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Title: Structural Sensitivity of Dry Storage Canisters

This LS-DYNA modeling study evaluated a generic used nuclear fuel vertical dry storage cask system under tip-over, handling drop, and seismic load cases to determine the sensitivity of the canister containment boundary to these loads. The goal was to quantify the expected failure margins to gain insight into what material changes over the extended long-term storage lifetime could have the most influence on the security of the containment boundary. It was determined that the tip-over case offers a strong challenge to the containment boundary, and identifies one significant material knowledge gap, the behavior of welded stainless steel joints under high-strain-rate conditions. High strain rates are expected to increase the material’s effective yield strength and ultimate strength, and may decrease its ductility. Determining and accounting for this behavior could potentially reverse the model prediction of a containment boundary failure at the canister lid weld. It must be emphasized that this predicted containment failure is an artifact of the generic system modeled. Vendor specific designs analyze for cask tip-over and these analyses are reviewed and approved by the Nuclear Regulatory Commission. Another location of sensitivity of the containment boundary is the weld between the base plate and the canister shell. Peak stressesmore » at this location predict plastic strains through the whole thickness of the welded material. This makes the base plate weld an important location for material study. This location is also susceptible to high strain rates, and accurately accounting for the material behavior under these conditions could have a significant effect on the predicted performance of the containment boundary. The handling drop case was largely benign to the containment boundary, with just localized plastic strains predicted on the outer surfaces of wall sections. It would take unusual changes in the handling drop scenario to harm the containment boundary, such as raising the drop height or changing the impact angle. The seismic load case was derived from the August 23, 2011 earthquake that affected the North Anna power station. The source of the data was a monitoring station near Charlottesville, Virginia, so the ground motion is not an exact match. Stresses on the containment boundary were so low, even from a fatigue standpoint, that the seismic load case is generally not a concern. Based on this study, it is recommended that high strain rate testing of welded stainless steel test samples be pursued to define the currently unknown material behavior. Additional modeling is recommended to evaluate specific dry storage cask system designs subjected to tip-over loads using a high level of model detail. Additional modeling of the canister interior components (basket, fuel assemblies, etc.) is also recommended, to evaluate the feasibility of fuel retrievability after a tip-over incident. Finally, additional modeling to determine how much degradation a system could undergo and still maintain the integrity of the confinement barrier should be performed.« less
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Resource Type:
Technical Report
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States