Concrete surrounding an electric tunnel at New Hampshire’s Seabrook Station nuclear power plant has lost almost 22 percent of its strength because ground water saturated it for more than a decade, according to a Nuclear Regulatory Commission (NRC) inspection.

What does that mean for the safety of the plant? The NRC said it found no impact on electric systems, piping, or any other components in the underground structure and the concrete walls are performing well above design specifications. The report, released May 23, concluded that the plant remains safe.

The problem, reported to the NRC by Seabrook owners NextEra Energy Resources last year, is the first confirmed instance of this type of degradation in a safety-related concrete structure at a U.S. nuclear plant.

Random cracks allow water to sometimes pool up to 2 inches deep in the tunnel interior, NextEra found, and the NRC said the company has had “limited success’’ in halting ground water seepage problems around the tunnel and elsewhere under the plant.

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If they can’t get Seabrook’s concrete degradation under control, it could be a problem for the plant’s ongoing efforts to extend its operating life another 20 years once its license expires in 2030. The degradation was in an NRC relicensing inspection report.

A NextEra spokesman said the company has not found concrete degradation anywhere else under the plant but an investigation is ongoing. The company also is revalidating the test results around the tunnel and is aggressively working to resolve the ground water problem.

The NRC said it is continuing to investigate the problem.

Nuclear plants have a myriad of tunnels and buildings underground, and many of those tunnels have an exterior layer of concrete that must protect it from water. Plants, like Seabrook, often have waterproof membranes to protect the concrete’s integrity and extensive efforts to drain ground water.

The tunnel and other areas where they found water seepage are part of a safety system used to help cool the reactor when they shut it down. In addition to the concrete tunnel problem, NextEra found corroded steel supports, piping, and anchor bolts in other areas they inspected but none have degraded concrete or in any way endanger the plant, said NextEra.

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