Analysis of burnup credit in fuel storage with CASMO

Recent trends in nuclear power plant operation have tended toward longer cycles with reload fuel of high (> 3.5 wt% 235U) enrichments. At the same time, the need for greater spent-fuel pool capacity has reduced storage canister spacing to the point where maximum allowable fresh enrichments are lower than those necessary for longer cycles. As a result, burnup credit analysis has been applied to spent-fuel storage rack criticality analysis. This allows the placement, after exposure, of higher fuel enrichments in the spent-fuel racks than would normally be allowed under the assumption of a fresh condition. The CASMO lattice physics code is used to analyze burnup credit on the Maine Yankee spent-fuel storage racks. Unit cell sensitivity analysis of rack criticality to in-core depletion effects and to out-of-core decay is considered. Also, large array criticality analysis is performed to study options for the placement of fresh and burned assemblies