The Tokaimura Nuclear Accident

Before Fukushima, the Tokaimura accident was the worst nuclear event in Japan and the third-worst in the world (behind Three Mile Island and Chernobyl). Here is the story.


Location of the Tokaimura nuclear facility

In 1979, nuclear power was all the rage. To get in on what was assumed to be a booming business, the huge Sumitomo Metal Mining Company in Japan established a subsidiary called the Japan Nuclear Fuel Conversion Company, later shortened to JCO. JCO was licensed to turn mined uranium into low-enriched fuel for nuclear reactors. It established its main plant next to the city of Tokaimura, in Ibarakin Prefecture, about 60 miles from Tokyo.

In 1983, JCO received a contract to make fuel for Japan’s planned fast-breeder reactor project, which was designed to produce plutonium as it burned its fuel for electric power, and thus make more fuel than it used. JCO constructed a new building at its Tokaimura plant to make the fuel. This was to be enriched to 20 percent Uranium-235.

In nuclear fission, an atom of uranium is struck by a neutron, which causes the uranium nucleus to split into two pieces, release a large amount of energy, and eject two or three new neutrons. In small amounts of uranium, those neutrons will simply escape and fly off. But if the mass is big enough, these neutrons are able to hit other uranium nuclei before they escape, releasing more energy and more neutrons. The reaction becomes a self-sustaining chain reaction. The amount of uranium necessary to produce a chain reaction is known as the “critical mass”. ¬†The actual amount needed for a critical mass depends on several things. One of these is geometry–the more surface area there is relative to the volume, the more easily neutrons can escape before hitting a uranium nucleus. If you take a suitable mass of uranium and shape it like a large thin flat pancake, therefore, it has a high surface area and will remain safe, while if you re-arrange the same amount of uranium into a sphere, which has less surface area, it can suddenly go critical. Another factor is what surrounds the uranium. When neutrons strike particular materials, they are slowed down by the collision, which makes it easier for them to hit more uranium atoms. In nuclear reactor design, this property is taken advantage of by surrounding the nuclear fuel with a “moderator” which slows the neutrons down and allows them to hit more uranium nuclei, requiring a much smaller critical mass. In the earliest reactors, the moderator was a lattice of graphite blocks surrounding the nuclear fuel; in most modern reactors, the reactor core is surrounded by ordinary water (which also acts as the coolant).

The primary danger in any nuclear fuel facility is accidentally arranging the uranium into a configuration in which enough of it is present in the same place to interact with each other to go critical and produce a nuclear fission reaction. This can happen if, for instance, you stack enough small containers of uranium together in a corner of the room, or if you accidentally flood a pile of uranium with water which then acts as a moderator. These are known as “criticality accidents”. When they happen, they in effect produce a small nuclear reactor, bathing the surrounding area with radiation from free neutrons, gamma rays, and fission products.

When JCO built its Fuel Conversion Building at Tokaimura in 1983, it carefully designed the whole process to prevent the possibility of a criticality accident. Each step of the process would be carried out in extremely small steps, using tiny amounts of material that would not be enough to go critical. Even the storage containers used to carry the diluted materials were carefully designed as tall thin cylinders to have the maximum surface area possible and allow as many neutrons as possible to escape. Because the process was so carefully designed, it was assumed that a criticality accident was impossible, and the new plant was brought into production in 1988 and ran for years without incident.

But by 1995, the nuclear industry was in decline, and JCO received fewer and fewer contracts. For three years, the Fuel Conversion Building sat idle. Finally, in 1999, JCO won another contract to convert a batch of 16.8 kilograms of 18.8-percent enriched enriched uranium into a liquid form called uranyl nitrate, for shipping to the experimental fast breeder reactor. In this process, small batches of 2.4 kilograms of uranium oxide powder were poured together with nitric acid into a dissolving tank, and uranyl nitrate slowly dripped out the bottom, to be carried by a stainless steel pipe into a tall thin storage container. From here, the uranyl nitrate could be drained from a valve into plastic bottles holding four liters of the diluted liquid, which were stored at a set distance from each other to prevent interaction. These small bottles were then dumped into a Precipitator Tank for further purification. As long as the process was followed, there was no chance of any accidental buildup of a critical mass of uranium.

But the plant did not follow the process.

