Calculation of Uranium-236 Penalty for Recycle Uranium Fuels

摘要

Spent nuclear fuel (SF) discharged from a typical 1000-MWe PWR [fueled with 3.3 % (weight percent) UO_2, burned to 33000 MWd/ton U, with an exposure time of 1100 days and a capacity factor of 80 %] contains approximately 95.5 % U, 0.9 % Pu, 3.5 % fission products and 0.1 % other actinides. Isotopic composition of U contained is 0.012 % U-234, 0.826 % U-235, 0.438 % U-236, and 98.724 % U-238 after 150 days from discharge. By reprocessing the SF, U and Pu contained can be recovered as pure and separate streams. Recycling of U and Pu in SF results in significant reductions in fresh fuel requirements (more than 40 %). For recycling, Pu recovered from SF by reprocessing (70% fissile) is to be diluted with natural or depleted U in order to produce mixed-oxide (U+Pu) fuel with an appropriate fissile content. U product of reprocessing (0.826 % fissile) is to be enriched before being recycled into the same reactor. The existence of U-236 in the recycle U requires an enrichment somewhat greater than 3.3 % in order to achieve the same discharge burnup. This additional enrichment is known as U-236 penalty. The negative effect of U-236 on neutron economy is not only due to its capture cross-section (5.2 barns) for thermal neutrons, but also because U-236 replaces U-238, which otherwise would be converted to Pu-239 in the reactor and contribute to the power production significantly.