A flat, uniform axial burnup assumption, preferred for its computational simplicity, does not always conservatively estimate the pressurized water reactor spent-fuel-cask multiplication factors. Rather, the reactivity effect of the significantly underburned fuel ends, usually referred to as the "end effect," can be properly treated by explicit modeling of the axial burnup distribution based on limiting axial burnup profiles. An alternative approach to this laborious explicit modeling is to augment the multiplication factor determined from an axially uniform calculation by an appropriate keff bias. Based on the observation that the end effect increases with a decrease in the cask size, conservative keff bias curves are determined by applying the limiting axial burnup profiles and assuming a single-assembly cask configuration. However, because of their conservative nature, the keff bias curves are not recommended unless there is a large reactivity margin in the particular cask of interest.

The horizontal burnup distribution poses less reactivity concern simply because the limiting arrangement in a cask is an unlikely event. The possibility of two or more assemblies with low burnup zones placed inward and next to each other is small, while the underburned fuel ends will surely be next to each other. Regardless, the reactivity effect of the horizontal burnup distribution is bounded by assuming a conservative horizontal burnup gradient within individual assemblies and the most reactive arrangement of multiple assemblies in spent nuclear fuel casks. This approach can have a significant effect on small cask designs where the orientation of fuel assemblies has a substantial influence on the calculated multiplication factor because of the large radial neutron leakage.