Nuclear power plant operators across the country should inspect steam generators more closely after finding evidence of developing cracks on the equipment at Seabrook Station in New Hampshire.
This nationwide advisory came out this month highlighting findings at Seabrook Station and another facility in Illinois, which use the same type of metal tube to carry hot water, said officials at the Nuclear Regulatory Commission (NRC).
During a routine inspection in 2012, Seabrook Station’s operator, NextEra Energy, found indications of cracking in two steam generator tubes. The indications ended up concentrated around dents or dings.
Officials removed both tubes were from service, and it does not pose any current safety threat. However, the findings illustrate potential weaknesses in the way officials conduct steam generator inspections, according to the NRC.
NRC spokesman Neil Sheehan said the findings at Seabrook were surprising because of the location of the developing cracks, and because inspectors could have missed the developing cracks if Seabrook Station had relied solely on a preliminary inspection.
The NRC described the situation in a July 3 memo, and asked power plant operators to consider implementing more rigorous inspection procedures for steam generators.
“We’re not requiring any action, but we are raising awareness about an issue that’s been identified at another plant or plants,” Sheehan said.
Steam generators are large components used to take water heated by a nuclear reactor and pass it through thousands of tubes. The water converts to steam and ends up piped into a turbine to generate electricity.
Seabrook Station has four steam generators, each of which has approximately 5,600 tubes, operating at temperatures of up to 621 degrees.
In the fall, NextEra inspected all of the steam generator tubes and found an indication of cracking about one-half inch long in one tube. The tube was not actually cracked, but a developing crack had penetrated about three-quarters of the way into the tube.
“The concern is that if left undetected, down the road, this could morph into an actual crack,” Sheehan said.
The finding prompted a closer inspection with a more sophisticated tool, which turned up two additional areas of concern on the same tube, about 6 inches away. Those areas would potentially have gone unnoticed under typical inspection procedures.
Signs of developing cracks were also on a second tube, in an unexpected place. Inspectors typically focus on the lower, hotter portions of steam generator tubes during inspections, since heat leads to stress corrosion. But one of the tubes at Seabrook Station had an indication of cracking near the uppermost support plate.
The finding underscores the fact that cracking ends up influenced not only by temperature, but also by factors like tube material and water chemistry, the NRC memo said. The NRC recommended using this knowledge to determine how frequently facilities should inspect the tubes.
In response to the NRC’s advisory, NextEra Energy spokesman Al Griffith said the steam generators at Seabrook Station continued to operate safely since they installed them in the 1980s. They monitor the steam generators at the power plant constantly, and steam generators undergo a rigorous safety inspection every 18 months.
“The situation described in the NRC’s Information Notice was identified by plant workers during the latest refueling outage, and involved only two of Seabrook’s 22,504 steam generator tubes,” Griffith said. “Both tubes were taken out of service, and there are no safety concerns with Seabrook’s steam generators.”
Seabrook Station is one of 17 facilities across the United States that utilize Alloy 600 tubing in steam generators.