United States Nuclear Regulatory Commission - Protecting People and the Environment

Initiating Events

Data for all unexpected reactor trips during power operations at commercial nuclear power plants were reviewed. Each event was reviewed and categorized according to the initial event and, additionally, was marked if certain other risk-significant events occurred, regardless of their position in the event sequence. The collected data were analyzed for time dependence, reactor-type dependence, and between-plant variance. Dependencies and trends are reported, along with the raw counts and the best estimate for initiating event frequencies.

The initial study is documented in "Rates of Initiating Events at U.S. Nuclear Power Plants: 1987-1995" (NUREG/CR-5750 ). The links below provide the current information for the study and latest results.

Additional information on fire ignition frequency is provided by "Nuclear Power Plant Fire Ignition Frequency and Non-Suppression Probability Estimation Using the Updated Fire Events Database" (NUREG-2169 ).

Current Results:

Supporting Information:

Historical Results:

Page Last Reviewed/Updated Thursday, June 15, 2017