Fuel enrichments

Other fuel besides that from U.S. commercial reactors may be disposed of in the ultimate repository. PossibiUties are spent fuel from defense reactors and fuel from research reactors outside of the United States. To reduce the proliferation of nuclear weapons, the United States has urged that research reactors reduce fuel enrichment in uranium-235 from around 90 to 20%. The latter fuel could not be used in a weapon. The United States has agreed to accept spent fuel from these reactors. 

Radiolytic oxidation is important to the design and operation of reactors because it adversely affects key graphite properties and, by removing moderator material, may bring about the need for increased fuel enrichment. As mentioned earlier, an inhibitor (methane) is added to the coolant to reduce radiolytic oxidation to acceptable levels. However, access of the inhibitor to the inner portions of the moderator brick must be assured. Two approaches have been adopted in the AGRs to provide this access. Vertical methane access holes are provided in the fuel bricks and in the later stations, Heysham II and Tomess, a pressure drop from outside to inside the brick was established to cause an enhanced flow through the brick. The amount of inhibitor added must be restricted, however, because the carbon inhibition reaction product deposits on the fuel pin and restricts heat transfer to the coolant, thus reducing reactor efficiency. 

The Canadian Deuterium Uranium reactor fissions with natural uranium, hence, no dependence on national or international fuel enrichment facilities that are needed to enrich uranium to about 3% U-235 to achieve criticality with light water moderation. 

Although protons are veiy efficient neutron moderators, they also efficiently capture neutrons to form hound proton-neutron pairs called detiterons. Reactors using ordinary water for the moderator compensate for neutron capture by using fuel enriched to about 3 percent U. 

In the light water reactor, the circulating water serves another purpose in addition to heat transfer. It acts to slow down, or moderate, the neutrons given off by fission. This is necessary if the chain reaction is to continue fast neutrons are not readily absorbed by U-235. Reactors in Canada use heavy water, D20, which has an important advantage over H20. Its moderating properties are such that naturally occurring uranium can be used as a fuel enrichment in U-235 is not necessary. 

Results are presented of studies undertaken in Italy by SAFI and Replastic of the gasification of refuse derived fuel enriched with post-consumer plastics for the production of electrical energy and gas for use in cement making. 11 refs. 

Two cores had 6-% fuel enrichment, the remaining cores 21 % enrichment. Mean fuel load therein was equal to 46.8 kg. According to specifications, maximal fuel bumup made up 20 % of the initial fuel load on average. 

Safety features at a nuclear power plant include automatic shutdown of the fission process by insertion of control rods, emergency water cooling for the cote in case of pipeline breakage, and a concrete containment shell. It is impossible for a reactor to have a nuclear explosion because the fuel enrichment in a reactor is intentionally limited to about 3% uranium-235, while almost 100% pure uranium-235 is required for a bomb. The worst accident at a PWR would be a steam explosion, which could contaminate the inside of the containment shell. 

The fissionable isotopes are U-233, U-235, Pu-239, and Pu-241. The fertile isotopes U-238 and Th-232 are converted to fissionable isotopes by neutron absorption (U-238 into plutonium isotopes and Th-232 into U-233). Natural uranium contains 0.71% U-235, 99.28% U-238, and 0.006% U-234. Fuel enriched in U-233 and plutonium must be produced from thorium and U-238, respectively (Fig. 1) by neutron capture the neutrons are provided initially by fission of U-235. 

The optimization reduced the fresh fuel enrichment required to satisfy the cycle energy requirements from 3.60 w/o to 3.40 w/o. As shown in Figure 8, the value of FAH predicted by FORMOSA-P over the fuel cycle was also reduced from 1.41 to the constraint limit (user input) value of 1.38. The optimized fuel pattern achieves substantial improvements in fuel cycle economics while at the same time improving the margin to thermal operating limits. 

Nuclear operating costs do not include the construction and operation of the U.S. government uranium fuel enrichment facilities. When all three of these enrichment facilities were operating at full capacity, their power consumption was similar to that of the country of Australia. Other excluded operating costs include Federal regulation, long term waste disposal and any health costs that are associated with people being exposed to radiation. 

Fuel enrichment. All practicable enrichment processes require the uranium to be in the form of a gas. UFg, which readily sublimes (p. 1269), is universally used and, because fluorine occurs in nature only as a single isotope, the compound has the advantage that separation depends solely on the isotopes of uranium. The first, and until recently the only, large-scale enrichment process was by gaseous diffusion which was originally developed in the Manhattan Project to produce nearly pure U for the first atomic bomb (exploded at Alamogordo, New Mexico,... 

Reaction front velocities are lower for fuel-lean mixtures and increase for fuel-enriched mixtures ... 

The nuclear fuel industry (see Section 2.5) uses large quantities of F2 in the production of UFg for fuel enrichment processes and this is now the major use of F2. Industrially, the most important F-containing compounds are HF, BF3, CaF2 (as a flux in metallurgy), synthetic cryolite (see reaction 12.43) and chlorofluorocarbons (CFCs, see Box 13.7). 

Nishioka, M., Aromatic sulfur compounds other than condensed thiophenes in fossil fuels enrichment and identification, Energy Fuels, 2, 214-228, 1988. 

Depending on conditions, in the course of the moderate combustion, organic acids may also be formed (at lower temperatures) in a thermal regime corresponding to 523-573 K, heterocyclic hydrocarbons are formed. The production of alcohols is sometimes also observed, particularly in fuel-enriched mixtures. Carbon monoxide, carbon dioxide and water are the main final products. 

During 1986 and 1987 the reactor was changed over to the modified core (01M) with the peak bumup as high as 8.3% h.a. to improve fuel performance and to increase BN600 reactor fuel bumup. The new peak bumup values were 6.5% h.a. for the lowly enriched fuel, 6.9% h.a. for the intermediately enriched fuel and 8.3% h.a. for the highly enriched fuel. The principal difference between the first load type core and the modified core was an increase in the height of a fuel fissile section from 750 to 1000 mm and utilization of three uranium 235 fuel enrichments, i.e. 17%, 21% and 26% instead of 21% and 33%. 

TABLE 2.THE DEPTH OF FUEL BURNING UP, FAST NEUTRONS DAMAGING DOSE, MICROCAMPAIGN AND CAMPAIGN DURATION AS THE FUNCTIONS OF MAKE-UP FUEL ENRICHMENT... 

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