Extreme weather and the absence of exterior sealant allowed cracks to develop in a shield building at the Davis-Besse nuclear plant in Ohio, according to an analysis FirstEnergy Corp. released today.

The Akron-based utility issued an investigative report that explores how the cracks developed in parts of the plant's shield building. The 900-megawatt plant is located in Oak Harbor, about 20 miles southeast of Toledo.

The analysis found that the portion of the building facing the southwest was exposed to wild temperature swings and driving rain following a blizzard in 1978. The lack of a weatherproof coating on the structure allowed moisture to migrate into the concrete, where it froze and expanded and caused cracks to develop.

FirstEnergy sent the report to the Nuclear Regulatory Commission along with correction action measures, and the agency said it is planning a public hearing on the matter.

The investigation was launched last year after plant workers discovered the cracks while replacing a corroding lid or "head" on the reactor. The plant was shut down at the time.

When workers cut a hole through the plant's 2-foot-thick concrete shield building to begin replacing the lid, they found a 30-foot crack in the shield building. The concrete structure is surrounded by a steel containment vessel that protects the reactor from natural disasters and terrorist attacks and serves as a secondary containment system. Different types of cracks in the structure were discovered through later investigations (Greenwire, Dec. 5, 2011).

In December, NRC allowed the plant to start operating again despite lingering concerns over the cracks and said the company had provided "reasonable assurance" that the facility was safe.

FirstEnergy said a team of structural engineers and other experts assisted plant personnel in examining the root cause of the cracks. Investigators found the 1978 blizzard was the only event capable of degrading the concrete and that the structure is "sound and in good condition."

FirstEnergy has agreed to apply an exterior protective sealant to the shield building, conduct additional inspections to ensure the cracks have not spread and develop a long-term monitoring plan.

The Davis-Besse plant has been shut down in past years for other repairs, including between 2002 and 2004 for an acid leak that later cost the company $5.45 million in fines and $28 million in civil penalties (Greenwire, Dec. 8, 2011).