FirstEnergy Nuclear Operating Co.’s (FENOC’s) conclusion that cracks in the shield building at the Davis-Besse Nuclear Power Station in Oak Harbor, Ohio, were caused by environmental factors resulting from a blizzard in 1978 and aspects of the shield building’s design is sound, the Nuclear Regulatory Commission (NRC) said on Thursday.
The root cause analysis report that FENOC submitted to the federal body in February “established a sufficient basis for its conclusions,” the NRC said. In the 119-page report, the subsidiary of Akron, Ohio–based FirstEnergy says that “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 0F and intense winds throughout the storm.
FENOC submitted the root cause analysis and proposed corrective actions in compliance with commitments made to the NRC. Corrective actions include additional tests and laboratory analysis of the shield building; applying a protective moisture coating on the building; and plans to restore the plant’s design and licensing basis.
The NRC “evaluated the effectiveness of the company’s proposed actions to ensure the continued safety of the shield building going forward,” said NRC Region III Administrator Charles Casto. If implemented properly, actions proposed by the company would “prevent recurrence of the laminar cracking,” the NRC concluded last week.
Davis-Besse is a 913-MW Babcock & Wilcox pressurized water reactor that began commercial operation July 31, 1978 (the blizzard occurred in January that year). 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 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.
Sources: POWERnews, NRC, FENOC
—Sonal Patel, POWER senior writer (@POWERmagazine)