Two individuals were detained Wednesday after breaching a fence at a GE Hitachi research reactor that uses “bomb-grade fuel” near San Francisco, the U.S. officials revealed Thursday.

According to the U.S. Nuclear Regulatory Commission, the intruders jumped the fence encircling the 1,600-acre Vallecitos site reactor in Alameda County, but fled after they were spotted. The pair was later arrested outside the plant.

The suspect’s identities are currently unknown.

Reuters reports:

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.”

Edwin Lyman, a nuclear power scientist at the Union of Concerned Scientists, said the incident “highlights the continuing danger posed by nuclear reactors fueled with nuclear bomb-usable materials such as highly enriched uranium.”

“These reactors should convert to safer fuels or shut down,” he added.

The Alameda County Sheriff’s Office has not issued a statement regarding the breach.