Duke Energy Progress shut down the Shearon Harris nuclear plant in Wake County on Wednesday after the company discovered that the reactor vessel, which contains the plant's nuclear fuel, showed early indications of corrosion and cracking.
The Nuclear Regulatory Commission reported this morning that the discovery was made this week during a review of ultrasonic data. It showed a 1/4-inch flaw in the reactor vessel head, the name for the lid that is bolted on top of the vessel to maintain superheated water under high pressure.
The defect did not penetrate the vessel wall, which is about 6 inches thick, and there is no indication that radioactive water leaked out of the vessel.
"The reactor is shut down and our repair plans are in progress," said company spokeswoman Kim Crawford. "There is no impact to public health or safety as a result of this issue."
The repair will take place in a highly radioactive area and will require the use of robotics, said NRC spokesman Roger Hannah. He said repairs will involve scraping out corroded material and welding the area.
The nuclear plant, which has been generating electricity since 1987, is located less than 25 miles from downtown Raleigh.
Duke Energy Progress, formerly known as Progress Energy, is the Raleigh-based subsidiary of Charlotte-based Duke Energy.
Water stress corrosion is a common form of degradation in nuclear plants, said David Lochbaum of the Union of Concerned Scientists.
"The stress comes from expansion of metal as it heats up," Lochbaum said. "The reactor vessel has carbon steel with a stainless steel overlay. Different metal and curved shapes cause stress as it heats up."
"Impurities in the water can collect in tiny cracks formed in metal by stress," he added. "These impurities, called corrosion, exacerbate stress factors accelerating the propagation of tiny cracks."