Explained | The Chernobyl nuclear disaster

The nuclear disaster and the ensuing radiation was responsible for 28 deaths in the succeeding weeks with 8.40 million people exposed to the radiation.

Overview: The United Nations General Assembly (UNGA) vide resolution 71/125 in 2016 designated April 26 as the International Chernobyl Disaster Remembrance Day. The resolution mentioned it was aimed at raising awareness on the long-term consequences of the disaster and strengthening coordination and cooperation to mitigate and minimise its consequences. 

On April 26, 1986, a nuclear reactor at the Chernobyl Nuclear Power Plant in the erstwhile Soviet Union was subjected to improper testing at low-power, resulting in loss of control. The ensuing fire and explosion destroyed the building where the reactor was kept – releasing radiation into the atmosphere. The International Atomic Energy Agency (IAEA) explains that as safety measures were ignored, uranium fuel in the reactor overheated and melted through the protective barriers.  

The radioactive elements released lasted in the air for about ten days, causing serious problems for populations in certain territories of present-day Belarus, Russia and Ukraine. According to the U.N., nearly 8.40 million people in the three countries were exposed to radiation. Two workers at the plant died immediately due to the explosion, with another 28 deaths in the following weeks as a result of the acute radiation syndrome. 

The World Nuclear Association says that the largest-ever uncontrolled radioactive release into the environment was the result of a “flawed” reactor design operated by “inadequately” trained personnel.  

The Chernobyl plant is situated about 130 km north of Kyiv (Ukraine) and about 20 km south of the border with Belarus, housing four nuclear reactors of the RBMK-100 design. Designed by the erstwhile Soviet Union, the tube type reactor is equipped with individual fuel channels using graphite as a moderator and water as a coolant. The World Nuclear Association mentions that the combination is found in no other power reactor in the world. Unit 1 and 3 were constructed between 1970 and 1977 while units 3 and 4 were completed in 1983. The event took place at Unit 4.  

A day before the disaster, prior to the routine shutdown, the crew wanted to determine how long the reactor could be cooled in the event of loss of power. The operators disabled the necessary automatic shutdown mechanisms before commencing the test. The peculiar design of the control rods (used for regulating fission inside a reactor) caused a dramatic power surge when inserted into the reactor. The heated fuel interacted with cooling water to create a fuel fragmentation (separation of fuel pellets into more than one piece), in turn, producing steam and increasing the pressure. 

This pressure ruptured the unit and caused a steam explosion, releasing fission products into the atmosphere. Seconds later, another explosion threw out fragments from the fuel channels along with hot graphite.