The San Onofre Nuclear Generating Station shut down its Unit 3 Tuesday afternoon because sensors detected a leak in the unit's steam generator tubes.

"The potential leak poses no imminent danger to the public or plant workers," according to a Southern California Edison statement. "There has been no release to the atmosphere." San Onofre spokesman Gil Alexander said sensors tripped this afternoon, showing mildly radioactive water was leaking from one of two water systems in the steam generator apparatus of Unit 3.

The water that touches the radioactive fuel rods--the water that is now leaking--is sealed in a series of tubes that, in turn, transfer heat to boil water in the second course. That non-radioactive water becomes the steam that turns the massive turbine blades. A third course of seawater condenses the steam in another part of the plant.

Alexander said it is the system with the contaminated water that shows evidence of leaking. All leakage is contained within the thick concrete containment dome, he said. The plant is set to shut itself down at the first sign of problems, that plunged Southern California into darkness in September.

But this leak was minor enough that it did not trigger the automatic shutdown of the unit; plant workers initiated it manually as a precaution at 5:31 p.m. Tuesday, Alexander said.

Alexander said Tuesday night that the dome is closed off and technicians are cooling down the reactor, a process that takes about 12 hours. After the cool-down, a crew will enter the dome, assess the leak and take steps to begin repairing it.

Visit Patch tomorrow for updates on this process. Unit 2 is currently offline for a planned maintenance, refueling and technology upgrade outage.