engleski

Upgrade of Krško Level 2 PSA Model for Regulatory Activities

In 2007, Slovenian Nuclear Safety Administration initiated the project on applications of Krško PSA model in the regulatory activities. As a part of the project, the Level 2 PSA model was to be upgraded in a way that Level 2 risk measures can be computed more directly, for the purpose of regulatory applications. In Krško PSA, the quantification of Level 2 consists of two distinctive stages: 1) propagation of Level 1 core damage sequences to plant damage states (PDS) and 2) mapping of PDS to Level 2 release categories. The first stage is performed by means of interfacing event trees or, so called, bridge trees, which are part of the RiskSpectrum PSA model. Each bridge tree is a model of response of containment systems to particular Level 1 core damage sequence or group of sequences. This part is also referred to as Level 1 – Level 2 interface. The end states of bridge tree sequences are PDSs. The second stage is performed by means of an accident progression event tree or a containment event tree (CET). The nodes of CET represent various relevant containment phenomena. Input into a CET is a PDS or a group of PDSs. The end states of CET are release categories. Therefore, by means of CET, each PDS is mapped into a set of release categories. The initial Krško Level 2 PSA study was performed in the 1993-1995 time frame, as a part of the Individual Plant Examination (IPE). One specific point of the IPE study was that only the top 40 Level 1 core damage sequences were propagated to the bridge trees and Level 2, out of the total of 184 Level 1 core damage sequences. At the time of the IPE study, these particular 40 sequences contributed, cumulatively, more than 90% to the total core damage frequency. This approach can be considered acceptable for the long term averaged base case results, but may be questionable for the applications such as PSA-based event analysis where Level 2 results may need to be re-computed conditionally on the specific (e.g. failed) status of some containment systems. Therefore, the model had to be upgraded in a way that all Level 1 core damage sequences are propagated to the Level 2. This involved Level 1 sequence grouping and upgrade to the bridge trees, which resulted in a number of PDSs which were not considered in the IPE study, since they were not involved in the top 40 core damage sequences. For these new PDSs, mapping to release categories had to be performed. The process resulted with a model which can be used for repeated computing of the Level 2 results with taking into account all Level 1 core damage sequences. The paper describes the model upgrade and the results of Level 2 re-quantification.

PSA Level 2; Regulatory Activities

nije evidentirano