Use for MELCOR in probabilistic safety assessments of level 2

The objective of the Probabilistic Safety Assessment Level 2 (PSA L2) is to determine the radiological source term risk of a nuclear power plant (NPP) based on core melt frequencies calculated in the PSA L1. As long as the containment is intact a core melt accident does not result in a large release of radioactivity to the environment. However, in the course of the accident different phenomena may result in a loss of the confinement. The probabilities of different failure modes of the confinement and containment respectively are determined by a Containment Event Tree (CET). The quantification of the branch probabilities and the determination of the fission product source term are supported by integral deterministic analyses. For PSA L2 for German NPPs AREVA NP GmbH uses MELCOR for these integral deterministic analyses. In addition, dedicated Monte Carlo programs have been developed by AREVA [Pau 2007] to derive probabilistic numbers from the deterministic results and to address physical phenomena, which cannot be adequately simulated by MELCOR. This paper presents the general approach how MELCOR is used within the frame of the PSA L2 for German NPPs. (orig.)