Bounding Estimates of Criticality Effects of Unburned LWR Fuel Assembly Tips for Casks Assuming Bumup Credit

The criticality scenario treated here of a large, (36 PWR Assemblies) dry, spent fuel cask is based on the puncturing and subsequent submersion of the cask into water which enters the cask at some reasonable rate. All assembly tips (one foot length, top and bottom) are assumed to be relatively unburned (2w/o enriched in U-235), whereas, the middle ten feet of each assembly has a residual discharge U-235 enrichment of 0.8w/o. The 36 PWR assemblies reside contiguously in the cask cavity without structural baskets or fixed neutron poisons. This, together with an enrichment of 0.8w/o for over 83% of the active fuel length, represent the factors for bumup credit in transportation. The tip enrichment with the in-leakage water are the essential ingredients necessary for super-criticality to occm. A comparison with the super-critical SPERT-OXIDE reactor experiments shows that the experiment and the cask-contents accident-environment are equivalent on a nuclear-criticality basis. Justification using this similarity argument is given in subsequent sections of this paper. Dollar step insertion of reactivity, initial reactor period, maximum energy released (mw-sec and total fission yield) from the nuclear excursion, maximum pressure, number of fuel rods ruptured and eventual shutdown mechanisms are all estimated for the cask directly from SPERT-OXIDE data. The SPERT experiments were performed in an open tank. The cask-water system is likewise considered an open tank. A step insertion of reactivity can be realized in the instance when a submerged cask is suddenly uprighted (recovery operations) allowing the water on the cask insides to quickly rush to the cask bottom - covering the assembly tips containing the higher U-235 enrichment