Nuclear safety officials have concluded that a fluke mishap last year at Progress Energy's Brunswick nuclear plant near Wilmington was caused by the lack of worker qualification for more than a decade.
The Nuclear Regulatory Commission issued its preliminary findings yesterday, but the federal safety agency is continuing its investigation to determine the safety significance of the incident.
The unusual mishap that shut down the Brunswick Unit 2 reactor last November may be the only such incident in U.S. nuclear history.
The Brunswick plant's boiling water nuclear reactor vessel functions like a giant teapot, producing steam to power an electric generator. The steam builds up under pressure contained by a tightly sealed lid.
But last November, mildly radioactive water bubbled out of the chamber rather than boiling inside under pressure to form steam. At one point the water was flowing out at a rate of over 10 gallons a minute, about 100 times more volume than would flow out under normal circumstances.
NRC investigators concluded the lid was improperly attached after a refueling outage because the maintenance workers were not trained in reactor vessel disassembly and reassembly. Qualification for this kind of work hadn't been provided since 2000, the NRC said in its assessment.
Nine of the 12 workers who performed the reactor vessel assembly were not qualified. Some received "just-in-time" (i.e., last minute) training before the Fall 2011 refueling outage, but not specifically in stud tensioning.
The workers didn't know how to read the instrumentation and torqued the reactor vessel lid's studs at 1,300 pounds per square inch instead of 13,000 psi. In other words, there were off by one zero, and screwed the studs at 1/10th the required pressure.
Some the bolts were left so loose they could be turned by hand, the NRC said. Others turned easily with a wrench.
Fortunately for Progress, the mistake did not cause damage to the reactor vessel.