Bias Estimates Used in Lieu of Validation of Fission Products and Minor Actinides in MCNP K_eff Calculations for PWR Burnup Credit Casks (NUREG/CR-7205, ORNL/TM-2012/544)

The U.S. Nuclear Regulatory Commission Division of Spent Fuel Management issued Interim Staff Guidance (ISG) 8, Revision 3 in September 2012. This ISG provides guidance for NRC Staff review of burnup credit analyses supporting transport and dry storage of pressurized water reactor spent nuclear fuel (SNF) in casks. ISG-8, Rev. 3 includes, among other things, guidance for addressing validation of criticality (k_eff) calculations crediting the presence of a limited set of fission products and minor actinides (FP&MA). Based on previous work documented in NUREG/CR-7109, ISG-8, Rev. 3 includes a recommendation to accept use of 1.5 or 3% of the FP&MA worth, depending on the criticality code and cross-section data used, to conservatively account for the bias and bias uncertainty associated with the selected insufficiently validated FP&MAs. This bias is applied in addition to the bias and bias uncertainty resulting from validation of k_eff calculations for the major actinides in SNF. The work described in this report involves comparison of FP&MA worths calculated using SCALE and Monte Carlo N-Particle (MCNP) with Evaluated Nuclear Data Files, Part B–V, –VI, and –VII based nuclear data. The comparison supports use of the 1.5% FP&MA worth bias when either SCALE or MCNP codes are used for criticality calculations, provided the other conditions of ISG-8, Rev. 3, Recommendation 4 are met. The additional conditions include that the cask design is similar to the hypothetical generic burnup credit-32 cask model and that the credited FP&MA worth is no more than 0.1 Δk_eff.