Tokaimura nuclear accidents facts for kids

The Tokaimura nuclear accidents were two serious events that happened near the village of Tōkai in Japan. The first accident was on March 11, 1997. It involved an explosion and fire at a facility that handled radioactive waste. More than twenty people were exposed to radiation.

The second, more serious accident happened on September 30, 1999. This was a criticality accident at a different facility that processed uranium fuel. It happened because workers handled the fuel incorrectly. This incident lasted about 20 hours. It led to radiation exposure for 667 people and sadly, two workers died. Many technicians had to go to the hospital with serious injuries.

It was found that these accidents happened because there wasn't enough safety checking. Also, the companies didn't have a strong safety culture, and workers weren't trained well enough. After these accidents, new safety rules were put in place. Japan's nuclear groups started checking facilities regularly. They also began teaching people more about how to handle nuclear materials safely. The company involved in the 1999 accident, JCO, lost its license. This was the first time a Japanese plant operator was punished by law for mishandling nuclear radiation.

The Tokai Nuclear Plant, Japan's first nuclear power station.

Japan's Nuclear Power Journey

Nuclear power was very important for Japan because the country doesn't have many natural resources. It helped Japan rely less on imported energy. Before the Fukushima nuclear disaster in 2011, nuclear power made about 30% of Japan's electricity. After Fukushima, nuclear power use went down a lot.

The village of Tōkai was a good place for nuclear power plants. It's about 70 miles from Tokyo and has enough land. So, experimental nuclear reactors were built there. Then, the Tōkai Nuclear Power Plant, Japan's first commercial nuclear power station, was also built in Tōkai. Many companies and government groups set up nearby. They did nuclear research, experiments, and made nuclear fuel. They also handled nuclear waste disposal. Nearly one-third of the people in Tokai work in jobs related to the nuclear industry.

The plant involved in the 1999 accident was built in 1988. It processed 3 tons of uranium each year. The uranium was enriched to a higher level than usual, up to 20% U-235. They used a special wet process for this.

1997 Nuclear Waste Accident

On March 11, 1997, the first serious nuclear accident in Tokai happened. It was at the PNC facility. This place turned liquid waste into a solid form using molten asphalt (bitumen) for storage. On that day, they were trying a new mix with 20% less asphalt. A chemical reaction inside one barrel started a fire at 10:00 a.m. The fire quickly spread to other barrels.

Workers couldn't put out the fire properly. Smoke and radiation alarms forced everyone to leave the building. At 8 p.m., flammable gases built up and exploded. This broke windows and doors, letting smoke and radiation escape into the area.

The accident exposed 37 people nearby to small amounts of radiation. The government called it the country's worst nuclear accident at the time. It was rated a 3 on the International Nuclear Event Scale. A week later, high levels of caesium were found 40 kilometers (25 miles) southwest of the plant. The building's roof was damaged, allowing more radiation to escape.

PNC managers told two workers to lie about what happened. They wanted to hide that there wasn't proper supervision. PNC leaders also didn't tell the Science and Technology Agency (STA) about the fire right away. This delay meant that emergency response teams were slow to arrive. This caused people to be exposed to radioactivity for longer. PNC officials first said radiation levels increased by 20%. Later, they admitted the true level was ten times higher. People in Tokai wanted PNC officials to be punished and the plant to be closed. After public anger, the facility closed. It reopened in November 2000 as a nuclear fuel reprocessing plant.

1999 Accident

Quick facts for kids Tōkai-mura Location of Tōkai-mura in Japan
Date	30 September 1999
Location	Tōkai, Ibaraki, Japan
Type	Nuclear criticality accident
Cause	Uncontrolled nuclear fission after uranyl nitrate overload
Outcome	INES Level 4 (accident with local consequences)
Deaths	2
Non-fatal injuries	667 contaminated
Inquiries	World Nuclear Association

The second, more serious Tokai nuclear accident happened about four miles from the PNC facility. It was on September 30, 1999, at a fuel processing plant run by JCO. This was the worst civilian nuclear radiation accident in Japan before the Fukushima disaster in 2011. The accident exposed people nearby to dangerous nuclear radiation. This happened after the uranium mixture became "critical." Two of the three workers mixing the fuel died. The accident was caused by a lack of safety checks, poor safety practices, and workers not being trained properly.

One reason for the accident was not enough safety checking. The company didn't install a special alarm for criticality accidents. They also weren't included in the National Plan for the Prevention of Nuclear Disasters. Because there wasn't enough safety technology, they had to rely on people to keep track of levels. This meant human mistakes could happen. Also, the safety checkers didn't do regular inspections. These inspections could have found the missing safety technology that might have stopped the accident.

Another reason was the safety culture at the company. The company didn't tell the safety management about a new operation. They knew it wouldn't be approved. A company spokesperson said their income was low, so they felt they had to open a new factory. They knew it wouldn't get approved, so they did it without telling the safety team.

