TOKYO (AFP) - The number of researchers exposed to low level radiation in an accident at a nuclear laboratory in Japan last week has hit 30, officials said on Monday, with human error likely exacerbating the problem.
The accident occurred on Thursday as 55 people were working at a laboratory in Tokaimura, 120 kilometres northeast of Tokyo, the Japan Atomic Energy Agency (JAEA) said.
The researchers were carrying out an experiment that involved firing a proton beam at gold when the accident happened, it said.
The agency, which had initially said six researchers were exposed to radiation, announced late Sunday that 24 more people were affected.
There was no widespread radiation leak, although two researchers were exposed to up to 1.7 millisieverts of radiation, a dose roughly equivalent to a strong medical X-ray.
"None of them required medical attention," an JAEA spokesman said.
The International Commission of Radiological Protection recommends a dosage limit of one millisievert per year, but says exposure to less than 100 millisieverts per year presents no statistically significant increase in cancer risk.
According to the agency, radiation was accidentally released during the experiment "due to overheating, which we suspect was caused by some technical problems".
Radiation then leaked from the facility to the outer atmosphere after workers used fans to lower levels of radioactivity in the laboratory, it said.
The agency spokesman said that the fans should not have been used, adding: "We don't know why they switched on the fan.
We suspect some wrong decisions were made by workers concerned."
Nuclear safety is a particularly sensitive issue in Japan, which in 2011 experienced the world's worst atomic accident in 25 years when a tsunami took out a nuclear power plant in Fukushima.