Deterministic and Probabilistic Effects of External Event in Safety Assessment

摘要

Safety assurance implicitly assumes a certain risk. The risk results from the cumulative contribution of many kinds of events. The safety design of nuclear facilities is expected to minimize the overall risk considering the deterministic and probabilistic effects from internal and/or external events. Conventional safety assessment for external events is based on a conditional risk insight, considering the severe hazard that will cause large deterministic effects on safety systems. When low probability effects are considered, the safety margins calculated by the conventional assessment are not clear, and it is not certain that the risks remain acceptably low. Assessment considering low probability effects should be performed for external events in order to strengthen the reliability of the risk constraint. These aspects of low probability effects in nuclear facilities are not yet fully integrated in a total risk management for external events. A rational safety design for external events is therefore required to comply with the risk constraint based on the probabilistic safety assessment. The cumulative probabilistic hazard is needed to evaluate the risk constraint and the risk significant effect of the hazardous events. A de-aggregation of the cumulative probabilistic hazard is necessary to develop design basis events that are risk dominant events for use in a deterministic safety design. The determination of design basis events based on the de-aggregation from the cumulative probabilistic hazard must not compromise the risk constraint. Aspects of the deterministic and probabilistic effects of external events are categorized by the indexes of damage rate and the probability of failure, respectively. This paper discusses a new methodology for enhancing the reliability of safety assurance by safety and risk assessments that comply with damage rate and probability of failure.