|Fig. 1: Abandoned Chernobyl Village. (Source: Wikimedia Commons)
The Chernobyl accident is one of the most infamous nuclear disasters in history. It happened on 26th of April 1986 as a result of a flawed reactor design and the staff that were not adequately trained. The resulting steam explosion and fires released at least 5% of the radioactive materials into the atmosphere. The explosion caused the nearby area to be completely uninhabited for years. Fig 1 shows an example of one of those abandoned villages. Resettlement of areas from which people were relocated is still ongoing. In 2011 Chernobyl was officially declared a tourist attraction.
On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested. The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The overpressure caused the 1000-ton cover plate of the reactor to become partially detached.  Intense steam generation then spread throughout the whole core causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite. Two workers died as a result of these explosions.
The Chernobyl accident's severe radiation effects killed 28 of the site's 600 workers in the first four months after the event.  Another 106 workers received doses sufficiently high to cause acute radiation sickness. Another 200,000 cleanup workers in 1986 and 1987 received doses of between 1 and 100 rem (the average annual radiation dose for a U.S. citizen is about 0.6 rem). 
The Chernobyl accident contaminated wide areas of Belarus, the Russian Federation, and Ukraine inhabited by millions of residents. Agencies such as the World Health Organization have been concerned about radiation exposure to people evacuated from these areas. The majority of the five million residents living in contaminated areas, however, received very small radiation doses comparable to natural background. 
The Chernobyl accident was a travesty that the entire world knows about. It killed numerous people and left severe health consequences for many others. Experts conclude some cancer deaths may eventually be attributed to Chernobyl over the lifetime of the emergency workers, evacuees and residents living in the most contaminated areas. However these health effects are far lower than initial speculations of tens of thousands of radiation-related deaths. The question now is how do we make sure an accident like that never happens again. Hopefully the current technology won't allow for this type of incident to ever happen again. The key differences in U.S. reactor design, regulation and emergency preparedness make it highly unlikely that a Chernobyl-type accident could occur in the United States. 
© Evgeny Moshkovich. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Gray, "The Human Consequences of the Chernobyl Nuclear Accident: A Strategry for Recovery," United Nations Development Programme, January 2002.
 "Chernobyl Nuclear Power Plant Accident," U.S. Nuclear Regulatory Commission, May 2013.
 "Chernobyl Accident and Its Consequences," Nuclear Energy Institute, November 2008.