FAS | Nuke | Intro | Nuke ||||| Index | Search | Join FAS ---------------------------------------------------------------------------- [Image] [Image] [Image] ---------------------------------------------------------------------------- Plutonium Production Plutonium, one of the two fissile elements used to fuel nuclear explosives, is not found in significant quantities in nature. Plutonium can only be made in sufficient quantities in a nuclear reactor. It must be “bred,” or produced, one atomic nucleus at a time by bombarding 238 U with neutrons to produce the isotope 239 U, which beta decays (half-life 23 minutes), emitting an electron to become the (almost equally) radioactive 239 Np (neptunium). The neptunium isotope again beta decays (half-life 56 hours) to 239 Pu, the desired fissile material. The only proven and practical source for the large quantities of neutrons needed to make plutonium at a reasonable speed is a nuclear reactor in which a controlled but self-sustaining 235 U fission chain reaction takes place. The graphite-moderated, air- or gas-cooled reactor using natural uranium as its fuel was first built in 1942. Scale-up of these types of reactors from low power to quite high power is straightforward. ccelerator-based transmutation to produce plutonium is theoretically possible, and experiments to develop its potential have been started, but the feasibility of large-scale production by the process has not been demonstrated. The “size” of a nuclear reactor is generally indicated by its power output. Reactors to generate electricity are rated in terms of the electrical generating capacity, MW(e), meaning megawatts of electricity. A more important rating with regard to production of nuclear explosive material is MW(t), the thermal power produced by the reactor. As a general rule, the thermal output of a power reactor is three times the electrical capacity. That is, a 1,000 MW(e) reactor produces about 3,000 MW(t), reflecting the inefficiencies in converting heat energy to electricity. A useful rule of thumb for gauging the proliferation potential of any given reactor is that 1 megawatt-day (thermal energy release, not electricity output) of operation produces 1 gram of plutonium in any reactor using 20-percent or lower enriched uranium; consequently, a 100 MW(t) reactor produces 100 grams of plutonium per day and could produce roughly enough plutonium for one weapon every 2 months. Light-water power reactors make fewer plutonium nuclei per uranium fission than graphite-moderated production reactors. In addition to production of plutonium, nuclear reactors can also be used to make tritium, 3 H, the heaviest isotope of hydrogen. Tritium is an essential component of boosted fission weapons and multi-stage thermonuclear weapons. The same reactor design features which promote plutonium production are also consistent with efficient tritium production, which adds to the proliferation risk associated with nuclear reactors. Reactors are generally purpose-built, and reactors built and operated for plutonium production are less efficient for electricity production than standard nuclear electric power plants because of the low burnup restriction for production of weapons grade plutonium. The types nuclear fission reactors which have been found most suitable for producing plutonium are graphite-moderated nuclear reactors using gas or water cooling at atmospheric pressure and with the capability of having fuel elements exchanged while on line. Several distinct classes of reactor exist, each optimized for one purpose, generally using fuel carefully chosen for the job at hand. These classes include the following: Research reactors. Usually operates at very low power, often only 1–2 MW or less. Frequently uses high-enriched uranium fuel, although most newer models use no more than 20-percent enrichments to make the theft of fuel less attractive. Fertile material ( 238 U for Pu, 6 Li for tritium) can be encapsulated in elements known as “targets” for insertion into the reactor core. The reactor can also employ a fertile blanket of 238 U in which plutonium can be bred. Cooling requirements and shielding requirements are relatively modest. Some research reactors can be refueled while operating, and such reactors are of special concern for plutonium production because they can limit fuel burnup, which enhances the quality of the plutonium compared to that obtained from reactors that require high burnup before shutdown and refueling. Research reactors using nearly 100-percent enriched material produce almost no plutonium in their fuel because the fertile species, 238 U, has been removed. These reactors can, however, be built with a surrounding “blanket” of natural or depleted uranium in which plutonium can be bred efficiently. The Osirak reactor built in Iraq and destroyed by Israeli aircraft was of this type. Propulsion reactors. Primarily found on submarines and large-surface combatant ships, nuclear reactors have given new operational freedom to the underwater navy and deliver increased time on station combined with high speed for both the submarine service and the surface navy. The United States and Russia have built most of the world’s shipboard reactors. The world’s first nuclear powered cargo ship was the U.S.N.S. Savannah; however, nuclear propulsion power has not been particularly successful in the commercial world. Today, the only operating commercial vessels using nuclear propulsion are Russian icebreakers. To keep the core size small, propulsion reactors generally use highly enriched uranium as fuel. In principle, a propulsion reactor core could be surrounded with a fertile blanket and used to produce plutonium. In practice, this has never been done. Space