The Chernobyl Nuclear Accident is one of the worst, debatably the worst, nuclear accident in world history. In 1986, there was an explosion at one of the power plant's reactors. It tops the charts with casualties and costs, winning it a level 7 classification on the International Nuclear Event Scale (a logarithmic scale to classify nuclear events). The only other event to attain a 7, the highest level on the scale, is the 2011 Fukushima Nuclear Disaster in Japan.
Chernobyl is a city in Northern Ukraine, about 80 miles north of Kiev, and 10 miles south of the Belarus- Ukraine border. When the nuclear accident occurred, Chernobyl was a part of the Soviet Union. Between Chernobyl itself, the more populated Pripyat near by, and other cities, there were approximately 130,000 people within twenty miles of the reactors. The power plant had four RMBK-1000 reactors, with two more under construction at the time of the accident. Reactor four was the reactor to explode, but the other three were shut down shortly after.
The RMBK-1000 reactor, pictured right, is one of two designs that originated from the Soviet Union in the 1970s. RMBK, or Reaktor Bolshoy Moshchnosty Kanalny, also, High-Power Channel Reactor is different from most other reactor designs, as it is the only to employ a graphite moderator and water coolant. Most reactors use a moderator and coolant of the same material. The role of a moderator is to make a chain reaction easier to attain, as fuel atoms absorb neutrons more easily the slower they move. The graphite moderator surrounds the pressure tubes and decelerates the neutrons released during fission. The water coolant cools down the reactor during fission. A final piece on RMBKs is the positive void coefficient. The void coefficient of reactivity is determined by the ratio of steam bubbles produced. When there is a decrease in steam bubbles the coefficient is negative because the reactivity has decreased. Vis versa, more steam causes an increased reactivity and results in a positive void coefficient.
The explosion occurred during a reactor systems test on reactor four. There was a power surge, which, in addition to other components, caused a positive void coefficient. A positive void coefficient can occur with these RMBK reactors. This is when, like in a RMBK, the moderator and coolant are different materials (a graphite moderator and water coolant) and the steam does not cool the reactor as much as necessary, which can enhance chain reaction. This overall design was proved to be unsafe and has been significantly changed since. But, due to the flawed design, an explosion ensued at 1:23 am on April 26, 1986 from over pressurization as more and more steam was generated. After the first explosion, a second and mush larger one occurred shortly after. The Chernobyl Firefighters arrived a few minutes later to extinguish the fire. All the firefighters on scene died trying to put out the flames or became sick due to high levels of radiation poisoning.
After a fire began at the power plant, water, clay, sand, and other materials were dropped onto the reactor to put out the fire and prevent the spread of radiation. The release of clay and sand onto reactor four continued for up to ten days after the accident. It took about 24 hours for the evacuation to be ordered. Authorities flew in to evaluate the area after receiving a vague phone call about a fire that was already extinguished. After surveying the power plant and hospitals in the area, the official evacuation of Pripyat was issued. The Soviet Union also cut down all the trees for a mile around the power plant to reduce contamination.  While only two of the reactor's workers died at the time of the explosion, 28 more perished from radiation poisoning. An additional 106 employees contracted radiation sickness. Outside the power plant workers, there were casualties in the surrounding area of Ukraine and Belarus. It is believed that the main long-term health effect from this accident is thyroid cancer.  This link is believed to originate from milk that was consumed in 1986 that had been contaminated by radioactive iodine. Approximately 6,000 cases of thyroid cancer have been detected from this population, and with 99 percent of them successfully treated, there were only 15 deaths between Ukraine, Belarus, and Russia. Finally, there were some design changes made to the RBMK. There was an increase in fuel enrichment from 2 percent to 2.4 percent and increased the number of control rods from about 26 to 45, to reduce the void coefficient of reactivity. In addition, the automatic shutdown mechanisms operate better and more efficiently.
© Mary Caballero. 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.
 "Backgrounder: Chernobyl Nuclear Power Plant Accident," U.S. Nuclear Regulatory Commission, May 2013.