A typical scenario, Station-Black-Out, which makes up an important part of contribution to core degradation frequency of three loop PWR Lingao phase II according to the PSA level 1 studies, was selected in this paper for Severe Accident Analysis with the application of combination of MELCOR and COCOSYS. Of the scenario, concurrent with postulated power recovery before degradation proceeded to failure of lower head, the evolution of in_vessel phase was studied; a detailed phenomena covering heat-up and dry-out of primary system, collapse and melt down of fuel assemblies, thermal hydraulic response inside the containment compartments, release and accumulation of H2 and fission product (FP) source terms, their leakage into environment were simulated and investigated. The effectiveness of preventive and mitigative measures, such as dedicated core depressurization system (DCDS), passive autocatalytic recombiner (PAR) system, are somehow validated through the calculation; the results also indicated that with the power recovered at 6 hour to start up ECCS, the corium could be retained above support plate.

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