Analysis of operating experience data in nuclear power plants

This report analyzes pressurized water reactor (PWR) and boiling water reactor (BWR) experience involving loss of decay heat removal (DHR) during reactor shutdown. Referring to USNRC's Licensee Event Reports (LERs), OECD/NEA-IRS reports, etc., we selected 206 loss of DHR events which have occurred in PWRs between 1976 and 1990 and 48 events in BWRs between 1985 and 1990. Analysis of 197 events which have occurred in U.S. PWRs indicates that the following direct causes are major contributions resulting in loss of DHR during reactor shutdown: 1) Inadvertent automatic closure of the suction/isolation valves in residual heat removal (RHR) system, 2) RHR pump cavitation due to air entrainment in reduced coolant inventory condition, and 3) Loss of power to RHR pumps. Human factors deficiencies involving procedural inadequacies and personnel errors were identified as the most significant underlying or root causes of the loss of DHR events. Most of the errors were committed during maintenance, testing and repair works. As for 48 events in U.S. BWRs, the leading category of loss DHR events was the inadvertent automatic closure of the suction/isolation valves in RHR system, most of which were caused by human errors. This report describes the above analysis results and presents the events description for the selected significant events. As well, the brief descriptions of the 206 events in PWRs and 48 events in BWRs are provided in Appendix. (author)