Improvements in the German Fire PSA Methodology Demonstrated at a German BWR Built to Earlier Standards

Fire PSA has to be performed as part of the Probabilistic Safety Analysis (PSA) for full power and low power and shutdown states within the mandatory comprehensive Safety Reviews for German nuclear power plants (NPP) at time intervals of ten years. The German guidance document on PSA methods requires a Fire PSA of several steps starting with alternatively a qualitative and quantitative or an advanced combined screening approach providing estimated values for damage frequencies. The existing combined approach has been enhanced by automating the screening process including a systematic calculation of fire propagation probabilities and standardized simplified fire simulations to roughly predict critical parameters. Further enhancements focus on fire induced cable failures and circuit faults and improvements in the uncertainty and sensitivity analysis. Thus the uncertainties in the results of fire PSA can be reduced as far as feasible and the predicted CDF values are more reliable. The results of a Fire PSA for a German NPP with boiling water reactor designed to earlier standards (BWR-69 type) with an overall fire induced core damage frequency of 1.9 E−06/a are outlined. This is in good agreement with results publicly available from Fire PSA for NPP in other countries. However, the CDF values are higher than those for some other German plants resulting from some pessimistic assumptions made.