Crash Analysis of Degraded Concrete Containment Structures
Dry storage casks are composite structures made of an inner steel liner and a concrete outerpack that houses the basket of the spent nuclear fuel bundle. The concrete of dry cask structures will degrade due to the natural aging and the environmental effect shrinkage, creep, heat transfer, moisture diffusion, and Alkali-Silica Reaction (ASR). Long-term creep and shrinkage of vertical dry cask are modeled using the B4 creep formulation for concrete. The results show that creep and shrinkage does not significantly affect the performance of the dry cask structure. A fully coupled thermo-hygro-chemo-mechanical (THCM) framework is developed to consider the effect of degradation on the concrete outerpack. The coupling between heat transfer and moisture diffusion is verified through an experimental study on concrete and then implemented in the dry cask structure. Also the coupling between the chemical (in form of ASR) and the mechanical processes is verified using experimental studies on concrete cylinders. First order kinetic approach is applied to model ASR for the concrete of dry cask structure. Environmental aging is considered in the form of deleterious mismatch effects of steel and concrete deformation during the cask performance. Environmental and mechanical degradation of concrete are determined for this dry cask storage structure. A de-convolution strategy is developed to extract the material properties from the structural stiffness properties of the cask. Using this strategy, the performance of continuous surface cap material (MAT-159) model is investigated, which is available in the material library of LS-DYNA. The behaviors of the intact and aged dry cask structure and the model and prototype cask are compared in the form of stresses, strains and failure modes. The 1:3-scale model cask and the prototype cask show similar behavior by applying the added mass to the 1:3-scale model cask calculated based on the dynamic similitude theory. Also, implementing the result of mechanical and environmental degradation due to ASR to the dry cask structure, introduces widespread damages to the cask during the tip-over scenario, while the damage is still localized in case of the initial reference concrete. Two main failure modes during tip-over event are concrete crushing and shear banding.