The shield building of FirstEnergy Nuclear Operating Co.’s (FENOC’s) Davis-Besse Nuclear Power Station in Oak Harbor, Ohio, lacked an exterior weatherproof coating, and this allowed moisture from the blizzard of January 1978 to migrate into the concrete and cause the hairline wall and subsurface cracks discovered during a reactor head replacement outage at the facility last fall, a root cause analysis report indicates.
The 119-page report was submitted to the Nuclear Regulatory Commission (NRC) on Tuesday. It “supports earlier analysis that concludes the structural integrity of the Shield Building remains intact, and the building is able to perform its safety function,” FENOC said in a statement.
The subsidiary of Akron, Ohio–based FirstEnergy says the report contains details of how the cracking could have occurred. “Absence of an exterior weatherproof coating on the Shield Building allowed moisture associated with the blizzard of January 1978 to migrate into the concrete, freeze and expand, causing tight, subsurface cracks in portions of the building. The root cause report concludes that the 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,” it said in a statement on Tuesday.
FENOC said it plans to apply a weatherproof coating to protect the shield building’s exterior walls, perform additional inspections to verify the cracks have not spread, and develop a long-term building-monitoring plan.
“In the course of the root cause investigation, an exhaustive list of potential cracking causes was examined, including possible design, construction, environmental and operational issues,” it said. “Thorough testing and analysis of numerous concrete samples conducted in support of the root cause demonstrated that the concrete in the Shield Building is sound and in good condition.”
Davis-Besse is a 913-MW Babcock & Wilcox pressurized water reactor that began commercial operation in July 31, 1978. The reactor has been besieged by a series of problems. In March 2002, maintenance workers found that corrosion had worn a football-sized hole into the reactor vessel head, prompting the NRC to keep the reactor shut down until March 2004. The NRC fined FENOC more than $5 million—its largest fine ever—for the actions that led to the corrosion.
In March 2010, during a scheduled refueling outage, an ultrasonic examination of the control rod drive mechanism nozzles penetrating the reactor vessel closure head showed that two of the nozzles did not meet standards. Later, investigators found new cracks in 24 of 69 nozzles. The plant resumed operation later that year.
FENOC told the NRC on Oct. 10, 2011, that its workers identified cracks in the shield building while the plant was shut down to replace the reactor vessel head. The shield building is a 2.5-foot thick reinforced concrete building that surrounds a 1.5-inch thick steel containment vessel that encloses the reactor. The two buildings are separated by a 4.5-foot space.
On Tuesday, the NRC said it had initiated a “rigorous review” of the causal analysis and would schedule a public meeting and issue an inspection report once the review was complete. Four NRC inspectors have reportedly been monitoring FENOC’s investigation of the root cause since it started.
“The NRC thoroughly reviewed the cracks after they were discovered and determined that they did not pose an imminent safety issue,” the federal regulatory agency said. “However, before the plant returned to service, the NRC issued a Confirmatory Action Letter to FENOC documenting its commitments to determine the cause of the shield building cracks by Feb. 28 and to implement long-term measures to ensure that the shield building continues to fulfill its safety function.”
Sources: POWERnews, FENOC, NRC