtained by ignition of of most trivalent compounds, AmF4 is produced by fluoridation of the dioxide or trifluoride. Its electronic configuration is 2-8-18-32-25-8-2 or (Rn)5f7-7s2.

Am-241 has a half-life of 433 years, and an estimated bare critical mass of 84 kg, my one-group lower bound calculation is 23 kg. It has a moderate spontaneous fission rate (2.4x10^2 F/sec-kg).

The most commonly used isotope of americium is Am-241, now produced in quantity by the decay of Pu-241:
Pu-241 -> (13.2 yr, beta) -> Am-241

Since Pu-241 is normally present in freshly made weapons-grade plutonium, Am-241 accumulates in the material as the Pu-241 decays.

Since Pu-241 is normally present in freshly made weapons-grade plutonium, Am-241 plays a significant role in weapons issues. Freshly made "weapons-grade" plutonium normally contains 0.5 - 1.0% Pu-241, reactor grade plutonium typically contains 5-15% Pu-241 to as much as 25%. In the course of a few decades, most of this Pu-241 will decay into Am-241. Am-241's energetic alpha decay and relatively short half-life give it a high specific activity and heat output. Much of the alpha and gamma activity and heat produced in older weapon grade plutonium is due to Am-241. Reprocessing WG-Pu reclaimed from old weapons eliminates this problem.

This high critical mass (almost double that of HEU) and relatively high cost make it unattractive for use as a weapon fissile material. It is made even more unattractive by the fact that a bare critical mass of Am-241 produces 9.6 kw of heat! It would be challenging to make a storable weapon that can tolerate this kind of heat output, probably requiring continuous active cooling in storage.

The longer lived isotope Am-243 (half-life 7370 years) is another possible fissile material. It has a higher critical mass, published estiamtes are unavailable but my one-group calculation is 38 kg. By comparison with the estimates for Am-241 this suggests a true value in the vicinity of 140 kg, which would have a heat output of 900 watts. Am-243 is a much less available and more expensive material however since it is produced by neutron irradiation of Am-241.

The isotope americium-241 is the most important because of its availability. It has been prepared in kilogram amounts from plutonium and has been used industrially in fluid-density gauges, thickness gauges, aircraft fuel gauges, and distance-sensing devices, all of which utilize its gamma radiation. Am-241 is used in very small amounts in smoke detectors (0.26 micrograms per detector) where its alpha emissions are employed. It has also been used as a diagnostic aid in bone mineral analysis. In Sept. 1998 the ORNL Isotopes Division charged $728.00 per gram of americium-241, and $180.25 per milligram of americium-243.

6.2.4.4 Californium

Californium, the sixth transuranium element to be discovered, was produced in 1950 by Glenn T. Seaborg, Stanley G. Thompson, Albert Ghiorso, Kenneth Street in 1950 by bombarding microgram quantities of Cm-242 with 35 MeV helium ions in the Berkeley 60-inch cyclotron to produce Cf-245. It was named after the State and University of California.

Metallic californium (Cf-249) has been prepared by reducing Cf2O3 with lanthanum metal. Two crystalline forms have been identified, a face-centered cubic and hexagonal close-packed structures. The melting point is 900 C (+/- 30). From theoretical models the density is 15.3 for the Cf-252 isotope.

Californium has valences II, III, and (rarely) IV. Californium (III) is the only ion stable in aqueous solutions, all attempts to reduce or oxidize californium (III) in solution having failed. In addition to the Cf(III) oxide (Cf2O3), several binary compounds with halogens are known -- the Cf (II) bromide and iodide; the Cf (III) fluoride, chloride, bromide and iodide; and the Cf(IV) fluoride. Other compounds that have been prepared include the oxychloride, CfOCl. Its electronic configuration is 2-8-18-32-28-8-2 or (Rn)5f10-7s2.

Known isotopes of californium include:

Isotope Atomic Mass Half-life  Decay Mode
Cf-242  242.06372     3.3  m   alpha to Cm-238
Cf-243  243.065      11    m   alpha to Cm-239
Cf-244  244.06599    20    m   alpha to Cm-240
Cf-245  245.06807    44    m   alpha to Cm-241
Cf-246  246.06884    36    h   alpha to Cm-242; SF (0.2%)
Cf-247  247.0712      3.11 h   electron capture to Bk-247 (99.97%)
Cf-248  248.07218 	334    d   alpha to Cm-244; SF (0.013%)
Cf-249  249.07485 	351    y   alpha to Cm-245; SF (5.2E-7%)
Cf-250  250.07640 	 13.1  y   alpha to Cm-246; SF (0.079%)
Cf-251  251.079580	898    y   alpha to Cm-247
Cf-252  252.08162 	  2.64 y   alpha to Cm-248; SF (3.08%)
Cf-253  253.08513 	 17.8  d   alpha to Cm-249; beta to Es-253
Cf-254  254.08732 	 60.5  d   SF (above 99%); alpha to Cm-250 (less than 1%)
Cf-255  255.0910 		  1.4  h   beta to Es-255