Instead of using the dissolving tank to mix the uranium oxide and nitric acid, the workers at the plant began mixing the two ingredients together directly by hand in large ten-gallon stainless steel buckets, which could then be poured right into the Precipitation Tank, bypassing the Dissolving Tank altogether. This was a lot faster than waiting for the mixture to drain through the Dissolving Tank, and it also allowed larger amounts of uranyl nitrate to be put through the Precipitator at one time. As a result, when work on the new contract began on September 29, 1999, the plant’s management decided to use this new improvised process (which had never been submitted to or approved by any regulatory agency) instead of the approved standard process. It was a fatal mistake.

For the first day, nothing happened. Three employees–Hisashi Oushi, Misato Shinohara, and supervisor Yutaka Yokokawa–were assigned the task of mixing up the uranyl nitrate and pouring it into the Precipitator. By the second day, September 30, they were almost finished. The Precipitation Tank was holding about 45 liters of liquid uranyl nitrate solution, with 18.8 percent enriched uranium. At 10:33 am, Oushi poured the last remaining solution into the tank.

The Precipitation Tank was a stainless steel vessel that was surrounded by another metal shell containing water, intended to carry away the heat from the chemical reactions that happened in it. Because the process was designed to use only small amounts of uranyl nitrate at a time from the Dissolving Tank, the Precipitator was made in a round shape–giving maximum volume for minimum construction material. Unfortunately, it was also the ideal shape for allowing the minimum amount of neutrons to escape from a mass of enriched uranium. When the workers by-passed the Dissolving Tank and poured their uranyl nitrate directly into the Precipitator, they had filled it with far more material at one time than it was designed for. The round shape of the tank formed a geometry that prevented neutrons from escaping, and the water coolant surrounding the tank acted like a moderator, slowing neutrons down, reflecting them back into the uranium mass, and increasing the fissions. When Oushi poured the final bit of liquid into the tank (through a glass funnel being held by Shinohara), it was enough to allow the liquid mass of enriched uranium inside to go critical. It had, in effect, created a small unshielded water-moderated nuclear reactor.

As Oushi poured, the entire room was suddenly lit by a bright blue flash, like a photographer’s flashbulb, as the “reactor” went critical. Instantly, billions of neutrons and gamma rays streamed out of the fissioning uranium. Oushi and Shinohara felt a wave of heat, staggered down the steps from the tank, and within seconds began feeling sick with stomach cramps and vomiting. Yokokawa, sitting at a desk a short distance away, turned to see what had happened, then helped Shinohara carry Oushi, who was losing control of his muscles, out of the room.

The impromptu nuclear reactor, meanwhile, continued to spew out neutrons and gamma rays. The radiation alarms in all three buildings at the plant screamed, and everyone evacuated. After a time, the heat from the nuclear fissions boiled the water in the surrounding cooling jacket, which made gaps in the water for the neutrons to escape and ended the reaction. But as the water then cooled, the bubbles disappeared, the water began moderating the neutrons again, and the reaction restarted. For the next 18 hours, the impromptu reactor started and stopped as the water cooled and reheated. The cycle continued until workers were finally able to find a way to reach the device from outside the building and drain the water to end the reaction and make the mass subcritical.

During this entire time, neutrons and gamma rays from the chain reaction were streaming outwards in all directions, and the boiling water and fission products were carried out of the building in the steam. Gamma radiation and fission products were detected outside the plant’s fences in the city of Tokaimura, and advisories were issued by the Japanese government, first evacuating everyone within 350 meters of the plant, then advising that everyone within 10 kilometers of the plant stay indoors.

Oushi, the plant worker who had been pouring the liquid uranium into the container, received the largest dose of radiation, and died of acute radiation poisoning 82 days later. Shinohara, who had been holding the funnel next to him, died 210 days after the accident. Yokokawa, the supervisor who was in the room with them, was hospitalized for two months and recovered. Over 200 plant workers, government officials, and emergency personnel were exposed to varying levels of radiation at the site, as well as an unknown number of Tokaimura’s 35,000 residents who lived near the plant. Measured radiation levels near the plant on the day of the accident reached as high as 1,000 times the normal background level. Within two weeks, however, levels outside the plant had decayed back to background level, while the inside of the building remained irradiated. The Japanese government undertook a longterm program to monitor the health of people exposed (including the 200 civilians who were closest to the plant), but as of 2014 there have been no abnormal increases in the number of sicknesses or deaths that could be attributed to radiation exposure.

The official investigation by the Japanese Government skewered JCO for deviating from established procedures, concluding that the unauthorized changes had been made to hurry the process and cut costs. Six of the company’s executives subsequently pleaded guilty to criminal charges of negligence resulting in the deaths of the two employees.

In 2003, with the Japanese economy in decline, JCO ceased operations and the Tokaimura plant was closed down.

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