Japanese nuclear authorities said Saturday that radioactive substances leaked from a nuclear physics laboratory of the Japan Atomic Energy Agency in Ibaraki prefecture after an incident on Thursday, local media reported.

About 55 researchers and others who engaged in an experiment to generate particles by applying a proton beam to gold may have been exposed to radiation as a result of inhaling the radioactive materials, said Japan's Kyodo News.

The Japan Nuclear Regulation Authority (NRA) said no one was sent to hospital and four have undergone checkups with the highest radiation dose detected at 1.6 millisievert.

The incident took place at 11:15 am Thursday when a malfunction in an equipment triggered overheat and resulted in evaporation and release of radioactive gold, said the NRA.

The nuclear regulation authority said they received the report on the incident late Friday and will conduct an on-site inspection Saturday.

Contamination in the lab building is estimated at 40 becquerels at most and all work in the lab has been suspended, said Kyodo.