The best known and most widely used isotope of californium is Cf-252. This isotope has an extremely high rate of spontaneous fission, a remarkable 3.08% of decay events being fissions, and a relatively short half-life (2.64 years) making it a potent emitter of fission spectrum neutrons (one microgram emits 170 million neutrons per minute). The half-life is long enough that a sample of Cf-252 can remain active for several years, allowing it to serve as a convenient compact portable neutron source. It has found use as a start-up neutron source for nuclear reactors, and in neutron moisture gauges and in well-logging (the determination of water and oil-bearing layers in bore holes). It is also being used as a portable neutron source for discovery of metals such as gold or silver by on-the-spot activation analysis. Cf-252 foils are used as a source of fission fragments for research purposes. Cf-252 has also been used as a source of fisson neutrons in a mock-up of the Little Boy bomb in Hiroshima dosimetry studies. As of May 1975 some 63 mg of Cf-252 had been produced and sold by ORNL.

Californium has acquired a reputation for having a remarkable small critical mass, usually alleged to be "in the gram range", creating much speculation about possible use in "pocket nukes" -- very small fission weapons. While this reputation for a small critical mass is partly justified, it is also exaggerated and appears to have taken on the character of an urban legend. In particular the assertion that californium has a critical mass "in the gram range" is at best very misleading.

As is true of other transuranic elements, the odd-numbered isotopes of californium have the most desirable fissile properties. Only two isotopes are plausible candidates for fission explosives - the long lived isotopes Cf-249 and Cf-251. Popular speculation about californium as a weapon material has usually centered on Cf-252 due to its neutron-emitting celebrity. Its comparatively short half-life (for weapons purposes), intense and penetrating neutron radioactivity, and high thermal output make it quite unsuitable for this however.

I have seen no actual published estimates for californium isotope critical masses, so I have done one-group calculations for the two californium isotopes that are most plausible for weapons use (Cf-249 and Cf-251) as well as the often discussed Cf-252 (see the beginning of this section for more on this method). I used theoretical estimates for the density of californium based on its position in the periodic table, which are fairly reliable, since experimental data is unavailable. The estimation model used tends to underestimate critical mass sizes but should be quite accurate for highly fissile materials like thse isotopes. The calculated bare sphere critical masses are:

By using a thick beryllium reflector the critical masses can be reduced to 40% or so of their bare value so that the most fissile of these isotopes, Cf-251, would have a reflected critical mass of 780 grams. This is arguably "in the gram range" since it is less than one kilogram, but it is obviously much, much closer to one kilogram than to one gram. Implosive compression can reduce this further. Just as a powerful and heavy implosion systems can produce low yield nuclear explosions from as little as 1 kg of Pu-239 (yield up to 100 tons, with the implosion system weighing on the order of 1000 kg), a relatively large implosion system could produce a low yield explosion from as little as 200 grams of Cf-251. The yield would be proportionately smaller of course, around 20 tons, and the total mass on the order of 200 kg. The U.S. has tested devices with 20 ton yields using the vastly cheaper plutonium (though much more of it) which only weighed 13 kg.

Experience with small nuclear devices indicates that it is impossible to make a nuclear device with a total mass less than the bare sphere critical mass of the fissile material used. Beryllium reflectors and high explosives can reduce the fissile mass required as indicated, but at the expense of adding more weight than is saved. Thus a nuclear device smaller than 2 kilograms or so using Cf-251 is almost certainly impossible.

The isotope Cf-249 results from the beta decay of Bk-249 while the heavier isotopes are produced by intense neutron irradiation of this product. Logically one would expect that Cf-252 would be more expensive then than Cf-249. However, in Sept. 1998 the ORNL Isotopes Division charged $180.25 per microgram of californium-249, and $56.00 per microgram of californium-252, indicating that factors other than ease of prodcution were setting the price. If Cf-251 could be manufactured in quantity for the 1998 sale price of Cf-252, then a critical mass would cost $11 billion.


6.3 Fusionable Materials

Fusion fuels are all light elements, which are enormously more abundant than the heavy elements relevant to fission. Isotopic separation is fairly easy in light elements since low masses mean relatively large mass ratios (Li-7 is only one atomic mass unit heavier than Li-6, but it is 17% heavier; U-238 is 3 amu heavier than U-235, but it is only 1.3% heavier).

