The Nuclear Regulation Authority said Wednesday that it provisionally assessed the severity of a nuclear exposure accident that occurred in June to be level 2 on the zero-to-seven international scale.
The June 6 accident at the Japan Atomic Energy Agency’s 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 that 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 that 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.
The Japan Atomic Energy Agency had assumed immediately after the incident that 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 that the man with the highest exposure to radiation had exceeded the annual limit for radiation exposure.
NRA Chairman Shunichi Tanaka has 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 that radioactive substances had been inappropriately stored for more than 35 years.
Events involving nuclear exposure previously assessed as level 2 incidents in Japan include a critical-state accident at the No. 1 reactor of Hokuriku Electric Power Co.’s Shika nuclear plant in 1999. The utility concealed the accident, which had occurred while the unit’s operations were suspended for regular checks, until 2007.
Tokyo also rated as level 2 a 1991 accident in the No. 2 reactor at the Mihama plant, run by Kansai Electric Power Co., in which a tube within a steam generator snapped, activating the emergency core-cooling system for the first time in the country.
A 1999 accident at a nuclear fuel processing plant in Tokai, Ibaraki Prefecture, which led to the death of two employees due to radiation sickness, was rated as level 4.