A nuclear research laboratory in northern Japan has reported a radiation leak that may have affected 55 people, though none has been hospitalised and no impact is expected outside the facility.

The Japan Atomic Energy Agency said on Saturday that the accident occurred on Thursday at a nuclear physics lab in Tokaimura, the site of at least two previous radiation accidents.

Four researchers were tested afterwards, with the highest radiation dose found to be two millisieverts. Nuclear workers generally are limited to 100 millisieverts of exposure over five years.

The leak occurred when experimental equipment overheated, causing the evaporation and release of radioactive gold.

Since the leak originally was thought to have been contained inside the lab, the incident was not initially reported, and workers used a ventilation fan, causing radiation to spread.

The JAEA said it was studying the potential environmental impact from the radiation leak.

Japan's nuclear industry has been in crisis since the March 2011 accident at the tsunami-hit Fukushima Dai-ichi nuclear plant, the country's worst-ever atomic energy disaster.

Most of Japan's nuclear plants remain closed after they were shut down for safety checks.

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 when they tried to save time by mixing excessive amounts of uranium in buckets instead of using special mechanised tanks.

Hundreds of people were exposed to radiation, and thousands of residents were evacuated as a precaution.

The government assigned the accident a level-4 rating on the International Nuclear Event Scale, which ranges from 1 to 7.

Source: Agencies