reactors and mobile power systems. Nuclear reactors have been used from time to time, usually by the former Soviet Union, to provide on-orbit electrical power to spacecraft. In principle, they will use HEU as fuel to keep the core mass and volume small. Other spacecraft have been powered by the heat released by the radioactive decay of 238 Pu. Power reactors. These are used to generate electric power. Few use fuel enriched to greater than 5–7% 235 U. Practical power levels range from a few hundred MW(e) (three times that in terms of thermal power output) to 1,000 or 1,500 MW(e)—meaning 3,000–4,000 MW(t). Power reactors designs have included water cooled-graphite moderated (the Soviet RBMK used at Chernobyl), boiling (light) water, pressurized (light) water, heavy water-moderated and cooled, graphite-moderated/helium cooled, and liquid metal-moderated. Most power reactors operate under pressure and cannot be refueled in operation. The RBMK and CANDU reactors are notable exceptions to this rule. The CANDU reactor was developed for the Canadian nuclear power program and is a deuterium oxide (heavy water) moderated reactor which can operate on natural uranium fuel. Breeder reactors. These reactors generate plutonium at a rate greater (numbers of nuclei per unit time) than they burn their fissile fuel (numbers of nuclei per unit time). Normally, breeders use fast neutrons and irradiate a fissile 238 U blanket. Plutonium produced in the fuel generally has a higher fraction of 240 Pu than that produced in other reactors, but the Pu made in the blanket of uranium surrounding the core is usually of a high quality, containing very little 240 Pu. Production reactors. These are used to make plutonium (and often tritium) efficiently. Production reactors are frequently graphite-moderated and either air-, CO 2 -, or helium-cooled. The longer a given sample of fuel is irradiated, the greater the build-up of 240 Pu, an isotope which decays by spontaneous fission and which should be minimized in weapon fuel. Consequently, plutonium production reactors usually are designed to be refueled while operating (on-line refueling) so that relatively little 240 Pu is found in the “spent” fuel. The first nuclear reactor, CP-1, went critical for the first time on 2 December 1942 in a squash court under Stagg Field at the University of Chicago. Construction on CP-1 began less than a month before criticality was achieved; the reactor used lumped uranium metal fuel elements moderated by high-purity graphite. Within 2 years the United States first scaled up reactor technology from this essentially zero-power test bed to the 3.5 MW (thermal) X-10 reactor built at Oak Ridge, Tennessee, and then again to the 250-megawatt production reactors at Hanford. The Hanford reactors supplied the plutonium for the Trinity test and the Nagasaki war drop. Clearly, reactor technology does not stress the capabilities of a reasonably well-industrialized state at the end of the twentieth century. Some problems did arise with the scale-up to hundreds of megawatts: the graphite lattice changed crystal state, which caused some deformation, and the buildup of a neutron-absorbing xenon isotope poisoned the fission reaction. This latter problem was curable because of the foresight of the duPont engineers, who built the reactor with many additional fuel channels which, when loaded, increased the reactivity enough to offset the neutron absorption by the xenon fission product. Finally, the problem of spontaneous emission of neutrons by 240 Pu produced in reactor plutonium became apparent as soon as the first samples of Hanford output were supplied to Los Alamos. The high risk of nuclear pre-initiation associated with 240 Pu caused the abandonment of the notion of a gun-assembled plutonium weapon and led directly to the adoption of an implosion design. Since each fission produces only slightly more than two neutrons, on average, the neutron “economy” must be managed carefully, which requires good instrumentation and an understanding of reactor physics, to have enough neutrons to irradiate useful quantities of U-238. Note, however, that during the Manhattan Project the United States was able to scale an operating 250 watt reactor to a 250 megawatt production reactor. Although the instrumentation of the day was far less sophisticated than that in use today, the scientists working the problem were exceptional. A typical production reactor produces about 0.8 atoms of plutonium for each nucleus of U-235 which fissions. A typical form of production reactor fuel is natural uranium metal encased in a simple steel or aluminum cladding. Because uranium metal is not as dimensionally stable when irradiated as is uranium oxide used in high burnup fuel, reactors fueled with the uranium metal must be confined to very low burnup operation, which is not economical for electricity production. This operational restriction for uranium metal fuel results in the production of plutonium with only a small admixture of the undesirable isotope, 240 Pu. Thus, it is almost certain that a reactor using metallic fuel is intended to produce weapons grade plutonium, and operation of such a reactor is a strong indicator that proliferation is occurring. Reprocessing Unlike fuel from fossil plants that discharge ash with negligible heat content, fuel discharged from nuclear reactors contains appreciable quantities of fissile uranium and plutonium (“unburned” fuel). These fuel elements must be removed from a reactor before the fissile material has been completely consumed, primarily because of fission product buildup. Fission products capture large numbers of neutrons, which are necessary to sustain a chain fission reaction. In the interest of