The efficiency of sequential accident management measures for a German PWR under prolonged SBO conditions

Abstract

In this paper, the results of ATHLET-CD simulations of a Station Blackout (SBO) accident for a German Siemens KWU type pressurized water reactor are reported. The developed model is used in a series of calculations to evaluate SBO coping time provided by a set of countermeasures relevant to the defense-in-depth Level 4. The analysed accident management measures (AMMs) cover a sequence of the bleed and feed procedures, starting/ending with secondary/primary side depressurization followed by the feeding of steam generators (SGs) in the passive (AMM-1) or active (AMM-2) mode and coolant injection from hydro-accumulators (HA) to the primary system (AMM-3).A sequential implementation of the first two measures with almost equal efficiency (AMM-1 and AMM-2) delays the core degradation onset (CDO) by 21.5 h compared to the case without AMMs, extending SBO coping time to 24 h. This time window can be further extended (more than twice) through sequential feeding of a single SG from the four emergency feedwater tanks of the plant. The third measure (AMM-3) is significantly inferior to AMM-1 and/or AMM-2 in contributing to the coping time, since it delays CDO by<1 h.