TOKYO — An atomic research lab in northern Japan has reported a radiation leak that may have affected about 50 people, though none were hospitalized and no impact was expected outside the facility, the lab’s operator said Saturday.

The Japan Atomic Energy Agency said the accident occurred Thursday in its Hadron Experimental Facility at the Japan Proton Accelerator Research Complex in the town of Tokaimura, where at least two previous radiation accidents have occurred.

Researchers were trying to generate particles by directing a proton beam at some gold when their equipment overheated, causing the evaporation and release of some radioactive gold, the government-run JAEA said in a statement. The leak originally was thought to have been contained inside the lab, and when a ventilation fan was switched on the radiation spread, it said.

The JAEA said it was studying the potential environmental impact from the radiation leak, but did not expect any effect on surrounding areas.

On Saturday, officials from the JAEA and the research complex apologized for the accident. The minister of Education, Science and Technology, Hakubun Shimomura, described it as “regrettable.”

Four researchers were tested after the incident, with the highest radiation dose found to be 2 millisieverts, the JAEA statement said. That’s near the average annual background dose for someone living in Japan. Nuclear workers generally are limited to 100 millisieverts of exposure over five years.