6.3.1 Hydrogen Isotopes

Hydrogen was identified as a distinct substance by Henry Cavendish in 1766, and was named by Antoine Laurent Lavoisier. Natural hydrogen consists of two isotopes: H-1 (light hydrogen or protium), and deuterium (D, H-2, heavy hydrogen) which occurs in nature in a concentration of 0.015% (one atom in 6760 of light hydrogen). Light hydrogen participates in fusion reactions extremely slowly (that's why the sun is still around). Deuterium fuses much more readily. In the smallest stars, known as brown dwarfs, only deuterium fusion can occur and once this is exhausted the star becomes inert and planet-like. All of the deuterium in the universe today was created in the first three minutes of the Big Bang, it has been slowly depleted by stellar burning. The unstable super heavy isotope tritium rapidly decays and thus exists in nature only in minute quantities.

In elemental form all hydrogen isotopes are gases with very low densities and boiling points. This often makes them inconvenient to incorporate into practical weapons, so lithium deuteride/tritide compounds are frequently used. 6.3.1.1 Deuterium (D)
This natural isotope was discovered by American chemist Harold C. Urey and his associates F.G. Brickwedde and G.M. Murphy in 1931. Urey was awarded the Nobel Prize for Chemistry for this achievement in 1934. Urey had predicted a vapor pressure difference between the molecular hydrogen (H2) and of a corresponding molecule with one hydrogen atom replaced by deuterium (HD) and, thus, the possibility of separating these substances by distillation of liquid hydrogen. The deuterium was detected (by its atomic spectrum) in the residue of a distillation of liquid hydrogen.

Deuterium was first prepared in pure form in 1933 by G.N. Lewis, using the electrolytic method of concentration discovered by E.W. Washburn. When water is electrolyzed--i.e., decomposed by an electric current (actually a water solution of an electrolyte, usually sodium hydroxide, is used)--the hydrogen gas produced contains a smaller fraction of deuterium than the remaining water, and, hence, deuterium is concentrated in the water. Very nearly pure deuterium oxide is obtained when the amount of water has been reduced to about one hundred-thousandth of its original volume by continued electrolysis. This was the standard method of preparation of D2O before World War II.

Deuterium is twice as heavy as hydrogen-1, a fact that is very noticeable in the elemental state. Deuterium oxide, D2O, is commonly called "heavy water".

Comparison Between Natural Hydrogen and Deuterium


                   H              D             H2O          D2O
density (sol.)  0.0763(13 K)                  0.917 (0 C)
density (liq.)  0.0700(20.4 K) 0.169(20.4 K)  1.000(4.0 C)  1.106(11.2 C)
dens (gas, g/l) 0.08987(0 C)   0.17960(0 C)   0.5974(100 C)
melting point   13.96 K        18.73 K          0 C          3.79 C
boiling point   20.39 K        23.57 K        100 C        101.41 C
critical temp.  33.26 K        38.41 K        374.2 C      371.5 C
crit. press.    12.8 atm       16.432 atm     218 atm

Two interesting properties of heavy water are that it releases significant amounts of heat upon mixing with light water, and that heavy water ice is slightly denser than light water. Because of this latter property, it will sink if dropped into a glass of regular water, making it useful as a (somewhat expensive) scientific puzzle demonstration. Heavy water is toxic only when enough has been consumed to replace a substantial percentage of light water from the body (several liters or more) so one glass with some heavy water ice cubes could be consumed safetly.

Deuterium is primarily manufactured for use as a nuclear reactor moderator. It is the best moderating material known. For reactor use it is enriched to 99.75% purity or higher. The U.S. has not manufactured heavy water since 1982. Its cumulative production is estimated at 7300 tonnes. The U.S. produced heavy water at Dana, Indiana (discontinued in 1957) and at the Savannah River Facility in South Carolina (discontinued 1982). Reactor grade heavy water costs about $500/kg.

Enrichment Methods
The normal procedure is for a heavy water plant to use a combination of separation techniques in series to achieve maximum economy. Dual temperature chemical separation is the method of choice for early stages of enrichment when the concentration is very low. This is because huge volumes of water are handled, and techniques that have very low energy consumption per unit of feedstock are essential. More energy intensive, but faster, methods may be used to achieve high purity.

Chemical Separation of Deuterium
The most economical method uses the fact that D concentrates to varying (small) degrees in hydrogen sulfide (H2S) gas at different temperatures. By sending liquid water and H2S mixtures through alternating hot and cold towers, D progressively builds up, alternating between the gas and liquid phases. This method, called the Girdler-Sulfide (GS) process, is usually employed until a concentration of 15% or so (13.5% at Savannah River) is reached. Since the 1950s the dual temperature H2S process has been the only process known to be used on a large scale anywhere in the world, except for the final concentration stages.

