The San Onofre Nuclear Power Plant is seen on April 6, 2012.
The groups sent a letter to the federal agency May 31 asking why the plant has been operating with flawed emergency diesel generators.
The request comes after plant operator told regulators the backup system could be shut down by an earthquake.
In the letter, the groups say the possibility that an earthquake could cause vibration sensors to shut down emergency diesel generatorsis a critical safety flaw.
Plant operator Southern California Edison reported to the Nuclear Regulatory Commission in early May that a vibration sensor at the San Onofre nuclear plant might incorrectly shut off emergency diesel generators if power went out during an earthquake.
Edison spokeswoman Jennifer Manfrè said engineers are still analyzing the vibration sensor and have not determined if it would actually shut off the generators during an earthquake.
She said there is a system designed to bypass the sensor and keep the generators operating during a power outage or plant emergency.
But Gary Headrick with San Clemente Green is not satisfied with that answer. He said the NRC has failed to properly examine earthquake impacts on critical safety equipment at San Onofre for three decades.
Headrick said the lack of federal oversight is a threat to the 8 million people who live within 50 miles of the plant.
“Community members deserve an explanation from the NRC for this safety failure,” said Headrick.
The San Onofre Nuclear Generating Station has been shut down since Jan. 31 due to unusual wear in steam generator tubes. A tube rupture sent radioactive steam into the atmosphere.
Jennifer Manfrè with Southern California Edison said earlier this week that inspectors haven’t figured out the cause of that problem.
“Within a matter of weeks the people of Southern California will be asked to trust the assurances of Edison that they can safely restart the San Onofre reactors," said Gene Stone with Residents Organized for a Safe Environment. "This latest revelation is one more reason why there is no trust in Edison claims and why the NRC is seen as a nuclear industry lapdog not capable of doing its job."