On April 26, 1986, a sudden surge of power during a reactor systems test destroyed Unit 4 of the nuclear power station at Chernobyl, Ukraine, in the former Soviet Union. The accident and the fire that followed released massive amounts of radioactive material into the environment.
Emergency crews responding to the accident used helicopters to pour sand and boron on the reactor debris. The sand was to stop the fire and additional releases of radioactive material; the boron was to prevent additional nuclear reactions. A few weeks after the accident, the crews completely covered the damaged unit in a temporary concrete structure, called the "sarcophagus," to limit further release of radioactive material. The Soviet government also cut down and buried about a square mile of pine forest near the plant to reduce radioactive contamination at and near the site. Chernobyl's three other reactors were subsequently restarted but all eventually shut down for good, with the last reactor closing in 1999. The Soviet nuclear power authorities presented their initial accident report to an International Atomic Energy Agency meeting in Vienna, Austria, in August 1986.
After the accident, officials closed off the area within 30 kilometers (18 miles) of the plant, except for persons with official business at the plant and those people evaluating and dealing with the consequences of the accident and operating the undamaged reactors. The Soviet (and later on, Russian) government evacuated about 115,000 people from the most heavily contaminated areas in 1986, and another 220,000 people in subsequent years
(Continue Reading from the United States Nuclear Regulatory Commission)
Chernobyl: 7 People Who Played a Crucial Role in the World's Worst Nuclear Disaster from The History Channel
The region is expected to remain uninhabitable for thousands of years. Many of the radioactive elements decayed quickly, but the most dangerous—iodine-131, strontium-90, and cesium-137—have half-lives of 8 days, 29 years, and 30 years, respectively. In chemistry, the definition of a half-life says that this means it will take 30 years for half of the initial cesium to decay. Then, it will take another 30 years for half of what you had left at the 30-year mark to decay. But in studying Chernobyl, scientists have learned that the “ecological half-life” of cesium—that’s how long it takes for the element to actually disappear from the local environment—is turning out to be much longer. While contamination in the water supply has improved, the levels of radioactivity in the soil remain higher than the 30-year half-life would predict. (Continue Reading from Scientific American)