The Chernobyl disaster occurred on April 26, 1986, when a series of explosions in Energy Block #4 of the Chernobyl Nuclear Power Station in Soviet Ukraine resulted in a core meltdown that contaminated a wide area, including about a quarter of the country of Belarus, with radioactive material. Initially, the accident was attributed to operator error, but subsequent investigations revealed major design and construction flaws in the reactor. There also were problems with emergency management.
While the nuclear plant meltdown was initially attributed to operator error, subsequent investigation by the International Atomic Energy Agency (IAEA) blamed design flaws and shoddy construction of the nuclear power plant itself.
This reactor was one of 17 Soviet-designed RBMK [a Russian acronym for Reactor Bolshoi Moschnosti Kanal'nyi "Channelized Large Power Reactors"], for which three generations of design can be described. They all evolved evolved graphite-moderated reactors optimized for the production of plutonium.  Soviet practice was to build many industrial facilities as dual-use, meeting both military and civilian needs. Western reactors intended purely for plutonium production are much smaller and generally are not significant contributors of electric power. Since the highest-grade plutonium is produced fairly early after fueling, access to the core of the operating reactor, both for extracting fuel rods from which plutonium will be extracted, and refueling the reactor, is necessary. A full containment building would make the usual sliding reactor shield and overhead crane impractical. 
Aside from the dual-use aspects, an enormous difference from reactors outside the Soviet Union is that the RBMK, while it has shielding, lacks a massive containment building around the radioactive parts of the plant. Although the steam explosions were quite violent, many, but not all, nuclear engineers believe a proper containment could have kept the disaster as a local event. This is not universally agreed: Globalsecurity observes "However, because the estimated energy released by the explosions was greater than most containment designs could withstand, it is highly unlikely that a containment structure could have prevented the release of radioactive material at Chernobyl."
The lack of a containment, however, involved a number of factors. In any event, Soviet metallurgy of the 1950s, the time of the first RBMK design, was not ready to build containments meeting Western safety factors. 
The disaster began in the first early hour of April 26, stemming from a scheduled shutdown and subsequent test of the reactor scheduled the previous day. The test was designed to determine how long turbines would spin and supply power if the plant lost its main electrical power.
At approximately 12:28 AM, the power level had dropped to 500 MW(t) in accordance with the test procedure, when control was transferred from the local to the automatic regulation system. At this point, a signal to hold the power at this level failed due to either operator or regulatory system response. Power produced by the plant rapidly fell to 30 MW(t).
Responding to this power drop, the operator retracted a number of control rods in an attempt to correct the situation. Fewer than the effective equivalent of 26 control rods were remaining in the reactor, a situation that should have happened only with the station safety procedure approval of the chief engineer. At 1:00 AM, the reactor power rose to 200 MW(t). Two pumps were switched into their opposite-handed cooling circuits in order to increase the water flow to the core. Although the operation of the additional pumps removed heat from the reactor core at a quicker rate, it reduced the water level in the steam separator.
Fifteen minutes later, the automatic trip systems of the steam separator were disabled by the operator to continue the operation of the reactor. The water flow was increased in an attempt to address the cooling system problem. In order to increase power, the operator withdrew some of the manual control rods to raise the temperature and pressure in the steam separator, disregarding an operating policy requirement that a minimum effective equivalent of 15 manual control rods were to be inserted into the reactor at all times. It is estimated that at this time the number of rods in the system was about half of the requirement.
Twenty minutes into the total process, the operator, trying to stabilize the steam separator water level, reduced the water flow rate to below normal and thereby decreased heat removal capability from the core. The increased amount of heat spontaneously generated steam in the core, abnormally giving the operator the appearance that the reactor was stable.
At 1:23 AM the actual shutdown process began. Turbine feed valves were closed off, and automatic control rods were withdrawn from the core. A 10-second withdrawal of the rods from the reactor was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feeds. Typically, a decrease in reactivity is the result of an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. This expected decrease did not happen due to the reduced feedwater to the core (the operator manually reduced the water flow rate).
Steam generation in the core increased, and because of the reactor's positive void coefficient, any further amount of steam would lead to a rapid increase in power output. Unfortunately, seconds later steam in the core began to increase uncontrollably. The emergency button was pressed by the operator which sent control rods into the core. It was too late, and the insertion of the rods from the top concentrated all of the reactivity to the bottom of the core, causing the reactor power to rise 100 times the allotted design value. Some fuel pellets shattered, reacting with the water to produce high pressure in the fuel channels. The channels ruptured, causing two explosions: one was a steam explosion and the other a result of expansion of fuel vapor. The two explosions caused a loss of integrity of the pile cap, allowing the entry of air that reacted with the graphite moderator blocks to form carbon monoxide gas. The gas ignited, causing a reactor fire.
