On 30 September, as workers at a Japanese nuclear fuel processing plant in Tokaimura were adding enriched uranium to a precipitation tank, they saw a blue flash—signaling the onset of a nuclear chain reaction. Exactly how did this accident occur? It will be some time before we get an official report and learn about the accident’s impact on Japan’s nuclear power program. (See previous story.) However, as Physics Today goes to press, independent analysts have already gleaned enough information from preliminary accounts such as those posted on the Web by Japan’s Science and Technology Agency (STA), which licenses nuclear facilities1, to piece together a picture of how a solution containing enriched uranium became critical, what power levels were reached, and what releases of radiation ensued.
The purification procedure licensed by STA for the Joyo fuel is shown by the blue lines in the figure above. The workers feed uranium oxide (U3O8) in powder form into a dissolving tank, where it is mixed with nitric acid to produce uranyl nitrate, or UO2(NO3)2, which is then transferred to a buffer tank. From there, it is sent into the precipitation tank, where ammonia is added to form a solid product (with contaminants remaining in solution). Uranium oxide is extracted from that solid, and the process is repeated until the oxide becomes sufficiently pure. At that point, the uranyl nitrate in the buffer tank gets shipped to another facility, where uranium dioxide is prepared and made into Joyo fuel. On the day of the criticality accident, workers were running fuel through the last steps of this process, according to Thomas McLaughlin of Los Alamos National Laboratory, one of three nuclear experts sent by the US Department of Energy to learn about the accident. The JCO plant only needed to mix some high-purity enriched uranium oxide (U3O8) with nitric acid to form uranyl nitrate for shipping. During this operation, the workers deviated from the licensed procedure in three basic ways. First, to speed up the process, they mixed the oxide and nitric acid in 10-liter buckets rather than in the dissolving tank (in doing so, they followed the practice that JCO had written into its manual—without STA approval). Second, for convenience, they added the bucket contents to the precipitation tank rather than to the buffer tank. That was a key misstep, because the tall, narrow geometry of the buffer tank precludes criticality. Third, in filling the precipitation tank, the crew added seven buckets, or roughly seven times more uranium than permitted by the STA license. It was the seventh bucket that caused the mixture to go critical.
According to Shunsuke Kondo, a nuclear safety expert from the University of Tokyo, who has done an independent analysis of the accident, the crew assigned to process the Joyo fuel that day was under time pressure: The crew chief was anxious to complete the current batch before a new team of workers arrived. Furthermore, Kondo reports, the workers were apparently not aware of the mass limitations on the uranium to be added to the precipitation tank.
A secondary effect of the water jacket may have been to prolong the chain reaction. Per Peterson and Joonhong Ahn of the University of California, Berkeley, point out that, without the water jacket, the heat generated by the chain reaction and the dissociation of water into hydrogen and oxygen would have expanded the solution, decreasing its density and slowing its reaction rate. With the water jacket in place to remove the fission heat roughly as fast as it was generated, however, the solution may have been kept just above the critical density.
Judging from the levels of gamma and neutron radiation measured near the plant perimeter, the criticality excursion seems to have lasted about 20 hours; after that time, the radiation levels dropped below detection limits. The chain reaction was shut off by draining the cooling water out of the jacket and made safe by adding boric acid.
According to STA, the three workers in the room at the time the precipitation tank went critical received doses of 17, 10, and 3 sieverts (the doses were deduced from the levels of radioactive sodium in the victims’ bodies). (One sievert, which equals 100 rems, is a measure of the biological response to the absorbed radiation.) Doses of 10 and 17 Sv are above the levels normally considered fatal, but the two workers who received such high doses were still alive at press time, perhaps because they were treated with blood stem cell transplants. The worker who received a dose of 3 Sv did not require transfusions and is expected to recover fully. In addition to these three severe cases, there were 66 individuals—plant workers, firemen, and others who responded to the accident, and a few city residents—who were exposed to measurable levels of radiation. Most criticality accidents in the past haven’t involved exposures of private citizens, but the JCO facility is sited quite close to the surrounding town. Within the 350-meter radius evacuated immediately after the accident, there were 47 houses and 150 people.
Monitors placed at a number of sites outside the plant detected the radiation levels. At one of the closest monitoring sites, STA reported dose rates of 4.5 mSv/hr for neutrons and 0.50 mSv/hr for gamma rays about 11 hours after the onset of criticality. That gamma dose rate was about 1000 times higher than the normal background level.
magnitude of the accident
McLaughlin has been working over the past year to update a report on criticality accidents around the world by incorporating data now available on accidents in the former Soviet Union. Although he can’t yet say what happened at the JCO plant, he did refer us to the list of “lessons learned” from past accidents. He noted that what many of the accidents have had in common have been failures in communications and operator training, improper procedures, lack of fissile-material accountability, and new or unfamiliar operations. Judging by the standards in the US, McLaughlin said, it appears that, in the Tokaimura incident, regulatory agencies and plant managers were not diligent in following approved procedures.
The entire JCO plant,
not just the purification operation, is now shut down, and STA has revoked
JCO’s operating license for the plant. Various investigations by government
agencies are under way.
--Barbara Goss Levi