Electrical and safety problems at the nuclear plant in September prompted an unusual visit by a team of federal inspectors.
Glitches in the equipment that runs and protects the Monticello nuclear power plant triggered an automatic shutdown in September and attracted the attention of federal authorities.
The plant returned to full operation Wednesday after a three-week shutdown, according to its owner, Xcel Energy, and no radiation was released at any time.
However, the malfunction of electrical and safety equipment prompted an unusual visit by a five-person special inspection team from the federal Nuclear Regulatory Commission (NRC). They will report their findings next month.
"When we have an unplanned shutdown followed by a couple of issues that we didn't expect to be there, we want to go into the plant," said Viktoria Mitlyng, regional NRC spokesperson. "We want to understand what led to the existence of not one equipment issue but two or three," she said, and whether they represent larger problems.
Assigning special inspection teams is somewhat unusual but not rare, according to David Lochbaum, a nuclear power safety expert at the Union of Concerned Scientists, an advocacy group in Washington, D.C. Lochbaum said the NRC dispatches inspection teams about six to eight times each year, on average. The United States has 103 operating nuclear plants.
Xcel officials said a special team investigated the plant once before, after an unexpected shutdown in 1991. The 600-megawatt plant is about 40 miles northwest of the Twin Cities. What happened last month at Monticello, according to NRC documents and Xcel site vice president Tim O'Connor, was that a cable broke and disconnected the plant from its main transformer and source of electricity. A backup transformer was out of service for maintenance. That triggered the nuclear reactor to scram, or shut down automatically, and the plant switched to a smaller third transformer with enough power to operate critical emergency systems.
One emergency system floods the reactor core with outside cooling water so the fuel doesn't overheat and melt. That high-pressure injection equipment turned on, O'Connor said, but it failed to switch off automatically as designed when water in the reactor vessel reached a certain level. Plant operators saw the rising water levels within a couple of minutes, he said, and manually turned the system off to avoid overflows into other pipes.
"Any time that you have a reactor scram, or trip, if there's equipment that doesn't work exactly the way it's supposed to, we do investigations in order to understand why," O'Connor said. Even if water had overflowed, he said, it would have been contained within the plant.
Xcel workers also had trouble turning off a diesel generator activated as part of another emergency system.
Some of the problems recurred on Sept. 17 when a contract worker in a lift was electrocuted as his equipment struck an overhead power line. The plant, still shut down but fully reconnected to the grid, again lost power from a main transformer.
The inspection team was already at the power plant at the time, and is looking at all of the circumstances from both incidents, Xcel's responses to them, and corrective actions, Mitlyng said.
O'Connor said Xcel has also identified and examined 20 other electrical cables about the same age as the one that failed, and made all other repairs and checks needed to put the plant back into service.
Lochbaum said that it may be a coincidence that the transformer and injection water level problems at Monticello occurred at the same time, or they may prove to be part of a pattern of negligence if equipment has not been maintained properly.
"There could be some other problems out there that are not yet flushed out and fixed," he said. "That's also what the NRC inspection team is looking into."
Tom Meersman • 612-673-7388