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:

• Initiating Event 2022 Summary (including data tables)
• Initiating Events Spreadsheet 2022

Supporting Information:

• Reference Material 
• Initiating Event Coding Guidance 

Historical Results:

• Initiating Event 2021 Summary (including data tables)
• Initiating Events Spreadsheet 2021

• Initiating Event 2020 Summary (including data tables)
• Initiating Events Spreadsheet 2020

• Initiating Event 2019 Summary (including data tables)
• Initiating Events Spreadsheet 2019

• Initiating Event 2018 Summary (including data tables)
• Initiating Events Spreadsheet 2018

• Initiating Event 2017 Summary (including data tables)
• Initiating Events Spreadsheet 2017

• Initiating Event 2016 Summary (including data tables)
• Initiating Events Spreadsheet 2016

• Initiating Event 2015 Summary (including data tables)
• Initiating Events Spreadsheet 2015

• Initiating Event 2013 Summary
• Data Tables 2013

• Initiating Event 2012 Summary
• Data Tables 2012

• Initiating Event 2011 Summary
• Data Tables 2011

• Initiating Event 2010 Summary
• Data Tables 2010

• Initiating Event 2009 Summary
• Data Tables 2009

• Initiating Event 2008 Summary
• Data Tables 2008

• Initiating Event 2007 Summary
• Data Tables 2007

• Initiating Event 2006 Summary
• Data Tables 2006

• Initiating Event 2005 Summary
• Data Tables 2005

• Initiating Event 2004 Summary
• Data Tables 2004

• Initiating Event 2003 Summary
• Data Tables 2003

• Initiating Event 2002 Summary
• Data Tables 2002

• Initiating Events 1995 Executive Summary  (from original NUREG)
• "Rates of Initiating Events at U.S. Nuclear Power Plants: 1987-1995" (NUREG/CR-5750 )