Reactivity accidents: A reassessment of the design-basis events

Abstract

This paper summarizes a study of light water reactor event sequences which have been investigated for their potential to result in reactivity accidents with severe consequences.The study is an outgrowth of the concern which arose after the accident at Chernobyl and was recommended by the report of the U.S. Nuclear Regulatory Commission (NRC) on the implications of that accident (NUREG-1251). The work was done for the NRC to reconfirm or bring into question previous judgements on reactivity events which must be analyzed for licensing. Event sequences were defined and then a probabilistic assessment was completed to estimate the frequency of the reactivity event and/or a deterministic calculation was completed to estimate the consequences to the fuel. Using the results of this analysis, analysis done by others, and a set of screening criteria developed within this study, judgements were made for each sequence as to its importance, and recommendations were made as to whether the NRC ought to be considering the important sequences as part of the design basis or for further, more detailed, investigation. SummaryThe required safety analysis for light water reactors includes various reactivity events which must 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 these 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 analyze these latter events, and events newly identified in this study, to reconfirm or bring into question previous judgements on the adequacy of the events selected as the design-basis, and to determine if any events needed more analysis in the context of severe accident research. This was done by performing probabilistic and/or deterministic calculations and, where available, using the research of others.

A SCOPING STUDY: Development of Probabilistic Risk Assessment Models for Reactivity Insertion Accidents During Shutdown In U.S. Commercial Light Water Reactors

“…A review of PWR and BWR operating history (LERs) and the reactivity insertion accident analysis reports were reviewed to identify the positive reactivity insertion events 1,2,3 . No attempt was made to identify additional paths or processes which would lead to positive reactivity insertion on the basis of a plant's detailed system diagrams or procedures.…”