Integrating internal events in an external event probabilistic risk assessment: tornado PRA case study

Abstract

Abstract External events (e.g. earthquake, wind, tornado) can be significant risk contributors in nuclear power plant probabilistic risk assessments (PRA). This paper reports the results of a PRA that completely integrates internal random failures and operator errors, and events that are externally initiated by tornado strike. Although this integrated approach is recommended practice, commerical PRAs typically treat internal and external events separately. The paper also illustrates how the results can be analyzed to prioritize plant upgrades, modify operator recovery actions, and/or modify equipment testing schedules to optimally reduce risk. For the plant studied, the results show that incorporating accident sequences that integrate random failures, operator errors, and the external event (tornado strike in this case) can significantly alter how the total damage frequency is distributed over the plant damage states. Integrating random failures and operator actions also resulted in identification of a key accident sequence that lead to modifying operator recovery actions and equipment test intervals at the plant. We conclude that not all plants would require a completely integrated external events risk assessment. However, we recommend that random failures be screened for inclusion in the external events analysis. Also, the human error analysis should be carefully reviewed to take account of the environment after occurence of an external event and to incorporate new recovery actions that would be taken as a result of external event-induced losses.