Criticality safety evaluation of spent fuel storage rack under normal condition using PHOENIX-P code

The high density spent fuel storage rack Boraflex was known to experience changes of its physical property and to dissolve under exposure to radiation in an aqueous environment for long period of time. In this study, the criticality evaluation for spent fuel storage rack of Ulchin unit 2 under normal condition was performed assuming complete loss of B-10 from the Boraflex and applying burnup credit. Benchmark calculations for PHOENIX-P and KENO-V.a. code was performed obtaining the calculation bias and bias uncertainties. The manufacturing tolerances of nuclear fuel and storage rack and their reactivity uncertainties were derived, as well. Considering those bias and uncertainties of calculation, the criticality of spent fuel storage under normal condition was conservatively evaluated. The spent fuel with initial enrichment of 5.0w/o was evaluated to meet the subcritical safety if its burnup is over 43,787 MWD/MTU. The criticality evaluation result also showed that spent fuels with the initial enrichment less than 1.56w/o were able to be stored in the storage pool regardless of their burnup. Conclusively, in the region II of the spent fuel storage pool, the maximum keff, considering all uncertainties, was calculated as 0.94834