Assessment of Reactivity Margins and Loading Curves for PWR Burnup-Credit Cask Designs

This report presents studies to assess reactivity margins and loading curves for pressurized water reactor
(PWR) burnup-credit criticality safety evaluations. The studies are based on a generic high-density 32-
assembly cask and systematically vary individual calculational (depletion and criticality) assumptions to
demonstrate the impact on the predicted effective neutron multiplication factor, keff, and burnup-credit
loading curves. The purpose of this report is to provide a greater understanding of the importance of
input parameter variations and quantify the impact of calculational assumptions on the outcome of a
burnup-credit evaluation. This study should provide guidance to regulators and industry on the technical
areas where improved information will most enhance the estimation of accurate subcritical margins.
Based on these studies, areas where future work may provide the most benefit are identified. The report
also includes an evaluation of the degree of burnup credit needed for high-density casks to transport the
current spent nuclear fuel inventory. By comparing PWR discharge data to actinide-only based loading
curves and determining the number of assemblies that meet the loading criteria, this evaluation finds that
additional negative reactivity (through either increased credit for fuel burnup or cask design/utilization
modifications) is necessary to accommodate the majority of current spent fuel assemblies in high-capacity
casks. Assemblies that are not acceptable for loading in the prototypic high-capacity cask may be stored
or transported by other means (e.g., lower capacity casks that utilize flux traps and/or increased fixed
poison concentrations or high-capacity casks with design/utilization modifications).