economic utilization of nuclear fuels and the conservation of valuable resources, several countries have constructed reprocessing plants to recover the residual uranium and plutonium values, utilizing a variety of physical and chemical methods. Plutonium is removed from spent fuel by chemical separation; no nuclear or physical separation (as for example in uranium enrichment) is needed. To be used in a nuclear weapon, plutonium must be separated from the much larger mass of non-fissile material in the irradiated fuel. After being separated chemically from the irradiated fuel and reduced to metal, the plutonium is immediately ready for use in a nuclear explosive device. If the reactor involved uses thorium fuel, 233 U, also a fissile isotope, is produced and can be recovered in a process similar to plutonium extraction. The first plutonium extraction (reprocessing) plants to operate on an industrial scale were built at Hanford, Washington, during the Manhattan Project. The initial plant was built before the final parameters of the extraction process were well defined. Reprocessing plants are generally characterized by heavy reinforced concrete construction to provide shielding against the intense gamma radiation produced by the decay of short-lived isotopes produced as fission products. Plutonium extraction and uranium reprocessing are generally combined in the same facility in the civilian nuclear fuel cycle. Although the United States no longer reprocesses civil reactor fuel and does not produce plutonium for weapons, other countries have made different choices. Britain, France, Japan, and Russia (among others) operate reprocessing plants. Heavy industrial construction. All operations are performed in a facility that is usually divided into two structural sections (hardened and nonhardened) and two utility categories (radiation and ventilation/contamination). The hardened portion of the building (reprocessing cells) is designed to withstand the most severe probable natural phenomena without compromising the capability to bring the processes and plant to a safe shutdown condition. Other parts of the building (i.e., offices and shops), while important for normal functions, are not considered essential and are built to less rigorous structural requirements. Radiation is primarily addressed by using 4- to 6-ft thick, high-den-sity concrete walls to enclose the primary containment area (hot cells). A proliferator who wishes to reprocess fuel covertly for a relatively short time -— less than a year would be typical -— may use concrete slabs for the cell walls. Holes for periscopes could be cast in the slabs. This is particularly feasible if the proliferator cares little about personnel health and safety issues. Fuel storage and movement. Fuel is transported to the reprocessing plant in specially designed casks. After being checked for contamination, the clean fuel is lowered into a storage pool via a heavy-duty crane. Pools are normally 30-ft deep for radiation protection and contain a transfer pool, approximately 15-ft deep, that provides an underwater system to move the fuel into an adjacent hot cell. Fuel disassembly. Fuel elements are breached (often chopped) to expose the fuel material for subsequent leaching in nitric acid (HNO 3 ). Fuel cladding is frequently not soluble in nitric acid, so the fuel itself must be opened to chemical attack. Fuel dissolution. Residual uranium and plutonium values are leached from the fuel with HNO 3 . The cladding material remains intact and is separated as a waste. The dissolver must be designed so that no critical mass of plutonium (and uranium) can accumulate anywhere in its volume, and, of course, it must function in contact with hot nitric acid, a particularly corrosive agent. Dissolvers are typically limited-life components and must be replaced. The first French civilian reprocessing plant at La Hague, near Cherbourg, had serious problems with leakage of the plutonium-containing solutions. Dissolvers may operate in batch mode using a fuel basket or in continuous mode using a rotary dissolver (wheel configuration). Fissile element separation. The PUREX (Plutonium Uranium Recovery by EXtraction) solvent extraction process separates the uranium and plutonium from the fission products. After adjustment of the acidity, the resultant aqueous solution is equilibrated with an immiscible solution of tri-n-butyl phosphate (TBP) in refined kerosene. The TBP solution preferentially extracts uranium and plutonium nitrates, leaving fission products and other nitrates in the aqueous phase. Then, chemical conditions are adjusted so that the plutonium and uranium are reextracted into a fresh aqueous phase. Normally, two solvent extraction cycles are used for the separation; the first removes the fission products from the uranium and plutonium, while the second provides further decontamination. Uranium and plutonium are separated from one another in a similar second extraction operation. TBP is a common industrial chemical used in plasticizers and paints. Solvent extraction usually takes place in a pulse column, a several-inch diameter metal tube resistant to nitric acid and used to mix together the two immiscible phases (organic phase containing TBP and an aqueous phase containing U, Pu, and the fission products). The mixing is accomplished by forcing one of the phases through the other via a series of pulses with a repetition rate of 30 to 120 cycles/minute and amplitudes of 0.5 to 2.0 inches. The metal tube contains a series of perforated plates which disperses the two immiscible liquids. U & Pu product purification. Although plutonium and uranium from sol-vent extraction are nearly chemically pure, additional decontamination from