The first process used for deuterium production in association with the U.S. weapons program was fractional distillation of water. The world's first heavy water reactor was loaded with D20 produced by this process. Water distillation was used as the second and final enrichment stage of the Savannah River Plant before shutdown, bringing the concentration to 99.8%. It is widely used around the world as a final stage, and has been probably the most widely used process after the GS process.

During the 1950s the U.S. used and liquid hydrogen distillation to produce deuterium for the hydrogen bomb program since it could be scaled up for production rapidly and has a high separation factor.

Electrolysis has been popular for the final stages of enrichment since it has a very high separation factor (D remains behind as water is electrolyzed), and when the volumes of liquid are small, the high energy cost is unimportant. Deuterium can be concentrated rapidly to 99.8% purity. The U.S. once used it as the final stage at Savannah River, taking 90% enriched feed from the water distillation cascade.

Two deuterium enrichment techniques, electrolysis and liquid hydrogen distillation, can produce substantial quantities of deuterium as a byproduct of industrial chemical processes or other endeavors. The famous Norsk Hydro plant at Vemork, Norway which was sabotaged by the allies during World War II, was an example of an industrial electrolysis plant that produced heavy water as a byproduct.

6.3.1.2 Tritium (T)
Discovered in 1934 by bombarding deuterium with accelerated deuterons (thus causing a fusion reaction), tritium (T, H-3) is radioactive with a half-life of 12.355 (+/- 0.010) yr. It exists in minute amounts naturally but significant quantities must be made artificially.

It is usually produced by the Li-6 + n -> T + He-4 reaction in nuclear reactors designed for isotope production. The lithium is enriched in Li-6 prior to irradiation, and is usually loaded in the form of aluminum canned lithium fluoride slugs or aluminum-lithium alloy rods. The latter type of target has been used at Savannah River. Tritium was been purified by heating the rods in a vacuum to release hydrogen and helium gases, followed by purification by palladium diffusion (to produce pure hydrogen) and cryogenic distillation (to produce pure tritium from the hydrogen isotopic mixture). These same refinement processes has been used in reclaiming tritium from recycled weapons reservoirs. Currently this is done using a new hydride process.

Substantial quantities are also generated in nuclear reactors moderated by heavy water through deuterium capture: D + n -> T. The Canadian CANDU (Canadian Deuterium) commercial power reactors collectively produce 250-500g of tritium annually.

Since it currently has no operating tritium production plant, the U.S. is exploring the possibility of producing tritium using a high energy (approximately 1 GeV) proton accelerator to be built at Savannah River (a scheme known as APT - Accelerator Production of Tritium). The APT approach uses the proton beam to generate neutrons through spallation from heavy nuclei - probably lead or tungsten. Each proton-nucleus collision produces about 30 neutrons, and leaves a single radioactive nucleus behind. At present the favored design for tritium production surrounds the heavy element target with a moderating blanket of heavy water, through which tubes circulate helium-3 gas. He-3 (itself produced by tritium decay) has a very high cross section for thermal neutrons (5000 barns) and forms tritium upon neutron capture. This system in effect recycles the existing tritium inventory. As an alternate approach a matrix of lead and lithium-6/aluminum alloy rods can be used for the target. Planned capacity is 3 kg/tritium a year, upgradable to 5 kg/yr.

T emits very weak radiation, 18.6 KeV betas with no gamma emissions. These are easy halted by thin layers of material (plastic, skin, etc.), which is why tritium is now used for glow-in-the-dark watches instead of radium. Tritium can only be a health hazard if taken internally. Tritium's specific activity is 9649 Ci/g. The heat output from tritium decay is 0.324 W/g.

When in the form of elemental hydrogen, inhaled tritium gas is not hazardous since the body does not readily absorb hydrogen, only about 0.004% of inhaled tritium gas is absorbed. Tritiated water (HTO) in liquid or vapor form is quite hazardous however, when inhaled nearly all of it is absorbed. There is also significant absorption of HTO vapor through the skin. The estimated LD50 (quantity that will kill half of the people absorbing it) is 10 Ci, given the high specific activity this makes the lethal dose about 1 mg. This is comparable to the toxicity of the nerve gas sarin.

6.3.2 Lithium (Li)

Lithium, element 3, is named after the Greek word for stone - lithos. Lithium is one of only three elements to have existed since the beginning of the Universe, having been synthesized during the Big Bang (the other two are hydrogen and helium). It is the lightest metal, both by density and by atomic weight. It was discovered in 1817 by Johan August Arfvedson in the mineral petalite. The first isolation of elemental lithium was achieved later by W.T. Brande and Sir Humphrey Davy by the electrolysis of lithium oxide. In 1855, Bunsen and Mattiessen isolated larger quantities of the metal by electrolysis of lithium chloride. In 1923 the first commercial production of lithium metal was achieved by Metallgesellschaft AG in Germany using the electrolysis of a molten mixture of lithium chloride and potassium chloride.

