The twin reactors — situated along the Pacific Coast in the densely populated corridor of millions of people between San Diego and Los Angeles — are the largest to shut down permanently in the U.S. in the past 50 years, federal officials said.
Southern California Edison's decision brings to a sudden end a dispute that began in January 2012, when a small radiation leak led to the discovery of unusual damage to hundreds of virtually new tubes that carry radioactive water. The plant hasn't produced electricity since then.
Edison has already spent more than $500 million on repairs and replacement power and had hoped to restart one reactor this year and run it at reduced power to eliminate the vibrations that had damaged the tubing. But the utility ran into resistance from regulators and was also facing various investigations and mounting political opposition.
Ted Craver, chairman of the utility's corporate parent, Edison International, said in a statement that the company concluded that "continuing uncertainty about when or if (the plant) might return to service was not good for our customers, our investors or the need to plan for our region's long-term electricity needs."
San Onofre, which opened in 1968, was capable of powering 1.4 million homes. California officials have said they can make it through the hot season without the plant as long as the summer is uneventful, but warned that wildfires or another disruption in supply could cause power shortages.
Environmentalists celebrated outside the front gates of the beachfront plant, and a pack of bicyclists shouted, "Shut it down!" as they went past.
Gary Headrick of San Clemente Green likened the news to births of his children: "The joy and the relief is comparable to something that big in my life, to know that 8 million people will be safe now from this supposed restart."
It wasn't clear how the electricity from the plant would be replaced permanently. The California Public Utilities Commission said it will work with governments to ensure Southern California has enough electricity, which could require increased energy efficiency and conservation, as well as upgrades to equipment.
Steve Kerekes, a spokesman for the Nuclear Energy Institute, an industry group, said San Onofre's shutdown "underscores the need for an efficient and effective regulatory process that results in timely decisions on the operation of these critical energy resources."
Mitsubishi Nuclear Energy Systems, which built San Onofre's steam generators, said it is disappointed with the decision and remains confident the plant can be operated safely.
The nuclear power industry has been encouraged in recent years by the development of several new plants in the Southeast. Nuclear Regulatory Commission chairwoman Allison Macfarlane said that the damage seen at San Onofre appears to be unique.
"At this point in time, we have not seen this particular kind of failure at any other reactor," she said.
It will take months, and possibly years, to complete the closing of the reactors. It will involve removing all fuel from the reactor cores.
Erich Pica, president of Friends of the Earth, said it is just the latest setback for the nuclear industry, which has been hit by safety-related shutdowns and faces steep competition from other energy sources, especially natural gas. He said San Onofre's closing represents an opportunity for California to use more clean energy, such as wind and solar power.
"This industry is on its final trajectory downward. And we should rename the Nuclear Regulatory Commission the Nuclear Retirement Commission," Pica said.
Sen. Barbara Boxer, D-Calif., chairman of the Environment and Public Works Committee, welcomed the news.
"This nuclear plant had a defective redesign and could no longer operate as intended. Modifications to the San Onofre nuclear plant were unsafe and posed a danger to the 8 million people living within 50 miles of the plant," she said.
The problems centre on four new, much-heavier steam generators that were installed during a $670 million overhaul in 2009 and 2010. Just a few years later, tests found some generator tubes so badly eroded that they could fail and possibly release radiation, a startling finding for nearly new equipment.
Each generator has 9,727 alloy tubes, which function somewhat like a radiator. The tubes circulate hot, radioactive water, which then heats a bath of non-radioactive water surrounding them. That makes steam, which drives the turbines to generate electricity.
In other nuclear-industry shutdowns over the years, the Shoreham plant on New York's Long Island was completed in 1984 for $6 billion but never opened because of community opposition. Decaying generator tubes helped push San Onofre's original reactor into retirement in 1992, even though it was designed to run until 2004. And in 1993, the Trojan plant near Portland, Ore., was shuttered years earlier than planned because of microscopic cracks in steam tubes.
Associated Press writer Matthew Daly in Washington, Ray Henry in Atlanta and Gillian Flaccus and Amy Taxin in San Clemente, Calif., contributed to this report.