Extended level 1 PSA. Interface to the level 2 PSA for the German PWR 69

This paper presents a methodology developed at AREVA to extend the modeling of the Level 1 (L1) Probabilistic Safety Analysis (PSA) as part of an extended interface to the Level 2 (L2) PSA. This methodology has been applied to the L1 PSA of the Series 69 Boiling Water Reactors (BWR 69). Since for most of the BWR 69 many years have elapsed between the L1 and L2 PSA projects, a conditioning of the L1 PSA modeling is required to couple it to the L2 PSA. An extended L1 - L2 interface is developed to model the propagation of core damage sequences of the L1 PSA using realistic success criteria for the L2 PSA. The L1 PSA model is extended before the plant damage states are grouped. All core damage sequences to the extended L1 PSA lead to core melt in the L2 PSA, avoiding unrealistic risk calculations with respect to the leading contributors and the resulting source terms probabilities in the L2 PSA. The first step of this methodology involves the identification of sequences of the L1 PSA with modeling conservatisms in view of the L2 PSA and those which lead to very unfavorable scenarios in the L2 PSA (e.g., large early release). Detailed information of the plant and specific deterministic analyses for the identified sequences are required to extend the modeling. Further possibilities to control the accident are considered, e.g. operator actions for the reactor feeding, for the containment heat removal (e.g. venting). This extended procedure also allows the identification of core damage sequences of the L1 PSA for which no core melt takes place in the accident progression. These sequences are identified as controlled sequences in the extended interface and to not need to be further considered in the L2 PSA. The modeling extensions suggested for the BWR 69 focus on sequences with a containment isolation failure and on sequences with a failure of the reactor safety water injection. The formulations of the extensions are however generic and can be used a guideline to extend the PSA modeling for other types of BWR. Numerical results for the BWR 69 show that the frequencies of sequences which lead to core damage decrease considerably. (author)