Actinide-only burnup credit for pressurized water reactor spent nuclear fuel - III: Bounding treatment of spatial burnup distributions

A flat, uniform axial burnup assumption, preferred for its computational si mplicity does not always conservatively, estimate the pressurized water rea ctor 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 bi explicit modeling of the axial burnup distribution based on limiting axial burnup profiles. An alternative approa ch to this laborious explicit modeling is to augment the multiplication fac tor determined from an axially uniform calculation by an appropriate k(eff) bias. Based on the observation that the end effect increases with a decrea se in the cask size, conservative k(eff) bias curves are determined by appl ying the limiting axial burnup profiles and assuming a single-assembly cask configuration. However, because of their conservative nature, the k(eff) b ias 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 bec ause the limiting arrangement in a cask is an unlikely event. The possibili ty of two or more assemblies with low burnup zones placed inward and next t o each other is small, while the underburned fuel ends will surely be nest to each other. Regardless, the reactivity effect of the horizontal burnup d istribution is bounded by assuming a conservative horizontal burnup, gradie nt within individual assemblies and the most reactive arrangement of multip le assemblies in spent nuclear fuel casks. This approach can have a signifi cant 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.