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FirstEnergy Corp. reported it found a small hole in the steel liner of its containment building surrounding the Unit 1 nuclear reactor at its Beaver Valley Power Station, the company told regulators.

The Akron, Ohio-based company was performing a routine inspection during a scheduled outage to refuel the plant when they found a paint blister about seven inches off the ground, indicating a potential problem with the liner.

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FirstEnergy reported the defect to the Nuclear Regulatory Commission (NRC) on Friday, but at the time the company didn’t know what was behind the corrosion. By Monday morning, officials suspected a piece of wood embedded in the concrete was the culprit.

“A piece of fibrous material, likely wood, decayed and created moisture,” said Jennifer Young, a spokeswoman for FirstEnergy.

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The wood may have come from the construction of the building four decades ago.

The Unit 1 pressurized water reactor sits inside a 4.5-foot-thick wall of concrete, lined with a .38-inch-thick sheet of steel. The steel liner would serve as a barrier for radiation in the event of an accident.

This is the second time a hole in the steel liner was discovered during an outage inspection at Beaver Valley’s Unit 1, and the third time corrosion was on the outside of the liner.

In 2009, FirstEnergy found paint blisters on the inside of the containment wall and uncovered a rectangular, 2.5 inches by 1 inch hole in the steel. Behind it was a piece of wood, 2 inches by 4 inches by 6 inches, wedged into the concrete.

In 2006, when the company cut a hole in the concrete wall to install a new steam generator and reactor head, it found three areas of corrosion (not a hole) in the steel lining exposed by the removed concrete.

The cause was never determined conclusively, but a 2011 report by the NRC examining steel lining problems across the industry, chronicled the incident and said “several pieces of wood were found during a subsequent inspection of the concrete debris pile.”

The report also noted officials discovered “peeled coating and spots of liner corrosion” at Beaver Valley’s Unit 1 in June 1992.

“If there’s any plant that has a high awareness of the issue of corrosion, it’s Beaver Valley, because they’ve been through this before,” said Neil Sheehan, a spokesman for the NRC.

FirstEnergy will likely deal with this hole, which measured at .4 inches by .28 inches, the same way it handled the last one — clean the area, grout the concrete, weld a new patch of steel to the liner and test its integrity.

Young said the inspection process “did exactly what it was supposed to do.” It identified a defect, not visible 18 months ago, before it developed into a problem.

“Public health and safety isn’t impacted,” she said.

Of the nation’s 104 nuclear reactors, 55 pressurized water reactors and 11 boiling water reactors have a similar containment system to Beaver Valley — a concrete wall in contact with a steel liner.

In a 2011 report on containment liner corrosion, the NRC wrote that such defects have been “observed at many plants,” and construction activities were responsible for a good chunk of that.

The agency’s scientists found in eight cases around the world, liner corrosion was the result of foreign objects such as wood planks and a glove embedded in the concrete.

“All eight known cases of embedded foreign material in the concrete occurred at plants built with only four companies as the primary construction contractor,” the NRC’s review said.

Stone & Webster, the contractor that built the structure for the Westinghouse pressurized water reactor at Beaver Valley’s Unit 1, built seven other plants with steel liners, the report said. A quarter of those had foreign objects whose decay corroded the steel.

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