SACRAMENTO — Power failed Thursday in the control room of the Rancho Seco nuclear power plant and, while employees worked to fix the problem, radioactive gas was released into the atmosphere, officials said.
But the amount of radioactive gas that escaped was not enough to harm anybody, according to Brad Thomas, a spokesman for the Sacramento Municipal Utility District.
A control room employee got sick during the shutdown, but the cause of his illness was not determined, he said.
The incident was called an "unusual event," the lowest level of emergency at the plant, 25 miles southeast of Sacramento.
It was the second time in five days that the plant was shut down while utility officials investigated what went wrong.
The plant was being brought back to full power after a shutdown Sunday night caused by a leaky valve, which let 3,000 gallons of radioactive water leak into a holding tank.
With the valve fixed, the plant was restarted Christmas Eve, and was at 75% of capacity at 4:15 a.m. Thursday when two circuit breakers tripped and shut off power to the Integrated Control System, Thomas said.
There was no immediate explanation of why the circuit breakers tripped.
Because of the power failure, which lasted about 15 minutes, control room operators had partial loss of control during an automatic shutdown, he said. A backup system was activated.
But while the operators were regaining control, a water pump valve failed and an ensuing leak released radioactive gas off into the atmosphere, Thomas said.
The amount was so small that a person standing outside the plant would have been exposed to less than 0.2 of a millirem. Residents of Sacramento are exposed to about 110 millirems a year from natural sources, he said.
Thomas said that during the shutdown, one of four operators in the control room complained of stomach and chest pains. He was taken by ambulance to a hospital where he was treated and then released.