Historical Perspective on Significance Relative to Safety Analysis and Actual Events

摘要

Since the early days of safety analysis there has been an overall goal to protect against accidents with significant risk. However, there has been considerable discussion and fluctuation in how to define 'significant.' This paper presents past and current attempts to define 'significant.' These include various siting evaluation guidelines. 'Small fraction' and 'well within' for futher division of the guidelines. These definitions were expaned to include identification of USQ, proper control functional classification and when a preliminary safety basis is required to authorize construction. Such schemes started with quantitative guidelines which have changed over time to today's climate of qualitative definitions. The differences, advantages and disadvantages are discussed and safety analysis expectations on what is significant are compared to a selection of actual events from the U.S Department of Energy (DOE), the Nuclear Regulatory Commission (NRC) and the chemical industry. The paper concludes with an assessment of causal factors common to those actual events that are not generally considered in typical accident sequence identification and, thus, are not usually considered 'significant' in the pre-event time frame. These common causal factors are still valid today and are important and recurring contributors to actual events.