Impact of integral burnable absorbers on PWR burnup credit criticality safety analyses

The utilization of credit for fuel burnup necessitates consideration of a wide range of fuel designs and operating conditions, including the use of integral burnable absorbers (IBAs). The Interim Staff Guidance on burnup credit (ISG-8) issued by the Nuclear Regulatory Commission’s Spent Fuel Project Office recommends licensees restrict the use of burnup credit to assemblies that have not used burnable absorbers. This restriction eliminates a large portion of the currently discharged spent fuel assemblies from cask loading, and thus severely limits the practical usefulness of burnup credit. This paper examines the effect of IBAs on reactivity to provide technical justification for relaxing the current restriction for dry cask storage and transport, and subsequently, to develop the necessary guidelines for relaxing the current restriction. Analyses have been performed for Westinghouse assembly designs with Integral Fuel Burnable Absorber (IFBA) rods, Combustion Engineering (CE) and Siemens assembly designs with UO2- Gd2O3 rods and CE assembly designs with UO2-Er2O3, and Al2O3-B4C rods. Analyses are presented for a realistic range of initial fuel enrichment and poison loading combinations based on plant data. The effects are quantified, and trends with initial fuel enrichment and poison loading are noted. The results demonstrate that assembly designs with IBAs other than IFBA rods are less reactive throughout burnup than their corresponding designs without the IBA rods (i.e., nonpoisoned, equivalent enrichment). On the other hand, IFBA rods were found to be more reactive at discharge burnup than assemblies without IFBA rods. Thus, neglecting the presence of IBAs other than IFBAs in a burnup-credit criticality safety evaluation will yield slightly conservative results; however, IFBAs must be accounted for in a safety evaluation.