The researchers suffered internal radiation exposure when the output of a proton beam increased unexpectedly and vaporized part of a gold target, resulting in radioactive substances leaking from the controlled area, the Japan Atomic Energy Agency said.
Investigators from Ibaraki Prefecture were at the facility in Tokai Saturday looking for the cause of the malfunction, the Kyodo News agency said.
The incident occurred Thursday at the Hadron Experimental Facility at the Japan Proton Accelerator Research Complex. Scientists were conducting an experiment aimed at producing elementary particles by aiming a proton beam at the gold target.
The extent of the researchers' exposure was not clear Saturday. Kyodo said they were exposed to as much as 1.6 millisieverts of radiation before the leak was stopped.
Kyodo said the Atomic Energy Agency initially thought there had been no radiation leak and did not immediately report the incident to the national Nuclear Regulation Authority.