Development of methods for internal event (level 1) multi-unit PRA

On March 11, 2011, the Fukushima Daiichi nuclear accident, triggered by the earthquake and tsunami, revealed the significant risk at a site with multiple nuclear power reactors. Since then, multi-unit probabilistic risk assessment (MUPRA) has attracted increased attention. Although several research groups have proposed frameworks for MUPRA, only a few MUPRAs have been conducted in the industry. This report provides an evaluation framework for an internal-event Level 1 MUPRA by extending single-unit probabilistic risk assessment (SUPRA) models to an MUPRA model. The proposed MUPRA framework is based on conventional SUPRA event tree/fault tree (ET/FT) and minimal cutset methods. The framework provides a procedure for extending the existing SUPRA models to an MUPRA model. In addition, the framework provides methods for (1) inter-unit common-cause failure analysis, (2) multi-unit human reliability analysis, and (3) multi-unit accident sequence development and frequency quantification. This report provides an example of MUPRA implementation using the framework. The example is a case study of two advanced boiling water reactors with a loss of off-site power as the initiating event, and the feasibility of the framework is confirmed through this case study. (author)