Analysis of reactivity accidents with diluted ECCS water

In a pressurized water reactor, power excursions leading to core melt, or possibly rapid fuel damage, can be postulated to occur if there is a large reactivity insertion due to a boron dilution coupled with the inability of the reactor trip system to respond adequately. In the past, sequences of this type had been considered for detailed analysis and then rejected because it was assumed that the required combinations of causative events were too improbable. The objective of the present study, performed for the US Nuclear Regulatory Commission, was to analyze these events and events newly identified, to either reconfirm, or bring into question, previous judgments on the adequacy of events selected for analysis. This was, in part, motivated by the accident at Chernobyl. Although the analysis could be done either probabilistically or deterministically, for the reactivity accidents of concern in this paper, a probabilistic analysis was performed. The source of diluted water is the emergency core cooling system, i.e., either the accumulators or the refueling water storage tank (RWST). Three categories of events were considered. In the first, it is the RWST water that enters the vessel during shutdown when the reactor has been depressurized. In the second, itmore » is the accumulator water during refueling. The third category includes sequences initiated by a loss-of-coolant accident (LOCA).« less