The Nuclear Regulatory Commission said Thursday that long work hours in cramped, noisy conditions likely contributed to an oversight that shut down the Harris nuclear plant in Wake County two months ago.
After a year-old problem came to light at the Harris plant in May, the federal agency conducted a special inspection. The agency issued its conclusions Thursday, explaining how the mistake was made and how to prevent it in the future.
One clue: The nuclear specialists who failed to detect the tiny scar that led to the shutdown of a nuclear plant had each worked more than two weeks straight without a day off.
The report suggests that the nuclear industrys tolerance of punishing work schedules creates conditions in which overworked technicians can miss clues that point to potential problems.
The NRC said the mishap, which idled the Harris plant for three weeks, did not pose a threat to public safety. The agency also credited Duke Energy Progress, which owns and operates the nuclear plant less than 25 miles from downtown Raleigh, for taking appropriate corrective measures, including restricting work hours and providing a better work environment for its contractors.
Raleigh-based Progress is a subsidiary of Charlotte-based Duke Energy, which acquired Progress last summer. The problems at the Harris plant preceded the utility merger.
The nuclear reactor at the Harris plant is capped by a 6-inch-thick lid, called the vessel head, thats perforated by dozens of nozzles. Inside the reactor, water is heated at high pressure and piped to a generator to make electricity.
The reactor and the containment building around it are designed to prevent radioactive material from escaping and contaminating the surrounding area.
In the spring of 2012, during a refueling outage, ultrasonic tests showed four tiny flaws in the nozzles, which were promptly fixed.
The same ultrasonic tests also indicated another irregularity. This one, however, went undetected by contractors and plant operators for 13 months. The flaw, suggesting that primary water stress cracking had begun to develop, was not detected until this May, forcing the shutdown of the plant.
The NRC noted that in May, a year after the initial ultrasonic discovery, the damage was a third bigger. The corroded area, located at a weld-point of a nozzle in the reactor lid, expanded from 1/4-inch to 1/3-inch, but never led to a leak of radioactive material.
The two specialists who missed the flaw were fully qualified, suggesting that time pressure or inadequate oversight caused them to miss the fifth flaw, the NRC said.
Progress concluded in its own review that the analysts work environment included tight quarters, noise and other distractions.
Whats more, one analyst had worked 24 days without a day off while the other had worked 17 days without a day off, the NRC said.
The high-performance culture of the nuclear industry, which attracts many from military backgrounds, accepts such conditions as normal.
The NRC wrote: The vendor analysts did not express concern with this type of working environment and responded that it was typical of working conditions during other outages at other nuclear facilities.