Mike Blake / Reuters The San Onofre nuclear power plant lies along the coast between Los Angeles and San Diego.

Security has been beefed up at the San Onofre nuclear power plant in Southern California after it was discovered that a generator there might have been tampered with, the plant operator said. The plant has been out of service since January due to a leak in a steam generator.

Southern California Edison 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.

Technicians later found excessive wear on hundreds of tubes in units 3 and 2, which had been taken offline earlier for maintenance.

The problems center on damage to alloy tubing in four steam generators that were installed during a $670 million overhaul in 2009 and 2010.

A three-month federal probe blamed a botched computer analysis for generator design flaws that ultimately resulted in excessive wear to the tubes.

Last month, Edison asked federal regulators for permission to restart the Unit 2 reactor and run it at reduced power.

The utility and the NRC scheduled a public meeting on the issue Friday in Southern California.

However, an NRC decision is not expected for months.

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 coast about halfway between Los Angeles and San Diego.

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

NRC officials were not immediately available for comment.

Reuters and The Associated Press contributed to this report.

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