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nuclear fuel[′nü·klē·ər ¦fyül]
a substance that is used in nuclear reactors to initiate a nuclear fission chain reaction. There is only one natural nuclear fuel, uranium. It contains fissionable 235U nuclei (primary fuel), which will maintain a chain reaction, and “raw” 238U nuclei, which, by capturing neutrons, are capable of converting to new fissionable nuclei of 239Pu (secondary fuel), which do not exist in nature:
Another secondary fuel not encountered in nature is 233U nuclei, formed as a result of neutron capture by raw 232Th nuclei:
Nuclear fuel is used in nuclear reactors. The fuel elements consist of the nuclear fuel, protected by hermetically sealed metal jackets of various shapes and sizes. Chemically, nuclear fuel can be in the form of a metal (including alloys) or in the form of an oxide, carbide, nitride, and the like. The principal requirements of nuclear fuel are good compatibility with the fuel cladding, high melting and vaporization points, good heat conductivity, weak interaction with the coolant, and minimum increase in volume (swelling) during irradiation in the reactor. In addition, the fuel should be easy and inexpensive to produce and simple to reprocess. Nuclear fuel used in fast breeder reactors must have a high breeding ratio, or conversion ratio, as well.
Uranium fuel for thermal reactors, which form the basis of the nuclear power industry, usually has an elevated content of the isotope 235U (2–4 percent by mass instead of the 0.71 percent in natural uranium). A considerable disadvantage of thermal reactors is the low thermal utilization factor of natural uranium. A considerably higher thermal utilization factor can be achieved in fast breeder reactors, which use uranium with a higher content of 235U (up to 30 percent). In the future, as stockpiles of 239Pu are accumulated, mixed uranium-plutonium fuel will be used, with 15–20 percent Pu. In this case, enriched uranium may be replaced by natural uranium or even by uranium that is depleted in 235U; the latter has already been accumulated in fairly large amounts around the world. Depleted uranium (without Pu) is also used in the shielded zone (breeding blanket) of a breeder reactor; the weight of the blanket is several times the weight of the core. In fast reactors that operate on uranium-plutonium fuel, the amount of 239Pu that is accumulated may be much greater than the amount that is consumed; that is, the breeding of nuclear fuel may occur. The breeding ratio depends on the composition of the nuclear fuel. Nuclear fuel can be arranged according to increasing ratio as follows: oxide (U, Pu)02, carbide (U, Pu)C, nitride (U, Pu)N, and metallic in the form of various alloys.
The production of uranium nuclear fuel (see Figure 1) begins with the processing of ores to extract the uranium. In preliminary ore grading by gamma radiation, 20–30 percent of the rocks, with a uranium content of 0.01 percent or less, is discarded (ordinary methods of enrichment are also used). Hydrometallurgical processing of the ore involves pulverization, acid leaching, ion-exchange or solvent extraction of the uranium from clarified solutions or slurries, and production of purified U3O8. Underground leaching in the deposit itself is used for uranium-poor and easily leachable ores, especially in deposits that are difficult to mine; in blanket deposits, the leaching is done through a system of boreholes, and in veins it is done in underground chambers after the ore is broken up and pulverized by blasting.
The U3O8 is then converted either to the tetrafluoride UF4 for subsequent production of the uranium metal or to the hexa-fluoride UF6 (the only stable gaseous uranium compound) for enrichment with the isotope 235U. Enrichment is accomplished by gas thermal diffusion or by centrifugation. The UF6 is then converted to uranium dioxide, which is used to make the cores of fuel elements or to produce other uranium compounds for the same purpose.
Fuel-element cores must meet high standards with respect to stoichiometric composition and impurity content. For example, in UO2 cores, the mass ratio of oxygen to uranium must be in the range 2.00–2.02; the permissible content of F and H2O (by mass) does not exceed 0.01–0.006 percent and 0.001 percent, respectively.
There are a number of reasons why thorium has not been widely used as a raw material for the production of fissionable 233U nuclei: (1) the proved reserves of uranium can meet the fuel needs of the nuclear power industry for many decades; (2) thorium does not form rich deposits, and the technology for its extraction from ores is more complex; (3) 232U is formed along with 233U, and the decay of 232U produces gamma-active (Bi-212 and Te-208) nuclei, which make such a nuclear fuel difficult to handle and complicate the production of fuel elements:
and (4) it is more difficult and costly to process irradiated thorium fuel elements in order to extract 233U than to process uranium fuel elements.
During the operation of fuel elements, the burnup of nuclear fuel is far from complete, and the breeding of nuclear fuel (plutonium) occurs in breeder reactors. Therefore, spent fuel elements are reprocessed so that the nuclear fuel can be used again; fission products are removed from the uranium and plutonium. The plutonium in the form of PuO2 is then used to make fuel-element cores, and the uranium, depending on its isotopic content, is also used to make cores or is converted to UF6 for enrichment with 235U.
Reprocessing of nuclear fuel is a complicated and costly method of treating high-level radioactive materials, requiring shielding from radiation and remote control of all operations, even after prolonged storage of the spent fuel elements in special receptacles. In this case, the permissible amount of fissionable material in each piece of equipment is restricted to prevent any possibility of a spontaneous chain reaction. Considerable difficulties are involved in the processing and disposal of radioactive wastes. Techniques have been developed for vitrifying and bitu-menizing wastes and “injecting” low-level wastes deep into the earth. The cost of nuclear fuel reprocessing and radioactive waste disposal considerably influences the economic factors pertaining to nuclear power plants.
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Patton, F. S., D. M. Gudzhin, and V. L. Griffits. Iadernoe goriuchee na osnove obogashchennogo urana. Moscow, 1966.
Vysokotemperaturnoe iadernoe toplivo. Moscow, 1969.
Zaimovskii, A. S., V. V. Kalashnikov, and I. S. Golovin. Teplovydeliaiushchie elementy atomnykh reaktorov. Moscow, 1966.
F. G. RESHETNIKOV and D. I. SKOROVAROV