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LOS ANGELES (Reuters) - Southern California Edison said an emergency generator at one of its San Onofre nuclear reactors may have been tampered with, prompting the utility to beef up security at the plant, which has been out of service since January due to a leak in a steam generator.

In a statement on Thursday, the Edison International unit said it discovered engine coolant in an oil system in the backup diesel generator in late October during routine monitoring. An internal probe found evidence of potential tampering, though it could not be confirmed.

The presence of coolant posed no safety risk, the company said.

The news is the latest blow to San Onofre's majority owner, which earlier this month said the cost of the prolonged outage at the damaged nuclear power plant has topped $317 million. San Onofre's Unit 2 and Unit 3 reactors have been shut down since January after operators discovered a leak in a steam generator tube in Unit 3.

The investigation into the potential tampering is ongoing, the utility said, adding that it has enhanced security at the nuclear power plant that sits on the Southern California coast about halfway between Los Angeles and San Diego.

The company informed the Nuclear Regulatory Commission of the issue on October 30 and told the agency of the potential tampering on November 27.

NRC officials were not immediately available for comment.

Last month, Southern California Edison submitted a plan to the NRC to restart Unit 2 and operate it at 70 percent of capacity for an initial five-month period, at which point it would halt production and look for signs of the premature tube-to-tube wear that led to the leak.

A meeting between the utility and the NRC to discuss the restart plan is scheduled for later on Friday in Laguna Hills, California.

(Reporting by Nichola Groom; Editing by Marguerita Choy)

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