History of Chernobyl and Tokaimura: Two of the Worst Nuclear Power Plant Accidents in Modern History – Part 3 of 3

Chernobyl – April 26, 1986

The accident at the Chernobyl nuclear power plant in the Ukraine was caused by a faulty reactor design, combined with mistakes made by power plant employees. A surge of power destroyed one of the reactors at the plant and released large amounts of radiation. Helicopters dropped boron and sand onto the reactor to prevent more radiation from leaking into the environment.

Six hundred employees were present at the time of the explosion. Of the 600 employees present, 134 were exposed to high levels of radiation. Two employees died within hours and another 28 employees died within the next four months. An additional 600,000 people who participated in the radiation clean up were also exposed to radiation. Approximately 200,000 of the people who participated in the clean up were exposed to levels of radiation that are deemed unsafe. The radiation exposure from the plant spread far and exposed approximately five million people who lived in the contaminated areas.

Even though most of the five million were only exposed to low levels of radiation, it is impossible to know the amount of health problems that can be blamed on the radiation exposure due to the large number of people exposed and the long term effects. Some deaths and illnesses, however, have been tracked, such as the cases of thyroid cancer in 4,000 exposed children, which have been attributed to the radiation exposure. Of those diagnosed with the painful and life threatening disease at least nineteen died early on.

Tokaimura, Japan – September 30, 1999

On September 30, 1999, there was an accident at a nuclear power facility ran by JCO Company in Tokaimura, Japan. The accident was caused as a result of an error made by JCO employees. The accident occurred when JCO employees used too much uranium in the uranium nitric acid mix the plant used to make nuclear fuel. The employees added 35 pounds of uranium to the tank that contained the nitric acid, instead of the 5.2 pounds that they were supposed to use. The improper mix caused a nuclear fission chain reaction explosion to occur. The company brought mass amounts of boron to the plant to absorb the radiation, but could not get near enough to the source to spread the boron. Instead, they broke the water pipes that led to the tank, to flood the area and stop the nuclear reaction. After approximately 20 hours, the nuclear reaction was stopped.

Approximately 39 employees were exposed to measurable levels of radiation as a result of the accident. Three employees were exposed to very high levels of radiation, two of which eventually died as a result of the radiation contamination.

Read About More Nuclear Power Plant Accidents in Part 1 and Part 2 of This Series

Read about the Chalk River and Mayak Plutonium Facility nuclear accidents.

Read about the Three Mile Island Nuclear Power Plant, Windscale & Lubmin nuclear power plant disasters.


International Atomic Energy Agency

World Nuclear Association

United States Nuclear Regulatory Commission

This post is part of the series: A History of Nuclear Power Plant Disasters

This three part series details the history of some of the worst nuclear power plant disasters in history.
  1. A History of Nuclear Power Plant Disasters – Part 1
  2. Three Mile Island Nuclear Power Plant: A History of Nuclear Disasters – Part 2
  3. Chernobyl & Tokaimura: A History of Nuclear Power Plant Disasters – Part 3