Heart-Stopping Moments: His Hands Held Death
On September 30, 1999, at the JCO Co. Ltd uranium processing plant in Tokaimura, Japan, three workers—Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa—were performing routine procedures. Unaware of what they were doing, they poured too much uranium into the mixing tank. In an instant, an uncontrolled nuclear chain reaction began. Ouchi, who was closest to the tank, received a lethal radiation dose of 17 sieverts—more than ten times the lethal dose. The world nearly witnessed a disaster that could have led to great destruction.
How Did the Chain Reaction Happen?
A criticality incident occurs when fissile materials such as uranium-235 or plutonium-239 accumulate in sufficient quantities to reach a critical mass. Normally, such a chain reaction only occurs in nuclear reactors specifically designed with strict controls. However, in Tokaimura, the workers bypassed safety procedures: they used a steel bucket to mix uranium nitrate with nitric acid instead of using an approved tank. As a result, the uranium solution reached a higher concentration than the safe limit, triggering an uncontrolled neutron explosion.
An Unexpected Hero: First Responders
When the radiation alarm sounded, the Japanese nuclear safety team arrived at the scene. However, the real hero was Dr. Toshio Saito, a nuclear physicist called from the Japan Atomic Energy Research Institute. Risking his own life, Saito entered the contaminated area to shut off the pump supplying uranium to the tank. His action stopped the flow of fissile material, preventing the reaction from continuing. He was later treated for radiation exposure, but thousands of lives were saved.
Avoided Destruction: The True Scale of the Threat
Although the Tokaimura incident did not result in a nuclear explosion like a bomb, it released a radiation dose that could be fatal to anyone within a 500-meter radius. If the reaction had not been stopped within 20 minutes, it could have caused a large-scale release of radiation into the atmosphere, contaminating agricultural and residential areas around Tokyo. According to a report by the International Atomic Energy Agency (IAEA), this incident registered a level 4 on the International Nuclear Event Scale (INES), making it one of the most serious in history.
Lessons Not Learned?
After this incident, Japan tightened its nuclear safety regulations. However, the Fukushima tragedy in 2011 showed that weaknesses still existed. Criticality accidents are not just theoretical threats—they can happen at any time if procedures are neglected. Heroes like Dr. Saito remind us that in a world increasingly dependent on nuclear energy, a small mistake can lead to a major disaster.
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*Reference: [Criticality accident — Wikipedia](https://en.wikipedia.org/wiki/Criticality_accident)*