Analysis of MSIV-ATWS events with the BNL plant analyzer

There are automatic safety features and operator-initiated emergency procedures which influence the sequence of events until the time when the standby liquid control system (SLCS), or other attempts to get control rods inserted, can effect shutdown of the core. One emergency procedure for a BWR/4 would require the operator to reduce the flow of high pressure coolant injection (HPCI) into the reactor. The core inlet flow rate at this time would be due to natural circulation and the reduced flow would lower the water level in the downcomer thereby reducing the natural circulation flow rate. This effect, and the reduction in core inlet subcooling due to mixing of the emergency feedwater with steam in the downcomer when the level was lowered, cause a sufficient increase in core void fraction so that the power would be reduced. A reduction in pressure might also be called for during this event in order to comply with the PSP heat capacity temperature limit (or possibly to prevent cycling of relief valves). In the past few years there have been several studies of this problem with the emphasis on calculating the power level in the core. In the present study we consider the power level as well as the resulting PSP temperature and take into account different assumptions regarding plant parameters and operator actions.