Risk-informed extension of the inservice inspection interval for pressurized water reactor vessel from 10 to 20 years

The Pressurized Water Reactor Owners Group (PWROG) methodology for extending the inservice inspection interval for reactor vessel welds in pressurized water reactors is described in technical report, WCAP-16168-NP, Revision 1. This report presents a risk-informed basis for extending the interval between inspections from the current interval of 10–20 years. This paper discusses the background of the PWROG inservice inspection interval extension methodology and the results of pilot plant studies on typical Westinghouse, Combustion Engineering, and Babcock and Wilcox plant designs. These pilot plant studies showed that the change in risk associated with the proposed inspection interval extension was within the guidelines specified in the United States Nuclear Regulatory Commission (NRC) Regulatory Guide 1.174 for an acceptably small change in risk. This paper also discusses the application of the extended interval to other pressurized water reactors and provides examples of the application.