It is distributed widely throughout nature. Lithium ranks 35th in order of abundance of the elements in the crust of the earth, being slightly less abundant than copper, but more abundant than lead. It is abundant and is mined commercially in many places including North America, and has wide commercial application. Natural lithium consists of two isotopes Li-6 (7.42%) and Li-7 (92.58%).

Lithium is an alkaline earth metal, and is the lightest of all metals. It is is softer than lead but harder than the other alkali metals. It can easily be drawn into wire and rolled into sheets. It is chemically very reactive, igniting readily in air and burning with a brilliant white flame. A freshly cut lithium chunk is silvery white but it tarnishes in a minute or so in air to give a grey surface. Since it corrodes very rapidly upon exposure to air, when it is stored it must be immersed in a non-hygroscopic liquid such as naphtha. Chemically, lithium resembles sodium in its behavior. It has the highest specific heat of any solid element. It has a body-centered cubic crystal structure. Its electronic configuration is 2-1 or 1s2-2s1.

Principal physical characteristics:

Density 0.534
Melting Point 180.54 degrees C
Boiling Point 1342 degrees C

Lithium is found in economically exploitable quantities in such minerals as spodumene, lepidolite, amblygonite, and petalite. After decades as the world’s leading producer of lithium and its compounds, the United States was surpassed in 1997 when Chile became the world’s largest lithium carbonate producer. Lithium has an enormous variety of uses in the chemical industry, it is widely used in small amounts by many other industries. The aluminum, ceramics and glass, lubricating grease, and synthetic rubber industries consumed most of the lithium minerals and chemicals sold in 1997. The largest use of lithium compounds in the United States are lithium carbonate additives in ceramics and glass manufacturing processes. The domestic manufacture of thermal shock-resistant cookware (pyroceramics) consumes the majority of lithium used in the ceramics and glass industry. The manufacture of black-and-white television picture tubes consumes significant amounts of lithium concentrates overseas. In the aluminum industry lithium is used in the electrochemical production process. The third largest end use for lithium compounds is as catalysts in the production of synthetic rubbers, plastics, and pharmaceuticals. Lithium has recently found extensive use in high energy lithium-ion and lithium-polymer batteries. Lithium carbonate, a common mineral, is used in the treatment of bipolar disorder and some forms of depression.

Lithium metal is obtained by the electrolysis of a mixture of fused lithium and potassium chloride. The metal is useful as an alloying agent with magnesium and aluminum to produce very light alloys. The alloys, which are 2% to 3% lithium by weight, are attractive to the aircraft industries because of their reduced density and superior corrosion resistance compared with those of conventional aluminum alloys. The metal is used as a deoxidizer and to remove unwanted gases during the manufacture of nonferrous castings. Lithium vapor is used to prevent carbon dioxide and oxygen from forming scale in furnaces in heat-treating steel.

Lithium is used in nuclear weapons in the form of lithium hydride. It can be used in weapons simply as a convenient means of storing deuterium fusion fuel (lithium hydride contains more hydrogen per unit volume than liquid hydrogen does), or it can serve as an essential fusion fuel in its own right. Lithium hydrides have also been used in weapons for neutron shielding. For weapons use it is usually desirable to enrich lithium in Li-6 to provide a more effective fusion fuel. This has been done using electromagnetic separation in the U.S. and Russia, but large scale production uses chemical enrichment methods.

The Y-12 Plant at Oak Ridge has been the major enrichment site in the U.S., using an electrochemical exchange process. This process relies on electromotive exchange, a differential affinity of mercury for Li-6. A lithium-mercury amalgam is agitated with an aqueous lithium hydroxide solution. Li-6 concentrates in the lithium-mercury amalgam phase, Li-7 in the hydroxide solution. Counterflow through a cascade of stages produces highly enriched Li-6 in the amalgam. Chemical reaction between the lithium amalgam and the water is prevented by placing anodes in the hydroxide solution, and using the amalgam as a cathode. The depleted lithium is removed from the aqueous solution through electrolysis. The enriched lithium is recovered from the amalgam by decomposing it with water in the presence of a graphite catalyst, producing an aqueous lithium hydroxide solution and regenerated mercury ready for amalgamation with fresh lithium feed material.

Originally the exchange was done using large shallow pans in the Elex (Electro-Exchange) Plant at Y-12, which operated from 1953-1956. The Elex plant began construction in autumn 1952 for use in weapons tests conducted in 1954. The plant reached initial operating capacity on 14 August 1953, with production expanding rapidly a few months later. The Elex process required repeated mixing of the amalgam and aqueous solution in the pans, followed by settling, and was thus not truly continuous.

