Use of PRA in assessing BWR vessel level instrumentation

This paper discusses the regulatory application of probabilistic risk assessment (PRA) in assessing Generic Issue 50, Reactor Vessel Level Instrumentation in BWRs. Results of this study led to the voluntary implementation of water level measurement improvements in boiling water reactors (BWRs) and formed the basis for a separate Generic Issue 101, BWR Water Level Redundancy. Reactor vessel level instrumentation is used in BWRs to perform a number of safety-related functions such as feedwater control and automatic scram and autostart of emergency core cooling systems (ECCS). Following a break in an instrument reference leg, if there is an additional postulated single failure in another level logic train, the related level instrumentation would indicate a full-scale high level regardless of the actual water level in the reactor vessel. The analysis of transmitter failures is for the plant analyzed and may differ for other BWRs. It illustrates the basic failure mode of transmitters in BWRs.