Study of Axial Burnup Profile Effects on BWR Burnup Credit*

Oak Ridge National Laboratory and the United States Nuclear Regulatory Commission have initiated a multiyear project to investigate the application of burnup credit for boiling-water reactor (BWR) fuel in storage and transportation systems. Many aspects of burnup credit must be considered if credit for burnup beyond peak reactivity can be accepted. This paper presents the results of a study examining the effect of axial burnup profiles on calculated cask reactivity for discharged BWR fuel assemblies.The axial burnup profiles used in this study are normalized end of cycle (EOC) profiles taken from a single cycle of operation at a modern BWR. This set of profiles was selected because it is from the same detailed operational data set used for other aspects of the overall project examining BWR burnup credit. A total of 624 profiles are available from a range of fuel assembly design types, and the EOC burnups range from 16.8 to 48.5 GWd/MTU. The profiles provide sufficient breadth to assess the generic impacts of axial burnup profiles on discharged fuel assembly reactivity. Cask reactivity is determined in the GBC-68 calculational benchmark model assuming a loading of 68 GE-14 assemblies with the same discharged burnup and axial burnup profile. The calculations are repeated assuming discharged burnups of 30, 40, and 50 GWd/MTU; and each of the 624 profiles is considered at each discharge burnup. The calculations are also performed both neglecting the presence of natural-enrichment axial blankets (a common practice in pressurized-water reactor burnup credit) and crediting the presence of these blankets. The trends are generally consistent at all burnups, both with and without axial blankets, although the overall reactivity level is much lower with explicit blanket modeling. Overall assembly reactivity is controlled by the reactivity of the top few nodes in the assembly, and it is noted that a larger number of top nodes is important when the blankets are modeled. The resulting keff values cover a broad range, indicating a strong dependence of discharged assembly reactivity on axial burnup profile. End effects as high as 12.7 % Δk are also noted. These results indicate that while the effects of axial burnup profiles on BWR burnup credit are similar to effects on pressurized-water reactor burnup credit, the reactivity impacts are larger. Appropriate axial burnup profile selection is essential for a conservative BWR burnup credit analysis beyond peak reactivity.