Damaged Davis-Besse Reactor Could Have Lasted 13 Months
WASHINGTON, DC, May 5, 2004 (ENS) - The Davis-Besse nuclear power plant in Ohio that was closed in February 2002 when a hole was found in a reactor head could have operated safely for an additional two to 13 months, the latest analysis and testing conducted by the Nuclear Regulatory Commission (NRC) has found. The plant is now open again and generating power.
The NRC's findings are summarized in a memo from Ashok Thadani, executive director for operations in the agency’s Office of Nuclear Regulatory Research to the Executive Director for Operations William Travers. The memo was made public Tuesday due to "the level of interest," the Nuclear Regulatory Commission said.
Laboratory tests on materials similar to the Davis-Besse reactor vessel were used to verify an analytical method which was then applied to the degraded condition found at the Davis-Besse plant, Thadani wrote.
"The results show that the reactor would have likely continued to operate safely for several months, at least until the end of its originally planned operating cycle, if the plant had not shut down for inspections in February 2002," he wrote.
On February 16, 2002, the Davis-Besse Nuclear Power Station in Oak Harbor, Ohio, began a refueling outage that included inspecting the head of the reactor pressure vessel, the container that houses the reactor core and the control rods that regulate the power output of the reactor.
In conducting its inspections, personnel employed by the licensee FirstEnergy found that three nozzles entering the head had indications of axial cracking, which had resulted in leakage of the reactor's pressure boundary. In addition inspectors found a hole in the head that may have been formed by boric acid.
The cavity did not penetrate the head, but only 3/8 of an inch of steel cladding was left intact. The steel cladding covering the head was found to contain a complex network of stress corrosion cracks having a total extent on the surface of two inches.
"The results also show the reactor vessel’s stainless steel cladding would have likely withstood pressures at least 125 percent of what is encountered in normal operation," Thadani wrote.
To provide an independent perspective on the extensive experimental and analytical work that Thadani details in his memo, an external review panel was formed. It includes:
The review panel met with the staff and the Oak Ridge National Lab in early December 2003 and had several discussions with the NRC staff since then. Each reviewer submitted an independent letter to the staff, but all reviewers raised the same issues about tests of the cladding.
While the clad disk tests provide useful information on the failure characteristics of the steel cladding of the reactor head, they should not be taken to represent the conditions that existed at Davis-Besse, the panel said.
Estimates of the Davis-Besse structural integrity should be based on a finite element analysis that represents much more closely the geometric conditions that existed at Davis-Besse on February 16, 2002, combined with laboratory data on the strength, toughness, and failure characteristics of the stainless steel cladding.
A better characterization of the crack network that existed in the Davis-Besse cladding is needed to support a realistic assessment of the as-found condition. Evidence does not suggest that failure was imminent on February 16, 2002, the panel concluded.
The plant is currently operating in Mode 1 at 100 percent reactor power. The plant achieved full power on April 4, 2004. The Davis-Besse Oversight Panel will continue to monitor plant activities utilizing enhanced inspection oversight coverage.
The Davis-Besse Oversight Panel will conduct a public meeting with the plant operator FirstEnergy scheduled at 3:00 pm on May 13 at the Ottawa County Courthouse, Lower Level 315, Madison Street, Port Clinton, Ohio. The meeting is to discuss plant performance and the NRC’s activities related to the process at Davis-Besse.
A final engineering and analysis report will be issued when the NRC report on the preliminary results and findings of the Accident Sequence Precursor analyses in early summer.
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