The vast majority of enriched lithium was prepared at the Y-12 Colex (Column Exchange) plant which operated from 1955 through 1961. This plant substituted packed columns for the pans, and operated by continuous counter-current flow. Enrichment operations ceased entirely in 1963 when a large stockpile had accumulated.

Three enrichment levels were produced: 95.5% Li-6, 60% Li-6, and 40% Li-6. The depleted lithium contained 1-4% Li-6. Lithium enriched to 95% Li-6 was used in the Castle Union test on 25 April 1954 that produced a 6.9 Mt yield (1.9 Mt from fusion). 40% Li-6 was used in the 15 Mt Castle Bravo test on 28 February 1954 (5 Mt fusion). But natural lithium was used in Castle Romeo (26 March 1954), giving a 11 Mt yield (4 Mt fusion).

The U.S. produced 442.2 tonnes of enriched lithium (of various grades) between 1954 and 1963, with 30,917 tonnes of depleted lithium hydroxide monohydrate as tails (this is a lithium content of 5152 tonnes). Much of this depleted lithium has been offered for sale as surplus since 1968. 10,467 tonnes of natural LiOH.H2O remain in inventory. In Sept. 1997 ORNL was offering Li-6 of 95-96% purity for sale at $1.30 per gram. High purity (>99%) Li-7, which is not the normal byproduct of Li-6 enrichment, was offered for $6.70 per gram.

Lithium hydrides are white crystalline solids (m.p. 620 deg C), of generally salt-like physical character. The term "lithium hydride" may refer specifically to a compound of light hydrogen or may be used generically to refer any lithium-hydrogen compound regardless of hydrogen isotope. They are usually prepared by direct reaction between hydrogen and metallic lithium at elevated temperature. Lithium hydrides have low density (0.82 for LiH, 0.92 for LiD). Lithium hydrides have no known solvent. LiH reacts violently with water, producing hydrogen gas (a fact that makes it useful for special purpose hydrogen gas generators, such as those used to inflate lifeboats). Pure lithium hydride quickly turns grey on exposure to light or air due to the formation of hydroxides and carbonates.

It can be formed into ceramic-like pieces for use in weapons, but extreme care is required to keep it dry to avoid deterioration. It is fabricated and handled in special "dry rooms" at Y-12, with workers wearing sealed suits to prevent body moisture from escaping. Fabrication of LiH weapon components has been performed by hot isostatic pressing. The LiH powder is first packed into a sealed plastic jacket. The jacketed LiH is then placed in an oil filled pressurized autoclave where pressures of 10,000 psi or greater and high temperatures compact it into a solid mass. This mass is later machined to its final shape. Thin film hydriding techniques to form thin lithium hydride layers may be used in some modern weapons.

In addition to its use as a thermonuclear fuel, lithium-6 has been used to harden nuclear weapons against outside neutron fluxes (from other nuclear weapons for example). Lithium-6 hydride has been used for this, the light hydrogen isotope presumably for its very strong moderating effect, and Li-6 for its low mass and high neutron absorption cross-section in the 0.1-1.0 MeV range.


6.4 Other Materials

Some other materials of special interest for nuclear weapons use are listed below.

6.4.1 Beryllium (Be)

Beryllium, element 4, is named after the mineral beryl, which is a natural ore (beryllium aluminum silicate, Be3Al2(SiO3)6, aquamarine and emerald are precious forms of this mineral). It was discovered discovered as an oxide by Louis Nicolas Vauquelin in 1798, and was isolated as the metal by Friederich Wöhler in 1828 (and independently by Antonine Alexandre Brutus Bussy) by the action of potassium on BeCl2 in a platinum crucible. Commercial production began in the 1950s as a result of the nuclear weapons program in the U.S.

Beryllium is an alkaline earth metal with a valence of two. It is steel gray and tarnishes only slightly in air, becoming covered with a thin layer of oxide. It is one of the lightest metals, being lighter than aluminum and only slightly heavier than magnesium. Nonetheless is still surprisingly dense considering its very low atomic weight. This is due to its having one of the densest packing of atoms per unit volume of any element (only the diamond form of carbon is higher). All of the beryllium in existence is thought to have arisen from cosmic ray collisions in interstellar space.

Beryllium and its compounds (particularly its oxide) have unique and remarkable combinations of physical, chemical, electronic, and nuclear properties. Accordingly despite its high cost it is widely used in a enormous variety of applications.

Beryllium has only one natural isotope with an atomic weight of 9. Its electronic configuration is 2-2 or 1s2-2s2.

