MELCOR analysis of loss of residual heat removal during mid-loop operation in a Westinghouse two-loop PWR

The possibility of an accident or component failure during mid-loop operation has been identified in probabilistic studies as a major contributor to core melt frequency and source term risk during shutdown conditions. The wide range of plant states encountered and the unavailability of certain safety features make it difficult to guarantee that safety systems operation will always be sufficient to terminate the accident evolution. In this context analyses are performed using MELCOR 1.8.5 for loss of Residual Heat Removal (RHR) at various times during mid-loop operation of a Westinghouse two-loop PWR. In the absence of recovery of RHR or other Accident Management (AM) measures, the sequences necessarily lead to a long term core uncovering, heat up and degradation, loss of geometry and eventual failure of the Reactor Pressure Vessel (RPV). The results show an extensive time window before uncovering and additionally before core damage, which increase progressively with increasing time after shutdown at which loss of RHR occurs. Significant oxidation of the cladding may result in concentrations of enough hydrogen for deflagration. The slow evolution implies an opportunity for the plant operators to initiate AM measures even after core uncovering has started. The analyses indicate a substantial time window during the uncovering within which the injection can recover the core without damage. The upper end of the window is determined by the temperature at which heat from cladding oxidation becomes a dominant factor, marking a critical point for the effectiveness of this recovery mode. The results provide confidence in the inherent robustness of the plant with respect to accident sequences of this type. (authors)