The major design flaw in the Chernobyl-4 reactor was the ability for the nuclear chain reaction and power output to occur in the a Loss Of Coolant Accident or the conversion of that water into steam. This flaw was the key factor that instigated the power surge which led to its destruction. The Chernobyl reactor was built with "a positive void coefficient. Soviet engineers sought to mitigate this tendency by backfitting RBMKs with faster-acting control rods and other improvements. Modifications made to all RBMKs are generally considered to be adequate to maintain this positive void defect at a low enough level to preclude the type of nuclear excursion--a sudden, rapid rise in power level--that occurred at Unit 4. U.S.-style light water reactors are designed with just the opposite characteristic--a negative void coefficient--so that the nuclear chain reaction automatically stops when coolant is lost."
Following the second explosion and subsequent reactor fire, approximately 14 EBq (1018 Bq) of radioactivity was released into the environment through the continuous nine-day burning of the graphite moderator, half of the radioactive material being biologically-inert noble gases.
It is estimated that all of the xenon gas, about half of the iodine and caesium, and at least 5% of the remaining material was released and deposited as dust and debris. Unfortunately the lighter material carried through the wind over Ukraine, Belarus, Russia, and parts of Scandinavia and Europe.
Primarily, the casualities were limited to the firefighters; those that worked to extinguish the flames on the roof of the turbine building on the first day absorbed an estimated range of up to 20,000 millisieverts (mSv).
The next task was performed by recruiting liquidators (200,000) all throughout the Soviet Union to assist in cleaning up the radioactivity at the site, which took course over two years, during 1986 and 1987. They were exposed to high levels of radiation, averaging 100 mSv. About 20,000 of them were exposed to 250 mSv and a smaller few exposed to 500. Over these two years the amount of liquidators increased to 600,000.
Overall, the highest doses were received by those immediately to the scene: dispatched policemen, firemen, soldiers, and liquidators (approximately 1000 people).
All totalled the amount of people dead from those immediately exposed to the Chernobyl accident is currently at 49; an initial 30 died from the accident itself (28 from radiation exposure). 19 of the 209 treated for acute radiation poisoning have died as a result of side effects of exposure, although out of those only 134 cases have been confirmed. Of 237 members of the staff and emergency workers who were at the scene as initial responders, acute radiation syndrome was diagnosed in 134 patients and as of 1998, 11 have died. It should be noted that the causes of deaths were due to coronary heart disease, myelodysplastic syndrome, lung tuberculosis, fat embolism, and one death from acute myeloid leukemia.
Consequences of radioactive exposure
The Chernobyl disaster was unique because of the relative size of the catastrophe compared to other events of this nature in history. However the impact of the fallout being distributed to other countries and outside of the contamination zone may have been overestimated.
In May of 2004, nuclear scientist Zbigniew Jaworowski, wrote about his experience with Chernobyl. While working on Poland, he observed that the amount of radioactive exposure had increased 2.6 millisievert(mSv) per year, a factor of only three times the average external radiation exposure the day before. This rate at the time was four times lower than places in Norway, where natural external radiation peaks to 11.3 mSv/year; approximately fifty times lower than in Ramsar(an Iranian resort) where the annual dose reaches 250 mSv/year; and more than three hundred times lower than Brazilian beaches and in southwest France where levels reach 790 and 870 mSv/year, respectively.
- Jim T. Smith, and Nicholas A. Beresford, Chernobyl: Catastrophe and Consequences. Springer, 2005. 310 pp.
- Svetlana Alexievich, Voices From Chernobyl (2006), Picador St. Martin's Press ISBN 0312425848, interviews with survivors
- RBMK Reactor, Globalsecurity
- Richard Rhodes (2007), Arsenals of Folly: the Making of the Nuclear Arms Race, Alfred A. Knopf, pp. 15-16
- Rhodes 2007, p. 6
- Chernobyl appendices, Simplified sequence of Events. Australian Uranium Association Uranium INformation Centre (September 2004). Retrieved on 2007-09-22. *This is a summarization of the simplified sequence of events.
- Chernobyl Accident. Australian Uranium Association, Uranium Information Centre (May 2007). Retrieved on 2007-09-22.
- Zbigniew Jaworowski, M.D., Ph.D.,D.Sc. (2004). "Lessons of Chernobyl: Nuclear Power is Safe". EIR: 58-63.