Computational Benchmark for Estimated Reactivity Margin from Fission Products and Minor Actinides in BWR Burnup Credit

This report proposes and documents a computational benchmark for the estimation of the additional reactivity margin available in spent nuclear fuel (SNF) from fission products and minor actinides in a burnup-credit storage/transport environment, relative to SNF compositions containing only the major actinides. The benchmark problem/configuration is a generic burnup- credit cask designed to hold 68 boiling water reactor (BWR) spent nuclear fuel assemblies. The purpose of this computational benchmark is to provide a reference configuration for the estimation of the additional reactivity margin for partial burnup credit (ISG-8) and document reference estimations of the additional reactivity margin as a function of initial enrichment, burnup, and cooling time. This benchmark model will also be used as the base case in future sensitivity studies, to be documented in a companion NUREG/CR. The geometry and material specifications are provided in sufficient detail to enable independent evaluations. Estimates of additional reactivity margin for this reference configuration may be compared to those of similar burnup-credit casks to provide an indication of the validity of design-specific estimates of fission- product margin. The reference solutions were generated with the SCALE 6.1 package. The SCALE depletion and criticality sequences have been extensively validated elsewhere for a variety of applications, including light water reactor fuel. Note that the reference solutions presented in this report are not directly or indirectly based on experimental results. Consequently, this computational benchmark cannot be used to satisfy the ANS 8.1, 8.24, and 8.27 requirements for validation of calculational methods and is not intended to be used to establish biases and bias uncertainties for burnup-credit analyses.