An atomic research lab in northern Japan on Saturday reported a radiation leak that may have affected about 55 people, though none were hospitalised and no impact was expected outside the facility.

The government-run Japan Atomic Energy Agency said the accident occurred on 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 radioactive gold, JAEA said.

Tokaimura was the site of Japan’s second-worst nuclear accident, in September 1999, when two workers were killed by a radiation leak at a fuel-reprocessing plant.

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