The chair of the Senate Environment and Public Works Committee accused Southern California Edison of dispensing "gobbledygook" about a 2004 internal letter that Boxer believes reveals possible criminal misconduct within the company.
"I don't have confidence in Southern California Edison, given what I now know," the California Democrat told reporters in a conference call, referring to the letter. "They ought to just tell the truth. Start now."
Boxer's statements reflect mounting tension around Edison's proposal to restart one of two reactors at the seaside plant, while the company faces a tangle of regulatory steps and federal and state investigations.
The twin-domed plant between Los Angeles and San Diego hasn't produced electricity since January 2012, after a small radiation leak led to the discovery of damage to hundreds of tubes that carry radioactive water in nearly new steam generators.
Meanwhile, Edison released a copy of a June 2005 letter that also shows the company had concerns about key aspects of the generator design, which appear to foreshadow eventual problems with tube wear.
Both letters were written to Mitsubishi Heavy Industries, which manufactured the generators.
The state Public Utilities Commission, which is conducting a separate probe into costs tied to the long-running shutdown, said a preliminary review showed Edison failed to provide either letter to the agency or parties involved in its investigation.
"We are considering whether Edison had a duty to disclose the letters earlier," commission executive director Paul Clanon said in a statement. "More importantly, we need to investigate whether Edison took unnecessary risks, or tried to evade regulatory oversight, when the steam generators were replaced."
Edison said the letters show the company "exercised responsible oversight" while the generator designs were being developed. It said both letters were given to the Nuclear Regulatory Commission in April.
"We believe that the determination for restart must be made based on technical merits," Pete Dietrich, SCE senior vice-president and chief nuclear officer, said in a statement.
"SCE's own oversight ... complied with industry standards and best practices," Dietrich added. "SCE would never, and did not, install steam generators that it believed would impact public safety or impair reliability."
Boxer said the 2004 letter, written by a senior SCE executive and obtained by her office, suggests "Edison intentionally misled the public and regulators" to avoid a potentially long and costly review of the generators before they went into service. The contents of the letter were first reported by The Associated Press.
Anti-nuclear activists trying to block the restart have long argued that Edison duped the NRC about extensive design changes to avoid a lengthy, trial-like review, known as a license amendment, that in some cases can take up to two years to complete.
The 2004 letter goes to a central issue at San Onofre, where Edison is seeking federal permission to restart the Unit 2 reactor and run it at reduced power in an effort to halt tube damage.
The replacement generators were different than the originals — they were 23.6 tons heavier and hundreds of additional tubes were added as part of design changes, for example. Edison installed the equipment in a $670 million overhaul in 2009 and 2010 without an extended NRC review after concluding the new machines met a federal test to qualify as largely the same as the ones they replaced, requiring little or no changes to safety systems or components in the plant.
Such equipment swaps, sometimes referred to as "like-for-like" replacements, are common in the industry.
But the Nov. 30, 2004, letter from SCE Vice-President Dwight E. Nunn states that "although the old and new steam generators will be similar in many respects they aren't like-for-like replacements."
Edison "told regulators one thing and did another," Boxer said.
The company said "at no time did SCE hide the differences from the NRC, nor did it seek to mislead the NRC."
It's not clear is what, if any, design changes were made after Nunn's 2004 letter.
The 2005 letter, released by Edison and also written by Nunn to Mitsubishi, warned about dry steam that could damage tubes and the potential for wear in an area where the tubes make a U-shaped turn.
The "industry's experience with tube wear in the U-bend region of the large steam generators is not encouraging," Nunn wrote. "This is of a great concern to Edison, because our steam generators are one of the largest in the Industry."
It's not clear is what, if any, design changes were made after Nunn's letters. Excessively dry steam in the generators and damage around the U-bend tube area both factored in damage at San Onofre, investigators found.
Boxer also raised questions about transparency at the Nuclear Regulatory Commission, which is considering Edison's restart plan.
She said her office asked the agency months ago for copies of correspondence between Edison and Mitsubishi, but never received the 2004 and 2005 letters. She did not say how her office obtained them.
She also said she was alarmed by equivocal statements from the NRC about whether it will conclude ongoing investigations at San Onofre before a restart decision.
The NRC has previously disclosed that its Office of Investigations and Office of the Inspector General are conducting probes into "allegations of wilful wrongdoing," but provided no details.
"I don't appreciate the way this has been handled," she said.
NRC spokesman Eliot Brenner declined comment.
Gradual wear is common in steam generator tubing, but the rate of erosion at San Onofre alarmed officials since the equipment is relatively new. Federal investigators last year concluded that a botched computer analysis resulted in design flaws that were largely to blame for the unusual tube wear.
Nunn said in 2004 that designing supports for the tubes would be tricky since larger generators appear more susceptible to tube wear, and added that he was "concerned that there is the potential that design flaws could be inadvertently introduced into the steam generator design that will lead to unacceptable consequences," including tube damage.
"This would be a disastrous outcome for both of us," he said.
Costs for the long-running shutdown have topped $553 million.
Last month, SCE's parent, Edison International, raised the possibility of retiring the plant if it can't get one reactor running later this year.
Follow Michael R. Blood at http://twitter.com/MichaelRBloodAPSuggest a correction