MELCOR Simulation of the TMI-2 Severe Accident and Initial Recovery Phases

MELCOR has become the preferred code package within the Swiss nuclear community for severe accident analysis of nuclear power plants, on account of its integrated systems-level approach and validation against experiments and more detailed codes. The present work extends previous MELCOR analysis at PSI from when a site emergency was declared, 18000 s, through to 70000 s, a point where recovery actions were initiated that eventually proved sufficient to restore the reactor to a safe and stable state. It arises out of a programme to assess MELCOR independently using empirical data consistent with the recommendations of the OECD/CSNI validation matrix for core degradation codes. It is the first successful attempt to simulate the whole plant sequence through to the recovery phase. The calculations were performed with code version 1.8.5RD, starting with the model for phases 1 to 4 reported at ICAPP-05. This was extended with a representation of the fission product release and transport pathways, and of the containment, as well as for the extended time period analysed, the so-called phase 5. Reference was made to original sources to obtain the appropriate time-dependent boundary conditions. This paper compares the results of the calculations with observed and deduced data for major accident signatures such as primary system pressure, hot leg temperatures; liquid levels in the vessel and in the pressurizer, and fission product release. The results show that the code can give a credible account of the accident, when reasonable assumptions are made regarding the input where uncertainties exist. The analysis therefore supports the use of the MELCOR-based strategy for severe accident plant transient analysis in Switzerland. Finally, observations are made regarding recent improvements in the code, on which further assessment will concentrate. (authors)