Estimating the probability of multiple misloads in spent fuel casks for light water reactor systems

The spent fuel dry cask should remain subcritical under normal, abnormal, and accident conditions. The cask may become susceptible to criticality if it is misloaded with assemblies that do not conform with the Certificate of Compliance (CoC). Based on the criticality calculations, the cask needs to be misloaded with more than one fuel assemblies and has an accident in order to become susceptible to criticality. Quantification of the risk of having a misloaded cask may help reduce the conservatism in the cask designs and eliminate unnecessary steps in the spent fuel handling and loading procedure. In this paper, the probability of misloading a cask with more than one light water reactor (PWR and BWR) fuel assemblies is quantified using a combination of event tree and fault trees. An event tree is built to predict all multiple misloads scenarios. Six multiple misloads scenarios are identified from the tree. The probabilities of the six scenarios and the total probability are calculated for casks for both reactor types. The multiple misloads probabilities are found to be 6.73E-07 and 7.55E-06 for PWR and BWR fuels, respectively. In addition, the probability of multiple misloads is calculated as a function of the cask capacity.