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

In this paper, the results of ATHLET-CD simulations of an SBO accident for a German Siemens KWU type PWR 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 cover a sequence of the bleed and feed procedures, starting/ending with secondary/primary side depressurization followed by the feeding of 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 hours compared to the case without AMMs, extending SBO coping time to 24 hours. 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 less than 1 hour.