NUREG-1150 risk assessment methodology

This paper describes the methodology developed in support of the US Nuclear Regulatory Commission's (NCR's) evaluation of severe accident risks in NUREG-1150. After the accident at Three Mile Island, Unit 2, the NRC initiated a sever accident research program to develop an improved understanding of severe accidents and to provide a second technical basis to support regulatory decisions in this area. A key product of this program is NUREG-1150, which provides estimates of risk for several nuclear reactors of different design. The principal technical analyses for NUREG-1150 were performed at Sandia National Labs. under the Severe Accident Risk Reduction Program and the Accident Sequence Evaluation Program. A major aspect of the work was the development of a methodology that improved upon previous full-scale probabilistic risk assessments (PRA) in several areas which are described.