Effects of control system failures on transients, accidents, and core-melt frequencies at a General Electric boiling water reactor

Pacific Northwest Laboratory (PNL) performed probabilistic risk analyses to estimate core-melt frequency and public risk associated with control system failures in a General Electric boiling water reactor. PNL also conducted value/impact analyses of proposed modifications of these control systems to prevent these failures. These analyses were based on failure modes and effects analyses previously performed by the Idaho National Engineering Laboratory (INEL). The control system failure modes identified by INEL and analyzed by PNL fall into three main scenarios: (1) failures that initiate feedwater overfill and also defeat the high level feedwater trip, (2) a failure of the condensate booster pump that results in increased flow to the vessel (overfill), and (3) an inadvertent actuation of the low pressure coolant injection system (LPCI) that also produces an excessive cooldown (overcool). For each of these modes, two failure sequences were postulated. The results of PNL's probabilistic analysis of failure progression to core damage and value/impact analyses of possible resolutions to prevent the occurrence of these failures are presented in this report. 16 refs., 28 tabs.