Accident sequence precursor analysis: level 2/3 model development

The US Nuclear Regulatory Commission’s Accident Sequence Precursor (ASP) program currently uses simple level 1 models to assess the conditional core damage probability for operational events occurring in commercial nuclear power plants (NPP). Not all accident sequences leading to core damage will result in the same radiological consequences. Therefore, it is necessary to develop simple level 2/3 models that can be used to analyze the response of the NPP containment structure in the context of a core damage accident, estimate the magnitude of the resulting radioactive releases to the environment, and calculate the consequences associated with these releases. This level 2/3 model development work was initiated in 1995, and several prototype models have been completed. Two versions of these prototype level 2/3 models have been developed. Simple level 2/3 models have been linked to the simple level 1 models to provide risk perspectives for operational events. Additionally, very complex and detailed models have been developed that take provide much greater flexibility in accommodating a much wider range of level 1 core damage accident sequences. These detailed models also make possible assessments that are integrated with the level 1 model, on the importance of different severe accident phenomena and containment performance characteristics. This paper describes the development and capabilities of these level 2/3 ASP models, and the linkage to the level 1 models.