Maximizing Allowable Cask Payloads Using Zone-Loading and Cooling Table Specifications

Utilities have recently been placing spent fuel into “dual-purpose” canisters, which can be stored on site in a concrete silo, and/or transported to another location (such as a repository) in a transportation cask. The newer dualpurpose canister designs generally have a higher fuel assembly capacity than earlier designs (e.g., 32 vs. 21-24 PWR assemblies). At the same time, the discharge burnup of spent fuel assemblies has been increasing, from typical values of ~35,000 MWd/MTU in the past to ~50,000 or more today. Finally, at many sites, much of the longer cooling time (and lower burnup) fuel in spent fuel pools has already been loaded into dry-fuel storage systems, leaving only “hotter” (higher burnup, lower cooling time) assemblies in the spent fuel pool. Taken together, these three factors represent a tremendous challenge for dual-purpose canister systems. Newer systems will be required to accommodate a larger number of higher burnup, lower cooling time fuel assemblies. Due to the resulting increases in thermal and radiological source terms from the assembly payload, this will generally result in higher cask system temperatures and cask external dose rates, making it more difficult to meet 10CFR71 and 10CFR72 thermal and radiological requirements. This may result in a significant reduction in the fraction of assemblies remaining in the spent fuel pool that qualify for loading into the dual-purpose canister, for either storage or shipment. One approach to addressing this issue would be to employ advanced, and potentially expensive, engineering features to enhance cask shielding and heat removal capabilities. Another approach involves the strategic loading of fuel assemblies in specific locations within the dual-purpose canister, along with a more rigorous (as opposed to simple and overly conservative) analysis of the specific assembly payload configuration inside the canister. This second approach, which does not involve difficult engineering design and fabrication, and which does not add to the cost of the canister or cask, is the subject of this paper.