Review of Axial Burnup Distribution Considerations for Burnup Credit Calculations

This report attempts to summarize and consolidate the existing knowledge on axial
burnup distribution issues that are important to burnup credit criticality safety calculations.
Recently released Nuclear Regulatory Commission (NRC) staff guidance permits limited burnup
credit, and thus, has prompted resolution of the axial burnup distribution issue. The reactivity
difference between the neutron multiplication factor (keff) calculated with explicit representation
of the axial burnup distribution and keff calculated assuming a uniform axial burnup is referred to
as the “end effect.” This end effect is shown to be dependent on many factors, including the
axial-burnup profile, total accumulated burnup, cooling time, initial enrichment, assembly
design, and the isotopics considered (i.e., actinide-only or actinides plus fission products). Axial
modeling studies, efforts related to the development of axial-profile databases, and the
determination of bounding axial profiles are also discussed. Finally, areas that could benefit
from further efforts are identified.