SCREENING APPROACH FOR SYSTEMATICALLY CONSIDERING HAZARDS AND HAZARD COMBINATIONS IN PRA FOR A NUCLEAR POWER PLANT SITE

摘要

As a lesson learnt from the investigations after the reactor accidents of Fukushima, probabilistic risk assessment (PRA) has to systematically address external and internal hazards including potential combinations of hazards with other events and hazards. Several international activities carried out in the recent past, in particular in the frame of the ASAMPSAE project, have revealed lists with several groups of different hazards, for which it has been checked site and plant specifically, if these need to be considered in the PRA. Moreover, a huge variety of potential event combinations of hazards and other events should be screened as well. In principle, three types of event combinations involving hazards can be distinguished: Consequential or subsequent events: A (external or internal) hazard induces one or more additional hazards; Correlated events: A common initiating event (including external hazard) results in one or more hazards, which even may occur simultaneously with a certain probability; Unrelated events: An initiating event (including hazard) occurs independently from, but simultaneously to a hazard. A screening approach has been recently developed by GRS for systematically screening those hazards and hazard combinations, which have to be addressed in PRA on a site and plant specific basis. The approach starts with a generic list of the entire hazards by screening out those ones that can be qualitatively excluded for the nuclear site under investigation. For the remaining hazards, probabilistic screening criteria are applied. Screening of the hazard combinations starts from those hazards not screened out site specifically as physically impossible. For these hazards not screened out, all physically possible event combinations are identified. Thereafter, detailed hazard combination screening is performed applying again quantitative criteria. The paper presents the systematic screening approach in general as well as an application example for a reference nuclear power plant (NPP) site in Germany.