Nuclear power plant in San Onofre (SONGS) in California is off due to a leak in the cooling water treatment system, soobshaet newspaper North County Times.
More leaked 31 January, but the media became aware of the incident until Friday.
Engineers have disabled third power station, after which the work to find the leak in the piping system.
"There will be days before we will have the final conclusions on the situation," — said Gil Alexander, a spokesman for operator Edison.
Leaking water from the cooling system plant can be dangerous, because water, which contains radioactive elements that can evaporate into the atmosphere.
Measurements carried out by specialists showed no elevated levels of radiation outside the plant.
According to the source the newspaper Wall Street Journal, one of the existing reactors will be started this weekend.
Total NPP SONGS, which was commissioned in 1968, had three reactor. The first unit was taken out of service in 1992 — the building is used as an additional reservoir for spent nuclear fuel (SNF). Power unit No. 2 was previously shut down for maintenance work.
At the plant in November, there was a similar incident with a leak in the cooling system. Breakage removed three hours after discovery of the leak.