Chernobyl Nuclear Accident

A serious accident occurred on April 26, 1986, at 1:23 a.m. at Unit 4 of the Chernobyl Nuclear Power Plant near Pripyat in the Ukrainian Soviet Socialist Republic. The power plant was located near the border of Ukraine and Belarus. A series of explosions sent a plume of highly radioactive fallout of iodine and cesium into the atmosphere. Cesium-137 deposition spread throughout Europe, and the radioactive plume reached the U.S. West Coast on May 6, 1986. Official word of the accident first reached the Western world when Sweden announced on April 28 that workers at its Forsmark nuclear plant had radioactive particles on their clothing and the source was traced back to Chernobyl. Initially, former Soviet ruler Mikhail Gorbachev downplayed the incident's severity, which is considered the worst nuclear power plant accident in history.

The Chernobyl accident was caused by a combination of human error and design flaws. It was initiated when power plant technicians conducted an experiment on the emergency core cooling system during its shutdown procedure. First, the emergency core cooling system was turned off, and 205 of 211 fuel control rods were removed from the reactor core. Following this, automatic safety devices that shut down the reactor when [Page 386]water and steam levels fall below normal and turbine stops were turned off because engineers did not want to spoil the experiment. Next, an additional water pump to cool the reactor was turned on. However, with only a 7% power output and an extra drain on the power system, water did not reach the reactor. Finally, since the reactor power output was lowered too much, it was very difficult to control. The result was a massive and catastrophic power excursion and steam and hydrogen explosions. These tore off the steel and concrete top of the reactor, exposed the core, and dispersed large amounts of radioactive particulate and gaseous debris, allowing oxygen to contact the superhot core, containing almost 2,000 tons of combustible graphite moderator. The graphite moderator fire increased the radioactive particle emissions. Much of the nuclear fuel supply in the reactor core eventually melted.

Inspectors just outside the Chernobyl Nuclear Power Plant's crumbling west wall take water and soil contamination samples and conduct land surveys to repair and build new access roads. Twenty years after the Chernobyl nuclear accident in 1986, radiation monitoring occurs on a regular basis on the grounds.

Source: Gerd Ludwig/Corbis.

There has been great controversy over the number of people who died from the Chernobyl accident, either directly or later through acute radiation syndrome or thyroid and other cancers. The official death toll stands at 56, while the estimated number of eventual fatal cancers ranges from 4,000 (World Health Organization) to 93,000 people (Greenpeace). Millions of other people became sick from Chernobyl, and many thousands have lost their homes and livelihoods. From 1986 to 1992, about 336,000 people were evacuated and resettled because of the accident. In addition, millions of acres of farmlands and forests have become contaminated, especially in Ukraine, Belarus, and Russia. Overall, 4.5 million people were affected. In December 2000, the Chernobyl Nuclear Plant was fully decommissioned.

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