FirstEnergy Nuclear Operating Company (FENOC), which operates the Davis-Besse nuclear power station in Oak Harbor, Ohio, has found that cracks in the station’s shield building were caused by a severe blizzard over thirty years ago.
The analysis confirms earlier findings that the structural integrity of the shield building remains intact, and that it is able to perform its safety function. However, FENOC has outlined a number of actions that will be taken to monitor the building and verify that the cracks have not spread.
The cracks in the Davis-Besse shield building were discovered last autumn, when the building was opened during an outage to replace the reactor vessel head. FENOC subsequently launched a detailed investigation to examine the potential causes of the cracks. On 28 February FENOC announced the completion of its root cause analysis report that had been completed with help from a team of structural concrete and causal analysis experts.
The absence of an exterior weatherproof coating on the shield building allowed moisture associated with a blizzard of January 1978 to migrate into the concrete, freeze and expand, causing tight, subsurface cracks in portions of the building, FENCO said in its 119-page root cause analysis report.
The report concludes that cracking occurred following the blizzard's combination of extreme weather conditions, which included three days of driving rain preceding a drastic temperature drop to around 0-degrees Fahrenheit and intense winds throughout the storm.
The report also outlines a number of actions that will be taken by FENOC, including applying a weatherproof coating to protect the shield building's exterior walls, performing additional inspections to verify the cracks have not spread and developing a long-term building monitoring plan.
The shield building is a 2 1/2-foot thick reinforced concrete structure that provides biological shielding, protection from natural phenomenon including wind and tornados and additional shielding in the event of an accident. This building surrounds a 1 1/2-inch carbon steel vessel containing the reactor.
FENOC has submitted the root cause analysis report to the Nuclear Regulatory Commission. NRC said that it has started a ‘rigorous review’ of FENOC’s analysis.
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