A timeline of the San Onofre nuclear power plant

A timeline of major events at the San Onofre Nuclear Generating Station:

1968: Unit 1 nuclear reactor begins commercial operation.

1983: Unit 2 begins operation.

1984: Unit 3 begins operation.

1992: Unit 1, needing millions of dollars in upgrades, is retired.

1997: Hundreds of dented and cracked water tubes are found in two steam generators, caused by the high temperatures of the operating reactors. The tubes did not rupture and are plugged.

2000: The U.S. Nuclear Regulatory Commission grants the plant a nine-year operating extension, permitting the twin reactors to run until 2022 instead of 2013.

2009-2010: The plant replaces aging steam generators for its twin reactors in a $670 million overhaul.

January 2012: Unit 2 is taken offline for routine maintenance.

January 2012: Unit 3 is shut down as a precaution after a broken water tube in a steam generator releases a trace amount of radiation.

January 2012: Plant operator Southern California Edison announces that Unit 2 and Unit 3 will remain shut down because excessive wear was found in hundreds of steam generator tubes. Edison eventually says it will take 1,300 of some 9,700 tubes out of service, either because of wear or as a precaution.

June 2012: Federal investigators say a botched computer analysis resulted in design flaws that are largely to blame for the wear.

October 2012: Edison announces plans to repair and restart Unit 2, operating at reduced power.

April 2013: Edison International, parent of Southern California Edison, says costs tied to the plant's shutdown and repairs have soared to $553 million.

May 2013: Federal regulators indefinitely delay a decision on whether to approve the restart of Unit 2.

June 7, 2013: Southern California Edison announces it will close the nuclear plant.