The JCO facility changed uranium hexafluoride into enriched uranium dioxide fuel. This was the first step to make fuel rods for Japan's power plants. Enriching nuclear fuel needs great care and can be very risky for workers. If done wrong, mixing nuclear products can cause a fission reaction, which creates radiation. To enrich uranium fuel, a special cleaning process is needed. This involves putting small amounts of uranium oxide powder into a tank. There, it mixes with nitric acid to make uranyl nitrate.

Then, this mixture is carefully moved to a special "buffer tank." This tank is designed to stop the fission activity from becoming critical. In another tank, ammonia is added to form a solid product. This tank is meant to catch any leftover nuclear waste. In the final step, uranium oxide is put into dissolving tanks until it's clean. This is done without enriching the isotopes, using a special "wet-process" technology.

JCO was pressured to work faster. This led the company to use an illegal method. They skipped several important steps in the enrichment process. Workers poured the product by hand in stainless-steel buckets directly into a precipitation tank. This caused a critical mass level incident. It started uncontrolled nuclear chain reactions that lasted for several hours.

Workers Affected

Two workers were at the tank when the accident happened. A third worker was in a nearby room. All three immediately saw blue-white flashes. They quickly left when they heard the gamma alarms. After leaving, one of the workers at the tank started feeling sick from radiation. He passed out but woke up 70 minutes later. The three workers were taken to the hospital. Doctors confirmed they were exposed to high doses of gamma, neutron, and other irradiation.

Besides these three workers who felt sick right away, 56 other people at the JCO plant were also exposed to radiation. Construction workers who were working nearby were also exposed.

How the Criticality Event Unfolded

JCO workers Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa were trying to speed up the fuel process. They needed to meet shipping deadlines. This was JCO's first batch of fuel for the Jōyō experimental reactor in three years. There weren't proper training rules for this process. To save time, the team mixed the chemicals in stainless-steel buckets. The workers followed JCO's operating manual for this. But they didn't know it wasn't approved by the STA. The correct way was to store uranyl nitrate in a buffer tank. Then, it should be slowly pumped into the precipitation tank in small amounts (about 2.4 kg).

Around 10:35 a.m., the precipitation tank reached critical mass. This happened when it contained about 16 kg of uranium. This dangerous level was reached after workers added a seventh bucket of liquid uranyl nitrate. This solution was enriched to 18.8% ²³⁵U. The amount added to the tank was almost seven times the legal limit set by the STA.

The 1996 JCO Operating Manual had rules for dissolving uranium oxide powder in a special tank. The buffer tank was tall and narrow. It was designed to hold the solution safely and prevent criticality. But the precipitation tank was not designed to hold large amounts of this solution. Its wide, round shape made it easy for criticality to happen. The workers completely skipped the buffer tanks. They poured the uranyl nitrate directly into the precipitation tank. An uncontrolled nuclear fission started right away. The nuclear fission chain became self-sustaining. It gave off strong gamma and neutron radiation.

When this happened, Ouchi was leaning over the tank. Shinohara was on a platform helping to pour the solution. Yokokawa was sitting at a desk four meters away. All three workers saw a blue flash (possibly Cherenkov radiation). Gamma radiation alarms went off. For the next several hours, the fission reaction continued.

Ouchi and Shinohara immediately felt pain, nausea, and trouble breathing. Both workers went to the decontamination room, where Ouchi vomited. Ouchi received the most radiation. He quickly had trouble moving, thinking clearly, and lost consciousness. When critical mass was reached, a lot of high-level gamma radiation set off alarms in the building. This made the three workers leave. All three workers didn't know how serious the accident was or how to report it. A worker in the next building saw the injured employees and called for medical help. An ambulance took them to the nearest hospital. The fission products contaminated the building and the area right outside the nuclear facility. Emergency service workers arrived and moved other plant workers to safe areas.

The next morning, workers stopped the nuclear chain reaction. They did this by draining water from a cooling jacket around the precipitation tank. This water acted as a neutron reflector. A boric acid solution was then added to the tank. This lowered all the contents to safe levels. boron was chosen because it can absorb neutrons.

Tokaimura Evacuation

By mid-afternoon, plant workers and people living nearby were asked to leave. Five hours after the criticality started, about 161 people from 39 homes within 350 meters of the building were evacuated. Twelve hours after the accident, 300,000 residents around the nuclear facility were told to stay indoors. They were also told to stop all farming. This restriction was lifted the next afternoon. About 15 days later, the facility put up sandbags and other shields to protect from leftover gamma radiation.

