Monte Carlo calculations for recriticality during the reflood phase of a severe accident in a BWR

Monte Carlo calculations were performed to assess the possibility of recriticality during the core relocation phase of a severe accident in boiling water reactors (BWRs). The MCNP Monte Carlo code was chosen both because of its inherent accuracy for such calculations and because many continuous-energy isotopic cross-section libraries based on ENDF/B-V were already available for it. These results suggest that a BWR core may be able to remain subcritical during reflood even when a large fraction of the control rod has been lost. In fact, it appears as if the core might remain subcritical even if 60 or 70% of the boron content of the affected control rods were lost. However, additional calculations at those conditions would be required to determine a realistic estimate of the amount of boron that could be lost without producing criticality