Beryllium's principal properties are:

Density 1.848 (20 C)
Melting point 1278 C (+/- 5 deg.)
Boiling point 2970 C (at 5mm pressure), above 3000 C at standard atmospheric

Beryllium has the unusual property of that many of its salts have a sweet taste, for which reason the element was originally called glucinium. It is also the most chemically toxic element. The OSHA permissible working exposure is limited to 2 micrograms/m^3 of air for an 8 hour shift, and about 100 micrograms/month. NIOSH recommendeds that airborne exposure never exceed 0.5 micrograms/m^3. Soluble compounds in the form of solutions, dry dust, or fumes may produce dermatitis or, when inhaled, acute potentially lethal effects similar to those caused by the poison gas phosgene.

It has many outstanding structural properties. Beryllium has a high strength per unit weight. Its modulus of elasticity exceeds that of steel (i.e. it is very stiff). It has the highest melting point of the light metals, and it has excellent thermal conductivity. It is also nonmagnetic and nonsparking. These factors combine to make it a superb material for heat shielding in ICBM reentry vehicles, and for inertial guidance system gyroscopes. It has good resistance to chemical attack (it does not oxidize at ordinary temperatures, and it resists nitric acid). It is very valuable as an alloying agent with copper. Beryllium-copper alloys (2% Be) have high electrical and thermal conductivity, high strength and hardness, good corrosion and fatigue resistance, and nonmagnetic properties. It is a superior material for springs and is used in many types of machinery, and for electronic connectors. About 75% of U.S. beryllium demand is for use in this alloy.

Its oxide has a very high melting point, excellent thermal conductivity, and high hardness (beryllium's ability to scratch glass is probably due to the formation of a thin layer of the oxide) and strength, making it useful in high temperature ceramics and as a substrate for high-density electronic circuits. It is also transparent to microwaves and used in many microwave applications. About 15% of U.S. beryllium consumption is for the oxide.

Beryl (which contains about 4% Be) is the most important beryllium ore throughout the world. Emerald, one of the rarest and most valuable gemstones, is a form of beryl. The major U.S. source has been low-grade bertrandite deposits in Utah. The United States is one of only three countries that process beryllium ore and concentrates into beryllium products and supplies most of the rest of the world with these products. U.S. beryllium production is exclusively handled by Brush-Wellman Inc's Delta Utah plant. It processes bertrandite and imported beryl. Beryllium hydroxide is shipped to the company's plant in Elmore, OH, where it is converted into beryllium alloys, oxide, and metal. One other company in the United States has the capability to produce beryllium alloys -- NGK Metals Corp., a subsidiary of NGK Insulators of Japan -- which produces beryllium alloys at a plant near Reading, PA. NGK Metals purchases its berylloium from Brush-Wellman however.

Beryllium compounds are generally white (or colorless in solution) and show great similarity in chemical properties to the corresponding compounds of aluminum. This similarity makes it difficult to separate beryllium from the aluminum that is almost always present in beryllium ores. Beryllium metal is usually prepared by reducing the fluoride with magnesium.

In 1997 U.S. beryllium consumption of 205 tons was valued at more than $70 million. Worlwide production in that year was 276 tonnes (beryllium metal content). The use of beryllium (as an alloy, metal, and oxide) in electronic and electrical components, and aerospace and defense applications accounted for more than 80% of consumption. The U.S. price of beryllium as domestic vacuum-cast metal ingot was $720/kg. Beryllium-copper master alloy was $350 per kilogram of contained beryllium, and beryllium oxide powder was $169/kg. The National Defense Stockpile (NDS) goal for beryllium metal was 363 tonnes.

It has many application in nuclear technology. It has high transparency to x-rays. It also has a remarkable range of desirable qualities for neutron-related applications. It generates neutrons when bombarded with gamma rays or alpha particles (about 30 neutrons/million alphas), through the He-4 + Be-9 -> C-12 + n reaction, which made it the first known source of free neutrons for laboratory study (the neutron energy is also a remarkably high 5.7 MeV). The existence of the alpha->n reaction made it useful in the neutron initiator in early fission weapons.

It has a very low absorption cross section for thermal neutrons and is thus used as a canning material for fuel elements in reactors. Due its low atomic weight it is an excellent neutron moderator, and has the largest macroscopic neutron scattering cross section of any material (due in part to its packing density). It also undergoes Be-9 + n -> Be-8 + 2n reactions, thus acting as a neutron multiplier; although this endothermic reaction (- 2.4 MeV) only becomes significant around 3 MeV.

These characteristics have made it a reflector/moderator of choice for extremely small, light nuclear reactors; and a neutron reflector of choice for low yield, very light nuclear weapons. The production of high quality beryllium forgings suitable for use as a weapon reflector requires the use of hot vacuum isostatic pressing.

6.4.2 Polonium (Po)
Polonium, element 84, was the first element discovered by Mme. Marie Sklodowska Curie in 1898, while seeking the cause of radioactivity of pitchblende from Joachimsthal, Bohemia. It is named after her native country Poland. It was extracted from the uranium ore pitchblende, where it is found in the concentration 0.1 ppm.

