Nuclear Reactor Baffle Bolt Problems Are Widespread Concern
Public Service Enterprise Group (PSEG) anticipates extending Salem Nuclear Generating Station’s Unit 1 refueling outage, which began on April 14, so it can inspect, repair, and replace damaged baffle bolts within the plant’s reactor vessel, according to information presented in the company’s first quarter earnings announcement.
A PSEG spokesperson told POWER that visual inspections at Salem had identified 18 of the metal insert liner’s 832 baffle-former bolts exhibit degradation, which means they had at least some indication of cracking. The news comes roughly a month after inspections at the Indian Point nuclear plant determined that 227 of its Unit 2 baffle-former bolts were degraded.
“As part of our license renewal commitments to the Nuclear Regulatory Commission [NRC], Salem Unit 1 was scheduled to conduct ultrasonic testing of the baffle bolts in 2019. Based on the visual inspections, we made the decision to conduct ultrasonic testing of the bolts now to determine the full extent of condition and to make appropriate repairs,” the PSEG spokesperson said.
Problems in Europe
It’s the first time that Salem has identified any problems with baffle bolts on either unit, but it is far from the first time for the industry. As far back as the 1980s, cracking was identified in French pressurized water reactor (PWR) baffle bolts.
In March 1998, the NRC issued an Information Notice to alert U.S. PWR license holders of the cracking found in reactor vessel internal baffle-former bolts at “several foreign PWRs.” The intent of the notice was to inform recipients of the problem so they could consider actions to avoid similar troubles. The suggestions in the notice were not necessarily requirements, however.
Are There More Potential Problems?
Neil Sheehan, public affairs officer for NRC Region I, told POWER that all U.S. PWRs—of which, there are currently 65 licensed to operate—utilize baffle plates as part of their reactor core internals. The baffle plates help direct water up through the nuclear fuel assemblies.
The design typically includes vertical plates bolted to the edges of horizontal former plates that are bolted to the inside surface of the core barrel. In some cases, there are eight levels of former plates located at various elevations within the core barrel.
The bolts that secure the baffle plates to the former plates are referred to as baffle-former bolts. In the case of the Indian Point and Salem reactors, which are both Westinghouse four-loop PWRs, the baffle-former bolts measure about two inches in length and have a 5/8-inch diameter. Irradiation-assisted stress corrosion cracking and fatigue in the 316-type cold-worked stainless steel bolts causes the cracking.
Corrective Actions
In January 2009, the Electric Power Research Institute submitted to the NRC a standard industry program for the aging management of PWR reactor vessel internals that it had developed. The NRC staff approved the approach in a safety evaluation issued in December 2011.
The program directs U.S. PWR owners to conduct ultrasonic examinations of all baffle-former bolts when a plant has been in service for between 25 and 35 effective full power years, with subsequent evaluations conducted on a 10-year interval. The initial examination is what led Indian Point Unit 2 to find the problems in its reactor.
“At Indian Point Unit 2, Entergy has begun replacing the baffle-former bolts. That work is continuing. At Salem Unit 1, ultrasonic testing is still under way to determine how many baffle-former bolts may be degraded. In both cases, the companies will submit analyses of the conditions, any replacement plans and the implications for the sister unit at the site,” Sheehan said.
The NRC is still weighing the significance of the recent inspection findings. It expects analysis performed by both Entergy and PSEG will help in its assessment.
—Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)