Tokaimura: Japan's Neutron "Bomb"
By John LaForge
In the middle of a crowded residential neighborhood in Tokaimura, Japan, manufacturers of nuclear reactor fuel caused a disaster September 30 that sent a billowing cloud of alpha, beta and gamma radiation into the atmosphere and spewed deadly neutrons* several kilometers from the site. It was the world's 60th uranium fuel fire, or "criticality," and it burned through the factory roof, according to National Public Radio. The 20-hour-long fire raised radiation levels to 20,000 times normal near the factory and 15,000 times normal 1.2 miles away.
|How It Happened
The Tokaimura factory completes one step in the process of making reactor fuel rods. The workers are supposed to remotely mix uranium oxide, enriched to 18.8 percent fissile U-235, with nitric acid. This high concentration of U-235 raises the risk of criticality, and the mixing team was doing it by hand using an illegal manual.
At 10:35 a.m. the men, using buckets, poured 35 pounds of the uranium -- eight times the proscribed amount -- in a tank where it formed a critical mass and started a chain reaction. Repeatedly for the next several hours, this mass fissioned and blew apart, producing neutrons and gamma radiation, then reformed in the tank to start another reaction.
The out-of-control reaction spewed radiation for hours, making the area too radioactive to allow monitors to approach the factory from which all staff had been evacuated.
Water in the mix and in a moderating jacket around the tank kept the reaction going by slowing neutrons and allowing them to split other atoms. The water jacket also worked to ricochet the neutrons back into the tank, splitting still more uranium atoms.
The daunting problem that went unsolved for 20 hours was how to drain away the water. New Scientist reports that it was 2:30 a.m. before JCO managers initiated a plan to remove the water. Then, after several failed attempts to remotely drain the tank, workers ultimately "rushed into the irradiated building for just minutes at a time" and deliberately broke apart the drainpipes.
This halted the chain reaction. The building itself remains so radioactive that it may never reopen.
The disaster was marked by an amazing lack of emergency preparedness and confused government responses. The federal authorities in Tokyo and the municipal government in Tokaimura issued conflicting advice on when it was safe for locals to return home. Seven days after the disaster, the Japanese Agriculture Ministry dared to say that iodine-131 and iodine-133 found 160 yards from the facility -- proof of radioactive fallout -- posed no threat to people eating local produce. Iodine-131 is dangerous for 80 days and damages the thyroid. Local police and television had initially warned people not to harvest crops.
Neutron radiation is extremely hazardous. It can penetrate cars, buildings and the body, where it can damage DNA and cause cancer. Signs of neutron radiation from the fire were found inside a bank 350 yards away, where coins were irradiated. A gold bracelet found 800 yards away was likewise irradiated. Greenpeace said it found radioactive sodium-24 in salt collected from inside houses 175 yards away. The passing neutrons caused the ionization of table salt. (The neutron bomb is designed to produce a huge burst of this radiation, poisoning people while destroying less property than an ordinary H-bomb.)
Hitoaki Koide of Kyoto University told the Los Angeles Times, "Of course, the people who were within 500 yards of the plant were irradiated. The only question is the degree."
Three factory workers had been illegally hand-mixing a refined uranium solution when it spontaneously began fissioning. Hisashi Ouchi, 35, and Masato Shinohara, 39, are in critical condition having received what is normally considered a fatal radiation dose. Ouchi received what's thought to be 17,000 times the average annual radiation exposure.
The team had combined eight times the usual amount of enriched uranium used in the operation and caused the mixture to go critical.
The government board responsible for the nuclear industry, the Science and Technology Agency, hadn't inspected the site for ten years. The agency confirmed that the workers had been illegally shortcutting the process since 1993. The disaster occurred in the JCO Co.'s "conversion experiment building," which processes highly enriched uranium into powder for nuclear reactor fuel. The fuel was being made for Japan's fast-breeder reactor program that has a disaster record of its own. In December 1995, the Monju fast-breeder leaked eight tons of radioactive liquid sodium and has been closed ever since.
During the Tokaimura "criticality," factory operators floundered without plans, equipment or emergency response help for nearly five hours before ordering the evacuation of houses within 350 yards. Monitoring of deadly neutron radiation did not begin until 61/2 hours later. It was 12 hours before hapless officials using municipal loudspeakers told people in a six-mile radius -- 310,000 residents -- to stay indoors, turn off fans and air conditioners, and wipe off any rain that fell on them. It was 20 hours before the out-of-control nuclear chain reaction and spreading of contamination was declared officially over.
The government estimates that 69 people were seriously contaminated: 59 workers, three rescue personnel and seven nearby golf course workers. Greenpeace and other environmental groups charge that the number is actually in the hundreds, and that the government is worsening the disaster by not providing adequate health checks.
Greenpeace has demanded that blood tests be conducted on everyone within 3/5 mile of the factory and that a registry of radiation victims be set up. State Geiger counter tests only detect surface radiation and cannot identify neutron exposure.
Tokaimura, a city of 33,000 known as "nuclear alley," is home to 15 nuclear fuel, radioactive waste and reactor facilities. In 1997 it was the site of Japan's worst accident -- a plutonium waste explosion that blew all the windows out of a four-story building and irradiated at least 37 workers.
The residents of Tokaimura have expressed shock at the haphazard response of the company and the government. The Boston Globe reports that many found it hard to believe radioactivity 20,000 times normal near the factory had suddenly evaporated a day later. Doctors in Japan told the Associated Press that no one can rule out long-term health consequences of exposure to even low levels of radiation.
*The splitting or fissioning of uranium releases neutrons that act like microscopically small bullets. Neutrons can penetrate the human body, aluminum, lead and concrete, and are among the most biologically destructive fission products. (Rosalie Bertell, No Immediate Danger)