Tokaimura: Accident Waiting to Happen
by Melissa Lovin
In September of this past year workers in Japan's Tokaimura plant unleashed a chain of events that resulted in the worst nuclear accident since Chernobyl. By bringing a critical mass of highly enriched uranium together they broke the first rule of working in a nuclear facility.
Nuclear experts have looked on in disbelief as it was revealed that the unit did not have built-in safeguards that would have made such errors impossible. The methods in use at the Tokaimura plant were also viewed as highly risky. Apart from Japan, the only other location to use similar procedures is Kazakhstan.
The accident occurred in the plant, run by the company JCO, located 150 km northeast of Tokyo with the actual event taking place in a building used for one step in the process of making reactor fuel rods. This involves adding an oxide of uranium, U3O8, to nitric acid. The solution is then mixed in a sedimentation tank with a solution of ammonium salt to for a precipitate of ammonium diurante. Later, this is processed to form uranium dioxide fuel.
The U3O8 and nitric acid were supposed to be mixed in an elongated container, designed to keep a critical mass of uranium from coming together, before being fed into the sedimentation tank in carefully controlled quantities. Automated checks should have ensured that a critical mass could not be added twice in the same process.
The details of what happened on September 30 are somewhat unclear, but it appears that workers mixed U3O8 and nitric acid and then poured the solution into the tank using buckets. Jinzaburo Takagi, head of the Citizens' Nuclear Information Centre in Tokyo until 1998, says "The way they did this was incredible."
After the seventh bucket, the sedimentation tank contained 16 kilograms of uranium, enriched so that 18.8 percent was the fissile isotope uranium-235. At 10:35 am, it formed a critical mass and started a chain reaction. The workers nearest the tank would have seen the flash of blue light as the air was ionized by a blast of radiation.
The uranium was forced apart by the first chain reaction and soon fizzled out. But the critical mass later reassembled, starting another chain reaction and releasing more neutrons and gamma radiation. This cycle repeated itself several times over many hours.
This process was facilitated by the water in the sedimentation tank and the cooling jacket surrounding it. Water acts as a "moderator" causing the neutrons released by decaying uranium-235 nuclei to slow so that they are more likely to trigger another nucleus to decay. Neutrons were reflected back into the tank by the water jacket as well.
The response of JCO and the Japanese government to this nearly catastrophic chain of events has been called sluggish at best. The initial reaction began at !0:35 am on September 30. At 11:15 the Science and Technology agency was informed of the accident. Evacuation of the residents within 350 meters of the plant began at 3:18pm. The governmental crisis management task force was not set up until 9:00pm. It was not until 2:30 am on October 1 that JCO staff began implementing their plan to bring the sedimentation tank under control.
The reactions should have immediately ceased if the water in the cooling jacket was drained. But this was realized far into the process and proved difficult to implement. Workers were forced to finally dismantle pipes leading from the jacket to drain it, rushing into the building filled with radiation for only minutes at a time. The chain reaction finally stops at 6:15 am on October 1.
One possible cause for the accidents is that workers were not used to handling highly enriched uranium. According to JCO, they only began processing the material to produce fuel for a fast breeder reactor on September 22 after a three-year break.
Even so, workers should not have been pouring the highly enriched uranium into the sedimentation tank by hand. In fact, the plant should have been engineer to keep them from doing so entirely. "What totally baffles me is why the technology allowed these workers to do what they did," says Malcolm Grimston, a nuclear analyst at the Royal Institute of International Affairs in London.
Fact is, this is not the first error in Japan's nuclear industry. In December of 1995 8 tons of molten sodium coolant leak from an experimental fast breeder reactor in western Japan. An investigation reveals that managers doctored the surveillance video to hide the extent of the damage.
In March of 1997 two fires broke out, ten hours apart, in barrels containing low-level radioactive waste at Tokaimura. Thirty workers are exposed to radiation, and seven maintenance staff are later found to have been out golfing. In August of the same year rainwater is discovered seeping into a storage pit containing drums of high-level waste at the Tokaimura plant. The investigation reveled that Donen, the Power Reactor and Nuclear Fuel Development Corporation, had cut millions of dollars from the budget for constructing and maintaining the pit. In May of 1998 Donen is disbanded and replaced by Genden (Japan Atomic Power Company); however, most Donen staff and offices are retained by Genden.
Finally, in July of 1999 20 tons of radioactive coolant water leaks from a crack in a cooling pipe at the Tsuruga power station northwest of Tokyo.
These lapses individually would have been bad enough, but together they are causing widespread fear that Japan's nuclear industry lacks a commitment to safety. This may be best witnessed in the primitive procedures for processing highly enriched uranium. Due to the extra hazards moderating with water brings, most manufacturers use a "dry" process to prepare highly enriched uranium. Only Japan and Kazakhstan still hang on to the aqueous processing.
It is likely that two of the most severely irradiated workers are certain to lose their lives. The extent of contamination inside and surrounding the plant are still unknown. According to Murdoch Baxter, editor of the Journal of Environmental Radioactivity, " You cannot eliminate the possibility of more deaths." Even if there are no more deaths the name Tokaimura will evoke fear for many in the years to come.