LOS ANGELES (AP) — The cost of repairs and inspections at the shuttered San Onofre nuclear power plant jumped to more than $400 million through December as the company that runs it contends with extensive equipment repairs, regulatory filings disclosed Tuesday.

“The scope of necessary repairs for the steam generators … or the length of the units’ outages could prove more extensive than is currently estimated,” wrote Edison International, the parent company of San Onofre operator Southern California Edison.

The plant hasn’t produced electricity since January 2012, after a tiny radiation leak led to the discovery of unusual damage to hundreds of steam generator tubes that carry radioactive water.

The filing touches on a series of broad questions about the plant as SCE pushes to restart one of two reactors.

“The cost of such repairs or the substitute market power that must be purchased during the outage could exceed estimates and insurance coverage, or may not be recoverable through regulatory processes or otherwise,” it said.

SCE is facing pressure from several sides, as it tries to get the planting running again. State regulators are determining if ratepayers should pay for costs tied to the long-running shutdown, and the Nuclear Regulatory Commission investigative arm quietly opened a probe in September tied to information Edison provided to the agency on its steam generators.

Meanwhile, environmental activists have been pushing to have the plant shut down permanently.

The regulatory filing also said SCE’s insurance coverage for wildfires that could arise from its operations might not be sufficient, and the parent company might not be able to obtain sufficient coverage on SCE’s behalf.

SCE has asked the NRC for permission to restart one of the reactors, Unit 2, and run it at 70 percent power for five months. Edison engineers predict the lower power level will halt vibration and friction that has damaged tubing in the steam generators.

In a conference call with Wall Street analysts, Edison Chairman Ted Craver said the company hoped the Unit 2 reactor could be online by summer but noted that preparations are being made if that doesn’t occur.

“We are convinced it is safe to run the unit,” he said.

NRC officials said late last year that operating rules require San Onofre to ensure that generator tubes retain structural integrity during “the full range of normal operating conditions,” including at full power.

The NRC said it wanted the company to demonstrate that Unit 2 could meet that threshold, or explain how generator tubes would interact with each other if the plant is operating at maximum capacity.

In a response to the NRC released Tuesday, the company argued, in essence, that 70 percent is full power.

Under its restart plan, full power “is 70 percent for the proposed operating period” and meets the federal requirement, the company wrote.

The problems at San Onofre center on steam generators that were installed during a $670 million overhaul in 2009 and 2010. After the plant was shut down, tests found some generator tubes were so badly eroded that they could fail and possibly release radiation, a stunning finding inside the nearly new equipment.

The ability of San Onofre to run safely at lower power — and whether that limit would require an amendment to its operating license — came up in December at a hearing of the Atomic Safety and Licensing Board, an arm of the NRC.

Administrative Judge Gary Arnold asked Edison attorney Steve Frantz if he was confident that the plant could operate at 99 percent power with its ailing generators.

“I do not say that,” Franz responded. He argued that running at 70 percent power would fall within San Onofre’s license and operating rules.

The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate something like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds and has with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter. Hundreds of the tubes have been taken out of service because of damage or as a preventative step.

Edison is planning further testing.

Spokeswoman Jennifer Manfre did not immediately respond to emails and a phone call seeking further comment.