Aftermath

There was no emergency plan or public communication from JCO. This led to confusion and panic. Authorities told local people not to harvest crops or drink well water. To calm public fears, officials started testing residents for radiation. These residents lived about 6 miles from the facility. Over the next 10 days, about 10,000 medical check-ups were done. Dozens of emergency workers and nearby residents were hospitalized. Hundreds of thousands of others had to stay indoors for 24 hours. Testing confirmed 39 workers were exposed to radiation. At least 667 workers, first-responders, and nearby residents were exposed to too much radiation. Radioactive gas levels stayed high even after the plant was sealed. On October 12, it was found that a roof fan had been left on, and it was shut down.

The incident was classified as an "irradiation" accident, not "contamination." It was rated Level 4 on the Nuclear Event Scale. This meant the situation was low risk outside the facility. The workers inside the facility were checked for radiation. The three main workers had much higher radiation levels than the safe limit for Japanese nuclear workers (50 mSv). Many employees and local people suffered accidental radiation exposure above safe levels. Over fifty plant workers were tested up to 23 mSv, and local residents up to 15 mSv. Sadly, the high doses of radiation caused the deaths of two workers, Ouchi and Shinohara.

Environmental Impact

The STA and Ibaraki Prefecture started checking gamma levels right after they were told about the accident. They took samples of tap water, well water, and rain within 10 km of the site. They also took samples of plants, sea water, dairy products, and seafood. All were checked for radioactivity after the accident. They found low levels of radioactivity in some plants very close to the nuclear facility. But they didn't find any in dairy products, water, or the sea.

Impact on Workers

According to STA radiation tests, Ouchi was exposed to 17 Sv of radiation, Shinohara to 10 Sv, and Yokokawa to 3 Sv. The two workers who received the highest doses, Ouchi and Shinohara, died several months later.

Their supervisor, Yutaka Yokokawa, 54, received treatment. He was released three months later with minor radiation sickness.

What Caused Both Accidents

The International Atomic Energy Agency said the accidents were caused by "human error and serious breaches of safety principles." Several human mistakes led to the incidents. These included careless ways of handling materials, workers who weren't experienced enough, not enough supervision, and old safety rules. The company hadn't had any accidents for over 15 years. This made employees too relaxed about their daily jobs.

The 1999 accident happened because of poor management of how operations were done. Workers and engineers weren't properly trained. Also, there were wrong ways of handling nuclear chemicals. Not enough communication between engineers and workers meant problems weren't reported when they happened. If the company had fixed the mistakes after the 1997 accident, the 1999 accident might have been much less serious, or might not have happened at all.

A 2012 report about the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission said that too much confidence in the nuclear industry and poor government oversight might also apply to the Tokaimura nuclear accident.

Help for Victims and Plant Closure

Over 600 plant workers, firefighters, emergency staff, and local residents were exposed to radioactivity. In October 1999, JCO set up places to help people with claims and questions. By July 2000, over 7,000 claims were made and settled. In September 2000, JCO agreed to pay $121 million. This was to settle 6,875 claims from people exposed to radiation and affected businesses. All residents within 350 meters of the accident who had to leave their homes received money. They agreed not to sue the company later.

In late March 2000, the STA took away JCO's license to operate. This was the first time a Japanese plant operator was punished by law for mishandling nuclear radiation. After this, the company president resigned. In October, six JCO officials were charged. They were accused of not training workers properly and knowingly ignoring safety rules.

New Safety Rules

In April 2001, six employees, including the head of the production department, admitted they were responsible for the deaths. Yokokawa was among those arrested for not supervising properly. The JCO President also admitted guilt for the company. During the trial, it was learned that a 1995 JCO safety committee had approved using steel buckets. Also, an unauthorized manual from 1996 suggested using buckets to make the solution. An STA report showed that JCO management had allowed these dangerous practices since 1993 to speed up the process. This was against approved nuclear chemical handling rules.

Because of the accidents, special laws were created. These laws set strict rules for safety and required quarterly inspections. These checks focused on how workers and leaders followed rules. This change made it mandatory to have safety education and quality checks for all nuclear power facilities and activities. Starting in 2000, Japan's nuclear commissions began regular checks of facilities. They also started widespread education about proper procedures and safety culture for handling nuclear chemicals and waste.

Efforts continued to follow emergency plans and international guidelines. New systems were put in place to handle similar incidents. This was done with new laws and organizations to try and prevent more accidents.

Japan relies heavily on imports for 80% of its energy. Because of this, there's still pressure to produce its own energy. In 2014, the Japanese government decided to create a "Strategic Energy Plan." This plan named nuclear power as an important energy source that can safely provide stable energy for the country. However, these accidents also led to movements against nuclear energy in Japan. Even today, there's a debate between the need for energy and the safety of the country's people. Groups around the world now work to help victims of nuclear sickness and prevent future nuclear accidents. They promote human well-being and protecting the environment.

See also

In Spanish: Accidentes de Tokaimura para niños

• Nuclear power in Japan
• Fukushima Daiichi nuclear disaster
• Rokkasho Reprocessing Plant, which was meant to take over from the Tokai Reprocessing Plant