Parametric Study of the Effect of Burnable Poison Rods for PWR Burnup Credit

The Interim Staff Guidance on burnup credit (ISG-8) issued by the United States Nuclear Regulatory
Commission’s (U.S. NRC) Spent Fuel Project Office recommends restricting the use of burnup credit to
assemblies that have not used burnable absorbers. This recommended 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. In the absence of readily available information on burnable poison rod (BPR)
design specifications and usage in U.S. pressurized-water-reactors (PWRs), and the subsequent reactivity
effect of BPR exposure on discharged spent nuclear fuel (SNF), NRC staff has indicated a need for
additional information in these areas. In response, this report presents a parametric study of the effect of
BPR exposure on the reactivity of SNF for various BPR designs, fuel enrichments, and exposure conditions,
and documents BPR design specifications. Trends in the reactivity effects of BPRs are established with
infinite pin-cell and assembly array calculations with the SCALE and HELIOS code packages, respectively.
Subsequently, the reactivity effects of BPRs for typical initial enrichment and burnup combinations are
quantified based on three-dimensional (3-D) KENO V.a Monte Carlo calculations with a realistic rail-type
cask designed for burnup credit. The calculations demonstrate that the positive reactivity effect due to BPR
exposure increases nearly linearly with burnup and is dependent on the number, poison loading, and design
of the BPRs and the initial fuel enrichment. Expected typical reactivity increases, based on one-cycle BPR
exposure, were found to be less than 1% Δk. Based on the presented analysis, guidance is offered on an
appropriate approach for calculating bounding SNF isotopic data for assemblies exposed to BPRs. Although
the analyses do not address the issue of validation of depletion methods for assembly designs with BPRs,
they do demonstrate that the effect of BPRs is generally well behaved and that independent codes and crosssection
libraries predict similar results. The report concludes with a discussion of the issues for consideration
and recommendations for inclusion of SNF assemblies exposed to BPRs in criticality safety analyses using
burnup credit for dry cask storage and transport.