23 Juin 2017
June 18, 2017
Another radiation exposure accident
A radiation accident earlier this month at the Japan Atomic Energy Agency’s facility in Oarai, Ibaraki Prefecture, underlines the need for operators of facilities handling radioactive substances to make sure there are no flaws in their safety systems and procedures. Such caution is all the more important since Japan will have to manage large amounts of radioactive substances in decommissioning nuclear power reactors, including the agency’s fast-breeder reactor Monju, which the government decided last December to take out of service.
The accident occurred when five workers were taking stock of 300 grams of uranium oxide and plutonium oxide put in a cylindrical stainless steel container at the Plutonium Fuel Research Facility in the agency’s Oarai Research and Development Center. The powdery substances had been encased in a double-wrapped plastic bag placed inside the container, whose lid was fastened with six bolts. When one of the workers opened the lid, the black powder sprayed out under pressure, exposing the men to radiation. The five workers were admitted to the National Institute of Radiological Sciences’s hospital in Chiba for treatment.
The fiasco brings to mind the 1999 criticality accident at a nuclear fuel processing facility operated by JCO Co. in Tokai, Ibaraki Prefecture, which killed two workers — the worst nuclear radiation accident in Japan prior to the 2011 meltdowns at the Fukushima No. 1 power plant. The fatal accident occurred when three workers were preparing a small batch of nuclear fuel using uranium enriched to 18.8 percent. They were handling the nuclear fuel in stainless steel buckets. The company apparently failed to give workers proper safety training, and sloppiness was the clear cause of the accident.
Sloppiness cannot be ruled out in the Oarai facility accident either. It must be noted that the incident occurred when the agency initiated work to examine the radioactive substances following an order by the Nuclear Regulation Authority to improve its operations. The NRA issued the order to the Oarai facility and six other facilities handling radioactive substances after it found that they had kept spent uranium and plutonium over an extended period in equipment not designed for long-term storage, in violation of the regulations. In one case, radioactive substances had been stored this way for more than 35 years.
The container at the Oarai facility had not been opened for 26 years. Plutonium emits alpha particles, which are helium nuclei, and it decays or transforms into a different type of radioactive substance. It is suspected that the extended storage caused helium, which was formed as a result of the plutonium’s alpha decay, to fill the container and pressurize the contents. The agency had no guidelines on how frequently the substances inside the container should be examined. In this connection, it must be pointed out that Japan has no official rules on the final disposal of radioactive substances used for research purposes like those at the Oarai facility.
In the room where the radiation exposure accident occurred, 55 becquerels of radioactive substances were detected — roughly 14 times the allowed limit of 4 becquerels. It was also found that the five workers were kept in the room for three hours following the accident until preparations for decontamination work were completed. One wonders whether the agency could not have acted more quickly. The five workers had been wearing masks, gloves and other protective gear when the accident occurred. It must be determined whether the agency had trained them in the proper use of the gear as they may have inhaled radioactive substances through small gaps between the masks and their faces.
At first, the agency reported that up to an unprecedented 22,000 becquerels of plutonium was detected in the lungs of the worker who opened the container — which translates into 1.2 sieverts over a year, far above the maximum of 0.05 sievert per year allowed by the government for designated nuclear workers. But later the agency corrected the report by saying that the plutonium may have been on his skin, not in his lungs. This raises the possibility that the agency failed to adequately decontaminate the worker’s skin.
A series of troubles at the Monju reactor since the leakage of sodium coolant in 1995 highlighted the lack of safety consciousness on the part of the agency and its predecessor. The Oarai accident points again to the same problem. The agency should fully disclose all information pertaining to the accident. To ensure safety, its operations should also be thoroughly examined, including the condition of stored radioactive substances and the work procedures for handling such materials. All the facilities in Japan using radioactive substances should follow suit. The NRA’s responsibility to oversee these facilities is heavy.