It took just seven particles of metal -- each the size of a speck of ground pepper -- to shut down Progress Energy's Brunswick nuclear power plant near Wilmington.
In a special inspection report describing the incident, federal nuclear regulators said Tuesday the malfunction that shut down the Brunswick plant near had a very low safety significance.
Progress Energy, the Raleigh-based power company that operates the nuclear plant, shut it down for a week and a half in September after one of the emergency backup diesel generators wouldn't start during a test.
The NRC sent in a special inspection team to review the malfunction. A nuclear reactor must be backed up by two emergency generators to provide electricity to emergency safety equipment in the event of a nuclear accident.
The NRC special inspection report issued Tuesday concluded the malfunction was most likely caused by the plant operators.
The problem disabled the diesel generator's governor, which controls fuel flow to the generator. Inspectors found that the governor was clogged with metal particles, cutting off fuel flow to the 3.8 megawatt generator.
The particles got into the governor in April when Progress Energy personnel replaced the governor on the generator. But the errant particles didn't disrupt the generator until September, when plant operators replaced the oil in the governor and moved the settled material.
Unplanned outages are the bane of nuclear power plant operators. They require electric utilities to find alternate sources of power for their customers, sometimes at premium prices.
This month the company's Shearon Harris nuclear plant in Wake County was shut down for five days after an oil spill in the generator room released 7,000 gallons, requiring an extensive cleanup.