(Adds GE statement, paragraph 5)

By Timothy Gardner

WASHINGTON, July 11 (Reuters) - Two people jumped a security fence at a GE Hitachi research reactor near San Francisco, the U.S. nuclear power regulator said on Thursday, raising concerns over a plant that is one of the few in the country that uses highly enriched uranium, a material that could be used to make an atomic bomb.

The intruders jumped a security perimeter fence at the Vallecitos reactor in Alameda County on Wednesday afternoon, a 1,600-acre (647.5-hectare) site about 40 miles (64 km) east of San Francisco, the U.S. Nuclear Regulatory Commission said on its website in a security threat notice.

They escaped security at the plant after being detected, but shortly afterwards suspects were detained outside the facility, the NRC said.

The Alameda County Sheriff's Office did not immediately respond to a request for comment.

The NRC notice did not mention that the plant is one of the few in the country to use highly enriched uranium, or HEU. Such plants have been under pressure from nonproliferation interests to convert to low-enriched uranium, or LEU, a material that cannot be used to make a bomb.

GE Hitachi Nuclear is a venture between General Electric Co and Hitachi Ltd. GE said in a release that the individuals "did not breach the inner perimeter fence nor access any buildings or operational areas and were immediately approached by security." GE did not respond to a question about the amount of HEU at the plant.

The NRC limits the amount of unirradiated HEU at research and test reactors to less than what would be needed to build a nuclear bomb device. The agency did not immediately respond to a request for comment about the amount of HEU at the plant.

The security significance of the event at Vallecitos was unclear, but "it highlights the continuing danger posed by nuclear reactors fueled with nuclear bomb-usable materials such as highly enriched uranium," said Edwin Lyman, a senior scientist and nuclear power safety expert at the Union of Concerned Scientists advocacy group.

"These reactors should convert to safer fuels or shut down," he said.

In a letter to the NRC in April, a copy of which was seen by Reuters, Jeffrey Smyly, a regulatory compliance manager at GE, said that there was not funding available from the U.S. Department of Energy to convert the reactor fuel to LEU. (Reporting by Timothy Gardner Editing by Susan Thomas and Lisa Shumaker)

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