SAMG-related Insights gained by Level 2 PSA Calculations

Leibstadt Nuclear Power Plant (KKL) is a BWR6/238 Mark 3 plant located in northern Switzerland. Current Emergency Operational Procedures (EOP) cover already numerous severe accident issues. In order to complete these procedures according to today's understanding of Severe Accident Management (SAM), KKL has initiated an internal effort to develop and use Beyond-Design-Basis Accident computer codes and models. The NRC MELCOR code has been preferred to MAAP for its nodalization flexibility and its state-of-the-art ability to integrally calculate entire accident progressions . After having performed numerous severe accident simulations for the KKL Level 2 Probabilistic Safety Assessment, the intended use of the Melcor deck is to develop accident scenarios for drills and support both technical staff training and Severe Accident Management. The actual KKL MELCOR environment is currently one of the most complete MELCOR applications worldwide. The KKL Level 2 PSA project is a good illustration of how computer codes may help plant engineers to better understand the physics and the progression of severe accidents and the plant specific response. They will be able to identify vulnerabilities under abnormal conditions, to quantify the consequences of operator actions by sensitivity calculations and to help the safety engineers establishing and validating SAM. The goal of a Level 2 PSA is not only to determine the containment response and source-term release, but, above all, to identify contingencies for improvement. The KKL development process towards these state-of-the-art SAM tool and method, as well as the main insights gained by the numerous investigations is presented. (authors)