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A green, or “very low safety significance” level, violation ended up issued to the NextEra Energy’s Seabrook nuclear power plant after a March 24 inspection, said officials at the Nuclear Regulatory Commission (NRC).

The issue at the Seabrook, NH-based facility does not indicate the plant is unsafe, according to the letter sent by the NRC.

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However, it refers to plant staff not initiating quickly enough the proper procedure to verify the capability of certain structures after getting new information that relates to alkali-silica reaction, a problem affecting the plant’s concrete.

“The violation describes two examples where initial and prompt operability determinations were not compelled by NextEra staff when additional information regarding the effects of ASR on safety-related concrete structures was identified,” wrote Mel Gray, the chief of the NRC’s engineering branch of the Division of Reactor Safety.

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Although operability determinations have since developed, Gray said the violation was similar to other very low safety significance violations identified by NRC inspectors in the past two years. Those previous violations ended up handled as non-cited violations at the time, but not this time.

The NRC does extensive inspections of the country’s nuclear power plants, averaging about 6,000 hours a year, according to NRC Region 1 spokesman Neil Sheehan. Those inspections include those done daily by the two NRC on-site inspectors stationed at every nuclear power plant, as well as more formal inspections.

NextEra Energy Seabrook’s green violation is of very low safety significance because the structures remained capable of performing their intended safety functions, Sheehan said.

“The NRC would normally issue a ‘non-cited violation’ for problems involving very low safety significance,” Sheehan said. “However, considering this issue is similar to findings issued by the NRC in the past two years at Seabrook, the NRC is issuing a ‘Notice of Violation’ in this case.”

Seabrook nuclear plant facing a green violation from the NRC.

Seabrook nuclear plant facing a green violation from the NRC.

By doing so, NextEra must respond in writing to the NRC as to the causes of the violation and how any corrective actions will be appropriately broad to prevent recurrence, Sheehan said. When the response is received, the NRC will perform additional inspections at the plant to verify the adequacy of NextEra’s actions, he added.

NextEra Energy Seabrook spokesman Al Griffith said the NRC team concluded Seabrook Station is safe based on its latest detailed inspection. The violation is of “very low safety significance,” he said, and staff is taking appropriate corrective action.

“To be very clear, this is not a safety issue,” Griffith said. “While we believe our plan to monitor and manage alkali-silica reaction for the long term is effective and comprehensive, we are committed to continuous improvement, and we will continue our efforts to ensure that the actions we are taking on ASR are appropriate and timely.”

A slow chemical reaction occurs between the alkaline cement and reactive silica found in some concrete aggregates when moisture is present. ASR forms a gel that expands, causing micro-cracks that can affect concrete properties. Commonly found in dams and bridges, ASR usually takes years, even decades, to make itself known.

Although nuclear power plants in Canada and Europe have experienced ASR, so far Seabrook Station is the only U.S. plant that has exhibited the phenomenon.

The NRC constantly monitors Seabrook Station and has assured the public that it remains structurally sound and operational, and that ASR has not affected the safety of the nuclear power plant. The plant’s walls — some of which are 4 feet thick, most at least 2 feet thick — still meet federal standards for load-bearing capacity due to the lattice of steel rebar reinforcing them.

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