each other, fission products, and other impurities may be required. Large plants use additional solvent extraction cycles to provide this service, but small plants may use ion exchange for the final purification step (polishing). Metal preparation. Plutonium may be precipitated as PuF 3 from aqueous nitrate solution by reducing its charge from +4 to +3 with ascorbic acid and adding hydrofluoric acid (HF). The resulting solid is separated by filtration and dried. Reprocessed uranium is rarely reduced to the metal, but it is converted to the oxide and stored or to the hexafluoride and re-enriched. Plutonium (and uranium) metal may be produced by the reaction of an active metal (calcium or magnesium) with a fluoride salt at elevated temperature in a sealed metal vessel (called a “bomb”). The metal product is freed from the slag, washed in concentrated HNO 3 to remove residue, washed with water, dried, and then remelted in a high temperature furnace (arc). Waste treatment/recycle. Reprocessing operations generate a myriad of waste streams containing radioactivity. Several of the chemicals (HNO 3 ) and streams (TBP/kerosene mixture) are recycled. All streams must be monitored to protect against accidental discharge of radioactivity into the environment. Gaseous effluents are passed through a series of cleaning and filtering operations before being discharged ,while liquid waste streams are concentrated by evaporation and stored or solidified with concrete. In the ultimate analysis, the only way to safely handle radioactivity is to retain the material until the activity of each nuclide disappears by natural decay. Early plants used “mixer-settler” facilities in which the two immiscible fluids were mixed by a propeller, and gravity was used to separate the liquids in a separate chamber. Successful separation requires that the operation be conducted many times in sequence. More modern plants use pulse columns with perforated plates along their length. The (heavier) nitric acid solution is fed in at the top and the lighter TBP-kerosene from the bottom. The liquids mix when they are pulsed through the perforations in the plates, effectively making a single reactor vessel serve to carry out a series of operations in the column. Centrifugal contractors using centrifugal force have also been used in place of mixer-settlers. The process must still be repeated many times, but the equipment is compact. New plants are built this way, although the gravity-based mixer-settler technology has been proven to be satisfactory, if expensive and space-consuming. A single bank of mixer-settler stages about the size of a kitchen refrigerator can separate enough plutonium for a nuclear weapon in 1–2 months. A bank of eight centrifugal contactors can produce enough plutonium for an explosive device within a few days and takes up about the same space as the mixer-settler. Hot cells with thick radiation shielding and leaded glass for direct viewing, along with a glove box with minimal radiation shielding, are adequate for research-scale plutonium extraction, are very low technology items, and would probably suffice for a program designed to produce a small number of weapons each year. The concrete canyons housing many smaller cells with remotely operated machinery are characteristic of large-scale production of plutonium. When plutonium is produced in a nuclear reactor, inevitably some 240 Pu (as well as heavier plutonium isotopes, including 241 Pu and 242 Pu) is produced along with the more desirable 239 Pu. The heavier isotope is not as readily fissionable, and it also decays by spontaneous fission, producing unwanted background neutrons. Thus, nuclear weapon designers prefer to work with plutonium containing less than 7 percent 240 Pu. A method for separating plutonium isotopes could be used to remove the heavier isotopes of plutonium (e.g., 240 Pu) from reactor-grade plutonium, thus producing nearly pure 239 Pu. Uranium isotope separation techniques [e.g., atomic vapor laser isotope separation (AVLIS)] might be applied to this task. However, this would require mastery of production reactor and reprocessing technologies (to produce and extract the plutonium) in addition to isotope enrichment technology (to remove the heavier plutonium isotopes). In practice, it is simpler to alter the reactor refueling cycle to reduce the fraction of plutonium which is 240 Pu. The plutonium must be extracted chemically in a reprocessing plant. Reprocessing is a complicated process involving the handling of highly radioactive materials and must be done by robots or by humans using remote manipulating equipment. At some stages of the process simple glove boxes with lead glass windows suffice. Reprocessing is intrinsically dangerous because of the use of hot acids in which plutonium and intensely radioactive short-lived fission products are dissolved. Some observers have, however, suggested that the safety measures could be relaxed to the extent that the proliferator deems his technicians to be “expendable.” Disposal of the high-level waste from reprocessing is difficult. Any reprocessing facility requires large quantities of concrete for shielding and will vent radioactive gases (Iodine-131, for example) to the atmosphere. Sources and Methods * Adapted from - Nuclear Weapons Technology Militarily Critical Technologies List (MCTL) Part II: Weapons of Mass Destruction Technologies ---------------------------------------------------------------------------- FAS | Nuke | Intro | Nuke |||| Index | Search | Join FAS ---------------------------------------------------------------------------- http://www.fas.org/nuke/intro/nuke/plutonium.htm Maintained by John Pike Updated Wednesday, October 21, 1998 4:35:26 PM