Effect of Fissile Isotope Burnup on Criticality Safety for Stored Disintegrated Fuel Rods.

摘要

If the fuel rods were to disintegrate and water added, a criticality could occur in a 13-in. PWR canister with fresh fuel enriched to 3.5wt % exp 235 U. The question is, ''If credit could be taken for burnup,could this indicate a subcritical condition.''In attempting to answer this question, a series of calculations were performed. A set of isotopic concentrations were generated for 5,000, 10,000, 15,000,and 20,000 MWD/MTU burnup levels. Four reflector materials,water,concrete and two types of soil,were considered. Results indicate that allowing credit for fissile isotope burnup does not completely remove the concern for criticality safety in the event of rod disintegration. Reactivities which are ''subcritical'' (k/sub eff/ = 0.95) would not occur for three of the four reflector materials at even the 20,000 MWD/MTU burnup level in the 13-in. canister. The water reflected canister would achieve the k/sub eff/ = 0.95level near 18,000 MWD/MTU. A smaller canister could be postulated. If a quarter inch gap is allowed, a Westinghouse 17 x 17PWR assembly requires a 12exp 1 /sub 4inch diameter canister. For such a canister with water reflection the ''subcritical'' (k/sub eff/ = 0.95) level would be reached near 15,000 MWD/MTU. The soil reflected canisters would reach this level between 18,000and 19,000 MWD/MTU. Considering the difficulties in taking credit for burnup,such modest gains in apparent safety are not encouraging. This situation might be improved,however,if credit were also taken for neutron absorption by fission product poisons produced during burnup. It is strongly recommended that other approaches to a solution of the criticality safety problem be considered. (ERA citation 04:009778)