Study on the enrichment limit of 5 wt% in light water reactor fuels

Increasing the average extracted burnup of spent fuel in light water reactors reduces the amount of spent fuel generated per power generation amount. For this purpose, it is effective to increase the fuel burnup, which requires a rise in the fuel concentration degree. The authors examined the regulation of the current enrichment limit of 5 wt%, physical meaning, and their validity, and thus clarified the meaning of enrichment limit. Using Monte Carlo N-Particle Transport code (MCNP), they calculated the critical weight using enrichment and atomic ratio as parameters. In the 'Legislation on the location, structure and equipment standards of processing facilities' etc. in Japan, enrichment degree of 5 wt% is considered as the boundary of a criticality accident. This is based on ANS/ANSI of USA. As a result of calculation based on the code MCNP, both uranium dioxide and metallic uranium have a convex maximum value in the hydrogen-to-uranium ratio (H/U) dependence of the effective multiplication factor, where the minimum critical mass exists. Therefore, it is reasonable to critically control the critical mass at the point where the effective multiplication factor becomes the maximum with respect to H/U, regardless of uranium enrichment, compound, or shape. When H/U infinitely approaches to 0 (zero), even uniform uranium with enrichment of 5 wt% does not reach criticality. (A.O.)