6.2.2 Plutonium (Pu)
Plutonium, element 94, named after the planet Pluto, was discovered by artificial production by Seaborg, McMillan, Kennedy, and Wahl in 1940 at Berkeley. 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), eventually becoming dark purpleblack. 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 is brittle in the alpha phase, which exists at room temperature in pure plutonium, but ductile alloys exist.
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.
It has significant solubility only in very acidic solutions, such as nitric and hydrochloric acid (as the nitrate and chloride). 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.
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 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).
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 US 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 (US 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 to power long duration deep space probes due to its high heat output and long lifetime.
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.
184.108.40.206 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 deg. 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 comparision, 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 Fatman 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 it is 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 as being silver), a process that was not very successful and led to blistering. The Fatman pit was 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. 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 unsuitable as an alloying agent in weapons.
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 US 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 an delectrorefining of plutonium oxides.
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.
220.127.116.11 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.
The 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 US laboratories.
18.104.22.168 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 with low MWD/tonne exposures since there are lower concentrations of the other 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 is produced with exposures of some 600 MWD/tonne, the Savannah River heavy water piles produce 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, producing super-grade weapons plutonium (0.9-1.0% Pu-240, and negligible amounts of other isotopes).
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. It rarely makes up more than a small fraction of one percent of plutonium composition, but the neutron production and heating can make it very troublesome even so. Its specific activity is 17.5 Ci/g.
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 US weapons program was produced at Hanford, Washington and Savannah River, Georgia. Soviet/Russian plutonium production is located at Kyshtym, near Chelyabinsk.
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 halflife (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.
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 selfheating 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.
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.
22.214.171.124 Weapon Grade Plutonium
This term is used by the US 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 <0.05% <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 <0.05% <0.05% trace
The US 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 US 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 US weapons designs (presumably older ones) required low Pu-240 contents for these reasons.
However, it is now clear that these issues are unimportant in advanced weapons designs used by the US 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.
126.96.36.199 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 Fatman 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 wepaons 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) 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.
188.8.131.52 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.