In the framework of a level 2 PSA for the German boiling water reactor type SWR 69, a probabilistic evaluation of low power and shutdown (LP&SD) states was performed by GRS on behalf of environmental ministry (BMU/BfS). This paper presents preliminary insights and results of the PSA for the low power and shutdown modes of this plant type. Dominant sequences are described in more detail.
A detailed evaluation of the German operating experience was performed, to find events, which can lead to initiating events or, which can influence the control of accidents during shutdown and outage. Precursor events, e. g. for tube rupture at the RPV due to heavy load drops, leaks at the reactor pressure vessel (RPV) bottom and the absence of control rods during core loading with fuel elements were found in the German operating experience.
Significant for this plant type is the risk contribution due to leaks inside the containment, because of the special containment design. During the outage, the containment bottom is open. Therefore the sump function is not available in case of leaks and the coolant can flood the rooms where the residual heat removal system is installed. The leakage can be returned to the reactor cavity by a one train system located in the sump of the reactor building.
A finding from thermo-hydraulic analyses was, that core damage can not occur as long as the operational systems for control rod drive flushing and pump seal water are in operation. The small operational injection rate of these systems is sufficient to compensate the vaporized coolant.
The calculated preliminary overall core damage probability (CDP) per outage for the considered BWR plant type in low power and shutdown modes is in the range of 5·10−6 and about 5 times higher than the CDP calculated for full power operation. The results show the importance of event sequences during low power and shutdown operation. The safety of this plant type can be improved by plant modifications. Such modifications can be improvements of the procedures for accident control or provision of more system redundancy during outage.
Highest contributors to the calculated core damage frequency yield the initiating events “loss of preferred power” and “loss of residual heat removal”. Each of these initiating events yields a core damage probability in the range of 10−6.