The demand for dry storage of spent nuclear fuel (SNF) is continually increasing due to competitive advantages such as expandability, economical efficiency, safety, security, and public acceptance. In recent years, the safety and security of SNF storage facilities received a considerable attention as these facilities are seen as an attractive target for vicious man activities. The safety of storage system under design-basis accidents should be verified during the license application process. For the extra-regulatory accidents, the consequence of the accidents needs to be quantified and a plan for recovery and mitigation should be prepared. In general, structural damage evaluation of a SNF dry storage system under an impact condition is a very complicated process due to limited knowledge on important parameters relevant to design safety. In this dissertation, a systematic risk assessment framework is developed for one of the most severe impact events on an interim dry storage facility, an aircraft impact (AI), using a probabilistic safety assessment. The proposed assessment procedure includes the determination of loading parameters and reference impact scenario, structural response analyses of facility walls, cask containment, fuel assemblies, and a single fuel rod composite, as well as radiological consequence analysis with dose-risk estimation. A descriptive case study of the AI is performed to demonstrate the practical applicability of the proposed framework.
In this work, new damage evaluation methodologies of a detailed generic cask model in a simplified building structure under AI were presented through numerical structural analyses and an analytical fragility assessment. A detailed storage metal cask model was developed in order to accurately and efficiently evaluate the structural response by a finite element code that can encompass various impact conditions. The structural analyses included evaluation of the dynamic response of a cask lid closure system to calculate the leakage path area, the dynamic response of a fuel assembly to estimate the fraction of rods that fail during the impact, and the dynamic response of a single fuel rod to estimate the load carrying capacity with a realistic and descriptive prediction. The risks are estimated for three structural levels considering aging effects for a storage facility, and cask, as well as post-irradiated conditions for a fuel assembly to evaluate the influence of parameters relevant to design safety.
The superiority of this framework is seen in its flexibility to consider a range of parameters in order to evaluate dry storage system capability to withstand AI and the prediction of potential realistic risks; the framework also provides insight into epistemic uncertainty in the available data, and into the sensitivity of design parameters in terms of risk. Further, this risk model can be used with any other representative detailed parameters and reference design concepts for other comparable direct or indirect impact conditions onto the cask body. Lastly, the procedure and methodology developed for the structural analysis in regards of fuel assembly characteristics can be further extended to support the development of guidelines for safe storage and post-storage transportation of SNF when augmented by realistic data obtained experimentally.