Duke Energy Progress shut down the Shearon Harris nuclear plant in Wake County on Wednesday after the company discovered that the reactor vessel – which holds the plant’s nuclear fuel and contains the nuclear reaction – showed early indications of corrosion and cracking.

The Nuclear Regulatory Commission reported Thursday morning that plant officials made the discovery earlier this week during a review of ultrasonic data that had been recorded in spring 2012.

The year-old data showed a one-quarter-inch flaw in the reactor vessel head, the term for the lid that is bolted on top of the vessel to maintain superheated water under high pressure.

It’s not clear why it took Progress a year to discover the corrosion, a question the NRC will attempt to answer in its review of the incident. General concerns about vessel head corrosion increased in the wake of the discovery in 2002 of a grapefruit-size cavity in the reactor vessel of the Davis-Besse Nuclear Power Station in Ohio, which is owned by FirstEnergy Corp.

The Davis-Besse incident, considered one of the most serious safety lapses in U.S. nuclear history, resulted in a two-year shutdown, more than $50 million in fines and penalties, and indictments against several utility employees and a contractor.

The Shearon Harris defect was caught early and did not penetrate the vessel wall, a protective barrier made of carbon steel and measuring about six inches thick. There is no indication that radioactive water leaked out of the vessel, the NRC said.

“The reactor is shut down, and our repair plans are in progress,” Progress spokeswoman Kim Crawford said. “There is no impact to public health or safety as a result of this issue.”

Progress offered no information on its repair schedule Thursday, but the work is likely to take at least several weeks. Additional information could be available Monday at a public meeting the NRC scheduled at the Holly Springs Cultural Center to present an overview of Shearon Harris’ safety record.

The public meeting was scheduled in advance to review the nuclear plant’s 2012 performance; however, NRC staff will be prepared to answer questions about the recently discovered vessel head corrosion and ensuing plant shutdown, NRC spokesman Roger Hannah said.

Because the vessel head repairs will take place in a highly radioactive area, the work will require using remotely operated robotics, Hannah said. He said repairs will involve scraping out corroded material and welding the area, not unlike filling a dental cavity, and will likely be performed by technical specialists brought in from an outside engineering company.

“I’m sure they’ll try to do the repairs as quickly as they can because as long as the plant is shut down they are not making electricity,” Hannah said.

The nuclear plant, which has been generating electricity since 1987, is 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. Duke acquired Progress in July 2012, and Duke officials have since been publicly critical of Progress’ nuclear plant performance during the run-up to the merger.

In February, Duke decided to shut down Progress’ Crystal River plant in Florida after concluding it would have been too expensive to repair. That facility was idled in 2009, never to restart again after the containment building wall was repeatedly cracked during a botched attempt to replace two steam generators.

The Shearon Harris vessel head corrosion was discovered in a welded section where a nozzle allows the insertion of a mechanism that’s used to control neutron-absorbing rods inside the reactor.

Such corrosion is a common form of degradation in nuclear plants, said David Lochbaum of the Union of Concerned Scientists.

“Impurities in the water can collect in tiny cracks formed in metal by stress,” said Lochbaum, a former NRC employee. “These impurities, called corrosion, exacerbate stress factors accelerating the propagation of tiny cracks.”

Murawski: 919-829-8932