Recommendations on the Credit for Cooling Time in PWR Burnup Credit Analyses

The U.S. Nuclear Regulatory Commission's guidance on burnup credit for pressurized-water-reactor (PWR) spent nuclear fuel (SNF) recommends that analyses be based on a cooling time of five years. This recommendation eliminates assemblies with shorter cooling times from cask loading and limits the allowable credit for reactivity reduction associated with cooling time. This report examines reactivity behavior as a function of cooling time to assess the possibility of expanding the current cooling time recommendation for SNF storage and transportation. The effect of cooling time on reactivity for various initial enrichments, burnups, and selected nuclide sets is shown and discussed. Further, the benefits of additional credit for cooling time are quantified based on a realistic high-capacity rail-type cask designed for burnup credit. While this report is primarily focused on cask storage and transportation, analyses are extended out to 100,000 years to understand the relevant concerns associated with long-term disposal and their possible influence on storage and transportation practice. The report concludes with a discussion on the issues for consideration and recommendations for expanded allowance of credit for cooling time in criticality safety analyses using burnup credit for cask storage and transportation.