A battery used to supply power to safety systems at the San Onofre Nuclear Generating Station was inoperable for about four years, federal nuclear regulators announced Monday.
The discovery prompted the U.S. Nuclear Regulatory Commission to announce it has stepped up oversight at the plant, which is operated by Southern California Edison Co.
NRC inspectors found the battery, used to supply power to plant safety systems under some accident conditions, didn't work between 2004 and when the problem was discovered on March 25.
According to the NRC, the problem was caused by loose electrical connections resulting from inadequate maintenance instructions.
"The problem stemmed from inadequate maintenance procedures,'' Elmo E. Collins, a regional administrator with the NRC, said in in a statement. "The problem was promptly corrected, but is troubling because the condition persisted for so long.''
Other emergency batteries were available to supply power to safety systems if needed, but the loss of one reduced the nuclear plant's safety margins, according to the NRC.
The commission uses a color-coded system to categorize inspection findings, ranging from "green'' to "white,'' "yellow'' and "red,'' depending on the safety significance.
The San Onofre Nuclear Generating Station was given a "white'' rating, or low to moderate safety significance, according to the NRC.