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.
Spent fuel from a power reactor is fuel which is no longer capable of efficient fission because of the loss of fissile material and the build up of fission products and actinides. It reaches this stage after about five years in the reactor. In some respects, the word 'spent' is inappropriate as only about 3% of the uranium originally in the fuel has been used up. Mainly because of the changes the fuel continues to undergo after removal from the reactor, the spent fuel from a power reactor is typically allowed to decay for about four years before reprocessing. This also reduces the amount of fission products which have to be dealt with by waste processing.
Spent fuel contains fission products and actinides produced when nuclear fuel is irradiated in reactors, as well as any unburned, unfissioned nuclear fuel remaining after the fuel rods have been removed from the reactor core. After spent fuel is removed from reactors, it is stored in racks placed in storage pools to isolate it from the environment.
An assumption of a cooling time of 160 days between the discharge of spent fuel from a production reactor and the reprocessing of the fuel is based on the optimum for recycling plutonium as well as uranium.
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.
The jargon 'head end' is applied to those operations which must be carried out before the separation process itself begins. They include receipt of the fuel in the head end plant and mechanical operations to reduce the fuel elements to a more suitable form for the processes which follow.
Once free of the supporting structure, the elements are cut up, placed in a dissolver and dissolved in hot nitric acid. During this process, the 'dissolver off-gases' krypton, xenon, iodine and carbon dioxide, together with nitrogen oxides and steam (from the nitric acid) are released. All of these, except krypton and xenon which are chemically inert, but radioactive, gases, are typically trapped or recycled for re-use. Any particles of fuel cladding or fission products which have not dissolved are removed by filtering them out in a centrifuge.
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. The chemical has both commercial and military applications, including as a plasticizer for cellulose esters, lacquers, chlorinated rubber, PVC, plastic and vinyl resins; heat exchange medium; solvent for nitrocellulose and cellulose acetate; pigment grinding assistant; painting additive; printing ink solvent; antifoam agent; dielectric; adhesive; and solvent extraction of rare-earth metal ions from solution of reactor products.
Tributyl phosphate is a very strong, aprotic and polar solvent and may be used for the production of synthetic resin and natural rubber solutions. It is used as a defoamer in concrete additives (especially in concrete containing lignin sulfonate as a fluidizer); textile processing chemicals (especially in combination with wetting agents); and glues and adhesives, paper coating slurries, plastic dispersions, drilling fluids, cementations, lubricants, coatings, electroplating. TBP is used as an extracting agent in liquid-liquid extraction, e.g. for the separation and recovery of rare earths and platinum metals, in the cleaning of phosphoric acid, sulfuric acid, nitric acid, and hydrofluoric acid, and in the concentration of uranium and thorium.
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 solvent 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.
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