The accident at the central Pennsylvania plant - the site of the nation's worst nuclear power plant disaster - exposed a dozen employees to radiation, but the public was in no danger, plant officials and government regulators said.
Plant officials likened workers' maximum exposure to the equivalent of two medical X-rays, while the U.S. Nuclear Regulatory Commission said the workers were exposed to a small fraction of the annual federal regulatory limit.
"We are back to work and back to normal right now," plant spokesman Ralph DeSantis said.
The accident happened at about 4:15 p.m. Saturday. A radiation monitor at an entrance to the reactor building "temporarily went up" slightly, but a later survey detected no contamination outside, DeSantis said.
About 150 workers in the reactor building were sent home, and plant officials contacted authorities a few hours later and decontaminated the building.
Any radiation on an external surface, such as safety suit, can be cleaned off, while it takes two to three days for radiation to naturally leave the body of anyone who breathed it in, DeSantis said.
The plant, which is on a Susquehanna River island about 10 miles south of Harrisburg, has two reactors. One suffered a partial meltdown in 1979 and remains mothballed. The other, owned by Chicago-based Exelon Corp., is still in use, but has been shut down since last month so steam generators could be replaced.
Workers removed the generators, nuclear fuel rods and the water from the reactor cooling system. Some radioactive particles were left behind in the reactor cooling pipes, which circulates water by the fuel rods in a closed loop.
DeSantis said workers were cutting cooling system pipes when an unexpected change in the air flow stirred up radioactive dust. The particles blew into the reactor building, he said.
The NRC is monitoring the plant's handling of the event. Agency spokeswoman Diane Screnci said it is not unusual for a small release of radiation to occur while a plant is refueling or replacing its generators.
Eric Epstein, chairman of the watchdog group TMI-Alert, said it is troubling that the plant would send workers back into the reactor building before identifying the root cause of the contamination.
Edwin Lyman, a senior staff scientist at the Cambridge, Mass.-based Union of Concerned Scientists, said it is too early to tell whether the accident is the result of negligence.
"In my mind, it's an unusual event and requires serious attention," Lyman said.