The Chernobyl disaster began on the 26th April 1986 with the explosion of the 4th reactor of the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of Ukraine, close to the border with Belarus, in the Soviet Union. It is one of only two nuclear energy accidents rated at seven (the maximum severity) on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident in Japan. The initial emergency response and subsequent mitigation efforts involved more than 500,000 personnel and cost an estimated 18 billion roubles (roughly $ 68 billion in 2019, adjusted for inflation). It is considered the worst nuclear disaster in history.

The accident occurred during a test of the steam turbine's ability to power the emergency feedwater pumps in the event of a simultaneous loss of external power and coolant pipe rupture. Upon successful completion of the test, the reactor was then shut down for maintenance. Due to a variety of factors, this action resulted in a power surge at the base of the reactor which brought about the rupture of reactor components and the loss of coolant. This process led to steam explosions and a meltdown, which destroyed the containment building. This was followed by a reactor core fire which lasted until 4 May 1986, during which airborne radioactive contaminants were spread throughout the USSR and Europe. In response to the initial accident, a 10 kilometres radius Exclusion Zone was created 36 hours after the accident, from which approximately 49,000 people were evacuated, primarily from Pripyat city. The Exclusion Zone was later increased to a radius of 30 kilometres, from which an additional aproximetly 68,000 people were evacuated.