Polonium has more known isotopes than any other element with 27 (all radioactive). Po-210 is the third longest lived (the longest lived isotope is Po-209 with a 109 year half-life). The isotope Po-210 (the only readily accessible isotope) is naturally formed from the decay of radium:

Ra-226 -> (1600 y, alpha) -> Rn-222
Rn-222 -> (3.823 day, alpha) -> Po-218
Po-218 -> (3.05 min, alpha) -> Pb-214
Pb-214 -> (26.8 min, beta) -> Bi-214
Bi-214 -> (19.7 min, alpha) -> Tl-210
Tl-210 -> (1.3 min, beta) -> Pb-210 ("Radium D")
Pb-210 -> (21 yr, beta) -> Bi-210 ("Radium-E")
Bi-210 -> (5.01 day, beta) -> Po-210 ("Radium F")

Radium samples more than 30 yr old (to allow the buildup of the Pb-210 parent) contain 0.013% (1 g per 7.5 kg of radium). Polonium can be conveniently extracted from radium by allowing it to deposit on to bismuth plates suspended in RaCl solution. Lead extracted from uranium ores contains Pb-210, from which Po-210 can be periodically harvested. In 1934 it was discovered that it could be manufactured by bombarding natural bismuth (Bi-209) with neutrons to form Bi-210.

Elemental polonium is silvery and has two allotropic forms, the alpha and beta forms, which can coexist between 18 C and 54 C. Its physical properties are:

Density: 9.196 (alpha form); 9.398(beta form); mixtures have intermediate densities
Melting point: 254 degrees C
Boiling point: 962 degrees C

Chemically polonium resembles bismuth and tellurium. Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. Due to its very short half-life polonium compounds tend to decompose quickly, particularly organic compounds. Metallic polonium can be prepared from polonium hydroxide.

Despite its fairly high boiling point, polonium is rather volatile - 50% of a sample will vaporize in 45 hrs at 55 C. Part of its volatility can be attributed to its radioactivity. When an atom decays, the recoil from the energetic alpha emission dislodges many atoms. Since 0.50% of it decays each day, a significant fraction of the atoms are ejected on a daily basis.

Po-210 is an alpha emitter with a half-life of 138.39 days. It is notable in that it emits almost no gamma rays, a property that made it important to the Manhattan Project and the early post-war weapon program of the U.S. By encapsulating Po-210 in a specially shaped capsule with beryllium, separated by a thin metal foil or film, a neutron initiator for fission bombs could be made. Turbulence created by sudden compression would cause the Po-210 and beryllium to rapidly mix, and produce neutrons by the Be-9 + alpha -> Be-8 + n + alpha reaction. The foil prevented the poorly penetrating alpha particles from creating neutrons prematurely. Other alpha emitters produce gamma rays in addition, which easily penetrate the foil and knock neutrons loose prematurely.

Po-210 is highly toxic and dangerous to handle. Its short half-life gives it an extremely intense radioactivity of 4490 Ci/g (4500 times as intense as radium-226, 65000 times as intense as Pu-239), its volatility makes it easy to ingest. The maximum allowable body burden is 0.03 microcuries (6.7x10^12 g).

The Manhattan Project found it impossible to prevent people who handled polonium from absorbing it. Fortunately absorbed polonium is not deposited in bone, but is excreted rather rapidly. Monitoring polonium content in urine, and frequent staff rotation, minimized the hazard. On the other hand, polonium deposited on inhaled particles is retained by the lung like plutonium, and can be extremely hazardous in this form. Much of the lung cancer hazard of tobacco smoking actually comes from polonium deposited on smoke particles, not chemical carcinogens.

In the chemical isolation from ore, the ore is treated with hydrochloric acid, and the resulting solution is heated with hydrogen sulfide to precipitate polonium monosulfide, PoS, along with other metal sulfides, such as that of bismuth, Bi2S3, which resembles polonium monosulfide closely in chemical behaviour, though it is less soluble. Because of the difference in solubility, repeated partial precipitation of the mixture of sulfides concentrates the polonium in the more soluble fraction, while the bismuth accumulates in the less soluble portions. The difference in solubility is small, however, and the process must be repeated many times to achieve a complete separation. Purification is accomplished by electrolytic deposition.

Po-210 produces heat at a rate of 140 w/g. A capsule containing 500 mg will reach 500 C. A milligram or so makes an easily visible blue glow from excitation of the air. Polonium has been used in devices that ionize the air to eliminate accumulation of electrostatic charges. In most applications (such as textile mills) this use has been replaced by safer beta emitters, but it has remained in use in applications where products are very sensitive to radiation exposure especially photographic film plants.