Reactivity accidents: A reassessment of the design-basis events

The required safety analysis for light water reactors includes various reactivity events which must be shown to result in acceptable consequences in order for plants to be licensed. Although it is possible to hypothesize reactivity events that might lead to unacceptable consequences, in the past such unanalyzed events were either precluded by specific design features or required combinations of causative events which were judged too improbable to be of concern. The objective of this study was to develop as complete a list as possible of hypothetical reactivity events and then to screen these events in light of current knowledge to determine if any might require detailed analysis. The goal of the study was to either reconfirm or bring into question previous judgments on the adequacy of the reactivity events required for analysis. The screening was done by performing probabilistic and/or deterministic calculations and, where available using the research of others. The focus of the study was influenced by the accident at Chernobyl. That event provided the incentive for this study to concentrate on accidents with relatively large reactivity insertions that could cause rapid fuel damage. In order to focus the study even more, the pressurized water reactor (PWR) analysis concentrated on a Westinghouse plant of four-loop design and the boiling water reactor (BWR) analysis on a BWR/4 design. However, reference is made to other designs throughout the report as the goal was to reach general conclusions that would be applicable to all PWRs or BWRs