Effect of axial exposure distributions in burnup credit criticality analyses

Burnup credit is the application of the effects of fuel exposure or burnup to nuclear criticality considerations in the design of spent fuel transport and storage facilities. One unique issue in this design approach is the proper treatment of the axial variation in burnup experienced by pressurized-water-reactor fuel assemblies. This paper describes calculations and results quantifying this effect in the criticality analysis of spent fuel array geometries. Recommendations are made to provide guidance in evaluating these effects via three different approaches. Final selection of the analysis methodology would be dependent on the specific application and the degree of accuracy required.