There is also a new proposal to test the plant's ability to withstand a big earthquake.
Diaz alleges up to 10 employees came to him after other managers wouldn't listen to their concerns about safety-related issues such as worker fatigue and excessive overtime.
"I took those concerns, raised them up to my management," said Diaz. "At that time I was told to not address those concerns. The exact words were 'They don't need you to be their superhero.'"
The 35-year-old says he then went to the U.S. Nuclear Regulatory Commission (NRC) last August. Diaz claims when his employer found out, he was fired.
He says the reason given for his termination by SoCal Edison was his performance.
"We have conducted a thorough review of the concerns and we are satisfied with how the employee was handled," said Gil Alexander, a Southern California Edison spokesman.
SoCal Edison won't talk about the specifics of the lawsuit.
"If we discover that anyone in management or supervision has retaliated against an employee for raising safety concerns, we consider that a termination offense," said Alexander.
The company admits it received a letter from the NRC last March that talked about the presence of a "chilling effect": employees who raised concerns about safety worried about retaliation.
"Since that time we think we've taken steps to address that issue," said Alexander.
The company says the steps include allowing employees to complain anonymously.
The increased scrutiny of the plant comes as it proposes a multimillion-dollar study that would use the latest technology to assess earthquake risks.
The planning was already in place prior to a 9.0-magnitude earthquake and tsunami damaged a nuclear plant in Japan. Lawmakers and residents have raised questions about the safety of the state's nuclear plants.
"We're not comfortable with having nuclear power this close to us with that many similarities to Japan, being on earthquake faults and this close to the ocean," said Gary Headrick, co-founder of San Clemente Green.
"We are very confident about the safety of San Onofre," said Alexander. "That doesn't mean there won't be lessons to be learned out of Japan."
SoCal Edison needs approval from the California Public Utilities Commission for the seismic study because it would be consumer dollars would pay for that research through higher rates. The studies could cost more than $20 million.