6 PSA Level 1 — NPP Ringhals 2 (PSAM-0155)

A comprehensive PSA Level 1 and Level 2 has been performed for the NPP Ringhals 2 from 2001 to 2004. After this there has been some minor updates by RAB. In this paper the Level 1 work and results will be presented. The PSA considered the operational states Power Operation, Start-up, Hot Standby Hot Shutdown.

As far as possible, a complete set of initiating events was determined for the event groups LOCAs inside Containment, LOCAs outside Containment (V-LOCA), Transients, Transients initiated by Common Cause Initiators (CCI), Area events (e. g. Flooding), Fire events, Risk of Outside Core Events, External events.

For each initiating event (except fire) an event tree has been constructed describing in a logical manner the accident sequences depending on the success respectively failures of the safety functions required to cope with the event in order to avoid core damage (core melt).

Based on a database with the routing of cables (power, signals) from the cubicle to the respective component the cable routes were integrated in the PSA model thus taking into account all cable related fire dependencies.

For all PSA relevant systems a Failure Mode and Effect Analysis (FMEA) was performed.

Based on the FMEA system fault trees were constructed for each safety function in the event trees. The fault trees model explicitly independent component failures, unavailability due to maintenance/repair, dependencies on support systems, e. g. power supply, start signals and implicitly common cause failures (CCF) grouping components into a CCF group for which a parametric CCF-model is applied.

The loss of the spent fuel pit cooling was identified as the outside core event that has to be analysed.

The external event scenarios to be modelled were selected in a detailed external events analysis. The relevant scenarios involving single and multiple external events were included in the PSA model and quantified.

Plant damage states (PDS) are the link between level 1 and level 2. Level 1 event tree sequences were grouped into particular plant damage states for which accident progression is similar. The link is performed in the same code (RiskSpectrum PSA Professional).

The “small event trees — large fault trees”-methodology was applied.

RiskSpectrum PSA Professional was used to model the event trees and the fault trees and to perform the calculations. LOCA frequencies were derived from the Swedish SLAP-database. Frequencies of Transients were determined using the operating experience of R2. CCI-frequencies were calculated on the base of component failure and repair rates. Fire frequencies were assessed for each fire cell using the Berry method with fire frequencies for buildings according to the Swedish X-Book. External event frequencies were estimated on the basis of site specific statistical data, e. g. extreme weather conditions. The frequencies of Area events (Steam discharge, Flooding) are determined on basis of frequencies given in the PRA Procedures Guide. The initiating event of the outside core events was defined as loss of fuel pit cooling function of the train in operation.. As far as possible plant specific component failure data from T-Book 5 were used. Where those were not available other reliability data sources or engineering judgement were used. Unavailabilities due to maintenance and repair were supplied by Ringhals based on R2 operating experience.

For common cause failures the alpha factor model was used and the alpha factors mostly taken from INEL-94/0064 which contains alpha factors including uncertainties derived from US nuclear power plants operating experiences. Human reliability analyses were performed using the THERP (NUREG/CR 1278) methodology and ASEP, a simplified THERP methodology.

The core damage frequency and its uncertainty is presented. The contribution of systems to the core damage frequency has been determined and is presented.