Shutdown decay heat removal analysis of a General Electric BWR4/Mark I: Case study

A General Electric Boiling Water Reactor (BWR4) with a Mark I containment has been evaluated as part of Task Action Plan A-45, ''Decay Heat Removal Requirements.'' Probabilistic risk assessment models were constructed to determine the dominant internal, randomly initiated accident sequences and special emergency sequences (e.g., earthquakes). The dominant sequences were reviewed to determine what modifications might be made to enhance the plant's ability to remove decay heat. Modifications which held promise went through a preliminary cost and design analysis. Additionally, the impact on the probability of core melt accidents was estimated given implementation of modifications. In the final step, these results were combined in a value-impact format according to NRC guidelines. The results indicate that feasible modifications to enhance decay heat removal do exist at the subject plant. The value-impact estimates for several modifications showed the potential for cost effectiveness under current guidelines. Alternate assumptions involving source term magnitude and interdiction criteria were found to significantly affect the results. The insights gained from this study will become part of an information base which will be used to develop generic recommendations regarding the adequacy of decay heat removal systems in light water reactors.