Two teenage intruders jumped over the fence, but after seeing security officers fled the territory. Shortly afterwards they were detained outside the site.
According to Alameda Ray Kelly County Sheriff, there is no suspicion of malicious intent.
“Looks like it was a childish prank,” he said.
The Vallecitos GE Hitachi nuclear research centre is home to highly enriched uranium, which can be used to create an atomic bomb.
According to information from open sources, radioactive materials are produced there for medical and industrial needs.