Federal regulators began a special inspection Wednesday of the Shearon Harris nuclear plant in Wake County to determine why a defect in the reactor vessel lid went undetected for more than a year.
The Nuclear Regulatory Commission flew in a pair of metallurgical experts from the agency’s Atlanta office to assist the two on-site resident inspectors assigned to the plant, which is located less than 25 miles from downtown Raleigh.
The inspection is expected to last through the end of the week, but regulators are prepared to continue next week. The NRC will issue a report and conclusions 45 days after it completes the inspection.
The flaw itself does not cause a risk to public safety, but it is a violation of NRC performance standards.
“For reasons yet to be announced the flaw was not caught as it should have been,” said Terrence Reis, the NRC’s division director for reactor safety in Atlanta. “We will stay there until we get the information we need to make sure the operator addresses the situation.”
Duke Energy Progress shut down the plant May 15 after discovering a one-quarter-inch flaw in a section of the massive lid that covers the nuclear vessel. The vessel holds the reactor and contains the nuclear reaction, where water is superheated to extreme pressures.
Duke Energy Progress is the Raleigh-based subsidiary of Charlotte-based Duke Energy. Duke acquired Progress last July, several months after the flaw was first recorded with ultrasound techniques.
The flaw went undetected for more than a year until Duke reported it last week. It’s indicative of primary water stress cracking, Reis said.
The flaw was one-quarter of an inch long in spring 2012, but it is not clear how much it has grown since then, Reis said. It did not penetrate the lid and did not cause leakage of radioactive water.
The flaw is located on a nozzle at a weld point on the underside of the lid, which is exposed to extreme heat under high pressure. The nozzle, No. 49, is used to insert a heat measuring device.
The lid itself is a six-inch-thick slab of metal that measures 15 feet across and is large enough to hold an automobile.
NRC officials do not know whether the flaw indicates corrosion or stress at the point of a weld. They will attempt to determine the facts relating to the degraded condition, the company’s response and the company’s corrective action.
“This is an unacceptable non-conformance, and if it had been found when it should have been found it would have been repaired,” Reis said.