Accident sequence precursor analysis: level 2/3 model development

The US Nuclear Regulatory Commission's Accident Sequence Precursor (ASP) pr ogram currently uses simple level 1 models to assess the conditional core d amage probability for operational events occurring in commercial nuclear po wer plants (NPP). Not all accident sequences leading to core damage will re sult in the same radiological consequences. Therefore, it is necessary to d evelop 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, est imate the magnitude of the resulting radioactive releases to the environmen t, and calculate the consequences associated with these releases. This leve l 2/3 model development work was initiated in 1995, and several prototype m odels 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 simpl e level 1 models to provide risk perspectives for operational events. Addit ionally, Very complex and detailed models have been developed that take pro vide much greater flexibility in accommodating a much wider range of level I core damage accident sequences. These detailed models also make possible assessments that are integrated with the level 1 model, on the importance o f different severe accident phenomena and containment performance character istics. This paper describes the development and capabilities of these leve l 2/3 ASP models, and the linkage to the level 1 models. (C) 1999 Elsevier Science S.A. All rights reserved.