The Tokaimura Accident:
Part I: Nuclear Power and Nuclear Fuel Reprocessing in Japan
Since the worldwide oil crises of the early 1970s, Japan's energy policy has focused on decreasing the country's reliance on foreign fuel imports. With few natural resources, Japan has embraced nuclear energy and currently derives approximately 36% of its electricity from nuclear power. Japan has made substantial investments in nuclear reactor technology. As part of its goal of energy self-sufficiency, Japan has maintained a policy of reprocessing spent nuclear fuel rather than treating or disposing of it as waste.
The Tokaimura nuclear fuel processing plant is operated by JCO Company Ltd. and is located approximately 120 kilometers (70 miles) northeast of Tokyo. JCO is a subsidiary of Sumitomo Metal Mining Company Ltd. and employs approximately 140 persons on a multi-building 0.16-km2 (40-acre) site. The JCO plant is one of 15 nuclear facilities in Tokaimura. Approximately one-third of the population of Tokaimura is employed by the nuclear industry. The main function of the JCO plant is to convert isotopically enriched uranium hexafluoride into uranium dioxide fuel. This is one step in the process of making reactor fuel rods for some of Japan's commercial nuclear power plants. The uranium used in the process has been enriched to contain up to 5% of the fissile isotope, 235U (compared to the relatively inert isotope 238U).
Heavy nuclei, such as uranium, can split into two lighter nuclei accompanied by an energy conversion of 200 MeV. This process is called fission and most of the energy released goes into the kinetic energy of the fission fragments. Some of the energy appears as decay products (beta and gamma rays) and the kinetic energy of neutrons emitted in the fission process.
The JCO plant occasionally purifies uranium to be made into fuel for an experimental fast-breeder reactor known as Joyo, which requires fuel enriched to 18.8% 235U. These higher levels of enrichment require greater precaution because of the higher probability of accumulating a critical mass. A critical mass implies that enough 235U has been amassed that at least one neutron from each fission, on average, stimulates another fission. Japan's Science and Technology Agency (STA), which licenses nuclear facilities, had adopted regulations that placed a mass limit of 2.4 kg (5.3 lbs) on the amount of 18.8% enriched uranium that could be processed at one time at the JCO plant. The license was issued in 1980.
The uranium fuel for the Joyo facility requires a chemical purification procedure. The uranium oxide purification procedure licensed by STA for the Joyo fuel involves feeding small batches of uranium oxide, U3O8, in powder form into a dissolving tank, where it is mixed with nitric acid to produce uranyl nitrate, UO2(NO3)2. This solution is then transferred to a buffer tank. The buffer tank serves a mixing function and has a geometrical design to prevent criticality. From there, it is sent into a precipitation tank, where ammonia is added in the form of an ammonium salt solution, to form a solid product, ammonium diuranate (NH4)U2O7. Any contaminants should remain in solution. Uranium oxide is extracted from the solid precipitate. The purified uranium oxide is reprocessed in the dissolving tanks until the uranium oxide becomes sufficiently pure. The uranium oxide is then converted to uranyl nitrate, transferred to a storage container, and shipped to another facility, where uranium dioxide, UO2, is prepared and made into Joyo fuel. It is important to note that the purification process does not enrich the uranium isotopes but simply removes the chemical impurities. In recent years only Japan and Kazakhstan have utilized this type of wet process technology.
Date Posted: 06/25/01 mb