Degradation to some concrete walls at the Seabrook Station nuclear power plant has caused them to lose more than 20 percent of their strength, according to a recent inspection report from the Nuclear Regulatory Commission. Concrete surrounding an underground electric tunnel has been saturated with groundwater for more than a decade, according to the report. The NRC found no impact on electrical systems or other components of the plant, however.

The issue could nonetheless present problems for Seabrook Station as it seeks a 20-year extension to its operating license, which expires in 2030. The problem is believed to represent the first confirmed instance of such concrete degradation in a safety-related structure at a U.S. nuclear plant.

The Boston Globe reported the story on May 30, noting that cracks in the walls have allowed water to pool up to two inches deep in the tunnel’s interior, and plant owners NextEra Energy Resources have had “limited success” in halting the seepage problems.

NextEra spokesman Alan Griffith told the Globe that all structures at Seabrook Station exceed design requirements and said conditions remain safe at the plant.