In-service wear measurement of B&WOG CRDM roller nuts

The Babcock & Wilcox Owners Group (B&WOG) has performed dimensional inspections on the critical wear-related internal components inside the control rod drive mechanism (CRDM) as a part of a life extension program (LIFEX) over the past 20 years. Under this program, CRDMs at Babcock & Wilcox designed pressurized water reactors (PWRs) were disassembled and inspected. The roller nut outer race tooth thickness is the only part found to be a potential wear concern for the functional life of the CRDM. The roller nut outer race is fabricated from Haynes Alloy 25, AMS 5759D. Some of the CRDMs have logged 84,470 ft (25.8 km) of travel after more than 20 years of service. Extrapolation of wear from the in-service inspections is in good agreement with the original accelerated life tests. Within the design travel limit, the wear data from the in-service inspections and the accelerated life tests are well fitted by a single logarithmic expression. However, based on the goodness of fit and observations of other wear tests, the roller nut wear is more conservatively modeled by a logarithmic function for the initial break-in period and a linear function beyond the break-in period.