A June nuclear exposure accident at a research and development center in Japan ended up assessed at a level 2 on the zero-to-seven international scale, officials at Japan’s Nuclear Regulation Authority said earlier this month.
The June 6 accident at the Japan Atomic Energy Agency’s (JAEA) Oarai Research and Development Center in Ibaraki Prefecture caused internal radiation exposure in five workers, although no harmful consequences were detected in the surrounding environment.
Level 2 on the International Nuclear and Radiological Event Scale is defined as an “incident.” The Fukushima nuclear accident, triggered by the huge earthquake and tsunami in March 2011, was rated at the maximum level 7, on a par with the 1986 Chernobyl disaster.
In June, one of the five workers — a male in his 50s — opened a container in a storage room at the Oarai facility, causing a plastic bag inside the container, which was holding plutonium and uranium powder samples, to rupture.
The worker continued the check even after he noticed the plastic bag swell, resulting in the inhalation of radioactive substances. Tests have found small quantities of radioactive materials — plutonium and americium — in the urine of the five workers, confirming they suffered internal radiation exposure.
The National Institutes for Quantum and Radiological Science and Technology, which suggested in mid-June that initial exposure estimates may not have been reliable, has since estimated the worker in his 50s will be internally exposed to a radiation dose between 100 and 200 millisieverts in total over 50 years, a level that could slightly increase the risk of cancer.
JAEA had assumed immediately after the incident the worker’s level of radiation exposure could extend to 12 sieverts over 50 years, a significantly higher figure and well above the legal limit set for workers who deal with radiation.
The agency initially detected up to 22,000 becquerels of plutonium 239 in his lungs, and up to 14,000 becquerels of radioactive materials in three other workers.
Local labor authorities also said they estimated the man with the highest exposure to radiation had exceeded the annual limit for radiation exposure.
NRA Chairman Shunichi Tanaka urged the JAEA to take measures to prevent further accidents, saying it is “responsible for ensuring the safety of workers.”
The agency came under fire when a number of sloppy work practices were revealed at the Oarai facility following the June 6 incident. The agency had not checked the container of powder samples for 26 years since it was sealed in 1991, and the workers involved in the accident had failed to use a special device to ensure there were no openings between the protection masks and their skin.
At the time of the accident, the agency was conducting checks on powder sample containers at the Oarai facility in response to inspections last year, when the Nuclear Regulation Authority found radioactive materials being stored in ways that contravened guidelines at the Oarai facility and six other facilities. In one case, the NRA found radioactive substances had been inappropriately stored for more than 35 years.