Probabilistic risk assessment of the spent nuclear fuel loading process in light water reactor systems

The spent fuel dry cask should remain subcritical under normal, abnormal, and accident conditions. The cask becomes susceptible to criticality if it is misloaded with assemblies that do not conform with the Certificate of Compliance (CoC). To avoid this scenario, the cask loading process involves several verification steps to make sure that all of the loaded assemblies satisfy the CoC requirements. However, most of loading and verification steps are carried out by humans with finite probabilities for errors, which need to be quantified. In this paper, the probability of misloading a cask with light water reactor (PWR and BWR) fuel is quantified using the event tree method. Probability distribution functions for all of the human errors are obtained using the SPAR-H human reliability analysis method. The Fussell-Vesely (FV) importance measure is performed to determine the tasks that contribute the most to the having a misloaded cask. The probability of misload is found to be 5.56E-06 for cask loaded with the PWR and 2.95E-05 for the cask loaded with the BWR fuel. Both of these are considered to be small.