Application of PSA results for prioritisation of cable layout reinforcement from seismic point of view

Anti seismic strengthening of safety and safety related systems cable layouts is one of the modernization program measures of Kozloduy NPP units 3 and 4. Considering the number of rooms these cables penetrate, this task becomes very difficult and expensive. Obviously not all layouts have the same contribution to the safety and many cable layouts strengthening may not contribute to risk reduction. The purpose of the work performed is to rank plant cable rooms, according to their risk contribution and to select only bigger contributors for strengthening. For detailed analysis of each particular cable room against negative seismic impact, seismic PSA results were used. Minimal cut sets and importance measures were analysed. As a result, core damage frequency were calculated for every cable room, assuming additional components failures due to seismic impact in cable rooms. These calculations were based on site seismic hazard characteristics, seismic capacity of the cable tray supports, conditional probability of component cable failure and conditional probability of component failure. The calculations were performed for six different seismic impact categories. Input data sensitivity analysis was performed to assess input data uncertainties to the results obtained. Value-impact analysis was performed, to support the decisions. It was concluded that only four rooms out of 38 contribute to risk meaningfully. As result, 34 rooms were discarded from anti-seismic reinforcement list. Four rooms have middle priority for seismic reinforcement. Cable tray reinforcement in these rooms would lead to seismic risk reduction by 7,46E-07 events/year. This is about 20% of the total seismic risk. (author)