Friends of the Earth based their claim on a report by the Japanese company which manufactured the plant’s steam generators.
The facility has been shutdown since January 31 because of problems with tubes in the steam generators.
The environmental group said a filing with the Nuclear Regulatory Commission (NRC) by Mitsubishi Heavy Industries means the San Onofre Nuclear Generating Station can’t restart this summer.
"Mitsubishi's filing with the NRC makes it clear that any plans Southern California Edison has for restarting either of the reactors this summer must be abandoned," said Shaun Burnie with Friends of the Earth.
Burnie said that's because the Japanese company is doing its own analysis, which won’t be completed until August 31.
"Our point to Southern California Edison and the Nuclear Regulatory Commission is, if there are assessments being done on the steam generator tube problems – which are serious – if they are going to be trusted by the public, all of those assessments have to be completed and assessed before there is discussion about restarting the plant," Burnie said.
But Jennifer Manfre with Southern California Edison said the filing by Mitsubishi is part of standard Nuclear Regulatory Commission requirements.
"The filing is related, but the filing itself has no direct impact on the timing for restarting the San Onofre Nuclear Generating Station," said Manfre. "Mitsubishi is performing their own internal investigation and the NRC filing is part of that process."
Last month, Friends of the Earth commissioned a study by a nuclear engineer which claimed design changes at the plant, made by Southern California Edison, are the cause of unusual tube wear.
The Nuclear Regulatory Commission said it had approved those design changes.
A special investigation team from the NRC and investigators from Southern California Edison are trying to find out why the nuclear plant is having problems with relatively new steam generators.
NRC Chairman Gregory Jaczko made a special trip to San Onofre on April 6. He also met with anti-nuclear activists.
Jaczko said until the exact cause or causes are known for the tube problems at the two generator units, the nuclear plant will remain closed.
The tubes are a primary barrier to prevent radiation from escaping into the atmosphere, which is what happened in Unit 3 January 31. A leak in one of the unit's steam generator tubes sent low-level radiation into the atmosphere but officials said there was no danger to the public. Subsequent inspections of Unit 2 found similar problems with unusual tube wear.
Southern California Edison is looking at contingency plans if the plant remains offline.
Edison spokeswoman Jennifer Manfre said those plans include buying energy on the open market.
Meanwhile, the California Independent System Operator or CAL-ISO, which operates the state's power grid, wants operators of a gas-fired power plant in Huntington Beach to restore two retired units to service.
AES Southland operates the plant in a lease agreement with a subsidiary of Southern California Edison.