Combined nuclear safety-security risk analysis methodology development and demonstration through a case study

Destruction of critical nuclear infrastructure would have a debilitating effect on national public health, safety, national economy and security. For this reason, analysts perform safety risk analyses on the performance of the nuclear system to quantify and understand the nature of unwanted events. Since the world has gone through many changes after the terrorist attacks of 9/11, nuclear security risk analysis also became a necessity. To date, the safety and security risk analyses have been done separately without a combined evaluation. Study results are presented for three types of risk analyses for a pure security initiating event, pure safety initiating event, and a combined analysis of safety-security risk for either a security or safety initiating event. The pure security risk analysis uses adversary sequence diagram and pathway analysis to calculate the initiating security event frequency of a successful adversary attack. The pure safety analysis represented a series of natural (random) safety system component failure events for which a safety system failure frequency was calculated using SAPHIRE probabilistic risk analysis code. On the other hand, the combined safety-security analysis considered a security initiating event followed by safety system failure or vice versa. The main outcome of the comparative study of three different types of risk analyses is that pure safety risk evaluation without considering the possibility of a simultaneous security attack would underestimate the risk value. Failure frequency due to a security event should be combined with the safety system failure analysis for a meaningful risk analysis and the Estimate of Adversary Sequence Interruption (EASI) model can be employed for this purpose. The usefulness of a combined safety-security risk analysis is demonstrated through a case study for the spent fuel storage pool facility.