A "Blue Flash" Hits Tokai-mura.
The accident at Tokai-mura points to the dangers inherent in an often-overlooked part of the nuclear industry -- nuclear fuel production. The production of fuel for electricity-generating reactors routinely exposes workers to radiation. Criticality incidents are among the types of accidents that can occur when handling low-enriched uranium.
According to news reports, the Tokaimura workers were handling uranium enriched to around 18 percent uranium 235, in preparation for fabricating mixed oxide fuel for the Joyo research reactor. Electricity-generating reactors usually use uranium enriched to around 4 percent.
A criticality accident can occur with uranium enriched to concentrations as low as 1 percent U 235 (natural uranium contains 0.71 percent uranium 235). The amount of uranium that can be stored safely in a given place depends on several factors. These include: the enrichment level of the uranium, the geometric shape of the material, and whether the uranium is close to any neutron-slowing substances -- e.g., organic solvents, plastics or the hydrogen atoms in water. Any well-regulated nuclear facility has rules specifying how much uranium can be in any given place.
Criticality incidents occur frequently in nuclear fuel plants. Of the 67 reportable events that occurred at the US Department of Energy's Paducah, Kentucky enrichment plant from October 1997 through September 1998, 37 involved nuclear criticality safety. (A reportable event is one that must be reported to the Nuclear Regulatory Commission.) During the same period, 155 of the 183 reported events at the DOE's Portsmouth, Ohio facility involved criticality safety.
Such incidents, which may involve nothing more than the careless juxtaposition of two containers of uranium, are accidents in the making. The only reason that they are not full-blown accidents is that criticality regulations include a significant margin of error designed to limit the amount of uranium workers handle to concentrations that are considerably less than critical mass. If a worker exceeds the rules by a small percentage, no accident occurs. Severe accidents happen when workers err on an unexpectedly large scale, as happened at Tokai, when workers poured about seven times the permitted amount of uranium into a container.
A criticality accident would not usually spread radioactivity far outside a plant as apparently happened at Tokai. Also it would not usually last anywhere near the 20 hours that the fission reportedly continued at Tokai. The criticality event tends to scatter the fissile material and fission products in the area of the accident, and the dispersal of the fissile material normally prevents a prolonged chain reaction.
There are eight stages in the normal fuel chain: (1) mining; (2) concentration of the mined ore; (3) refining and conversion of the ore to uranium tetrafluoride; (4) conversion of the uranium tetrafluoride to uranium hexafluoride (a solid at room temperature; a gas when heated); (5) enrichment of the uranium hexafluoride; (6) fabrication of uranium hexafluoride into fuel rods and fuel assemblies; (7) installation of the fuel rods in a nuclear reactor; (8) management of the used fuel rods and other radioactive waste.
A criticality may occur during conversion or at any of the later stages. The NRC underlined the need for safe management of enrichment plants, when it insisted that United States Enrichment Corporation (USEC) pay a higher annual certification fee than is usually required from facilities that handle low-enriched uranium. In the June 2, 1997 issue of Nuclear Fuel, the NRC explained that it was concerned that gaseous diffusion plants "are subject to a relatively large number of credible accidents, most of which have multiple initiating events. The potential on-site and off-site consequences posed by these accidents are significantly greater than those applicable to low-enriched uranium facilities."
The Tokai-mura incident was called Japan's "worst nuclear accident." Ironically, Japan's last "worst nuclear accident" also occurred at Tokai-mura. On March 11, 1997, a fire and explosion at the plant released a cloud of smoke possibly containing uranium and plutonium. Investigations disclosed that no fulltime plant employees were on duty at the time of the fire and seven maintenance workers were off-site playing golf. Smoke detectors had been turned off and the sprinkler system was not automatic -- it had to be operated by hand.
Plant operators failed to warn nearby residents and even failed to evacuate 64 visitors who were on a tour of the plant when the fire broke out. When Prime Minister Hashimoto learned that the plant operators had lied to the government, he ordered a police raid on the company's headquarters. Five officials eventually were demoted for falsifying reports.
The Tokai-mura accidents were not isolated events. On October 4, 29 workers were exposed to high levels of radiation during a leak at a power plant in South Korea. Greenpeace International observed that such accidents demonstrate "the lack of a safety culture within the nuclear industry globally" and has called on world governments to "change their energy policies by ending nuclear programs ... and moving into energy efficiency and renewable energy."
The Uranium Enrichment Project [PO Box 131 Georgetown, KY 40324 (502) 868-9074, www.earthisland.org/yggdrasil] conducts research and educates the public about nuclear issues, in particular those relating to the US enrichment establishment. For the latest news on Tohai-mura, see the website of Japan's Citizens' Nuclear Information Center: www.jca.apc.org/cnic.
Mary Byrd Davis, Ph.D. (firstname.lastname@example.org), is the director of the Yggdrasil Institute's Uranium Enrichment Project, an Earth Island Institute project.
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|Author:||Davis, Mary Byrd|
|Publication:||Earth Island Journal|
|Article Type:||Brief Article|
|Date:||Mar 22, 2000|
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