At least four people, and possibly up to 55, were exposed to radiation when radioactive material leaked during a laboratory experiment in Japan.
The leak at the nuclear physics laboratory in Tokaimura, 110km northeast of Tokyo, occured on Thursday although the operators did not tell the Japan Atomic Energy Agency until Saturday.
Radioactive material also leaked outside the facility during the experiment, because a ventilation fan was being used.
A statement issued by the agency on Saturday said researchers were using a proton beam on gold to generate particles during the experiment.
They were exposed to radiation of between 0.6 - 1.6 millisieverts. Another 51 people were in the building at the time.
All four of those confirmed to have been exposed were involved in the experiment, but none needed to be taken to hospital.
Japan has imposed stricter safety measures since the meltdowns at the Fukushima Daiichi nuclear station north of Tokyo after the earthquake and tsunami on 22 March 2011.
Nuclear operators have only two of 50 reactors running while they await safety checks.
Two people died after an accident at a uranium reprocessing facility at Tokaimura on 30 September, 1999.
