United States Nuclear Regulatory Commission - Protecting People and the Environment

Baffle-Former Bolt Degradation

Sponsored by the Office of Nuclear Reactor Regulation

In spring 2016, two U.S. nuclear power plants identified a large number of Type 347 stainless steel baffle former bolts with indications of degradation during the performance of ultrasonic inspections following the guidelines in MRP-227-A, “Materials Reliability Program: Pressurized Water Reactor Internals Inspection and Evaluation Guidelines.” This poster will show the degradation of the baffle former bolts is attributed to irradiation assisted stress corrosion cracking. At this time, significant degradation of the baffle former bolts has only been observed in Westinghouse four loop pressurized water reactors with a “downflow” configuration and bolts made of Type 347 stainless steel. Seven U.S. reactors match this configuration. Both units have replaced all the potentially degraded bolts with Type 316 stainless steel bolts in an improved design. The NRC performed a risk informed evaluation of the issue and concluded that it did not warrant the immediate shutdown and inspection of any facilities.

Page Last Reviewed/Updated Friday, June 09, 2017