Worst nuclear reactor disaster
Who
Chernobyl Nuclear Power Plant
What
10,900 PBq (petabecquerels) people
Where
Ukraine (Chernobyl)
When

The largest accidental release of radioactive material took place at 01:23 a.m on 26 April 1986 at the Chernobyl Power Plant, located around 100 km (62 miles) north of Kiev in the Ukrainian SSR (then part of the Soviet Union, now the nation of Ukraine). The reactor explosion and subsequent fire created a plume of radioactive dust and gas that rose to an altitude of more than 1 km (0.62 miles) before dispersing with the prevailing winds. The radioactive particles contained within that plume released an estimated 10,900 petabecquerels (PBq) of radiation into the environment.


The 2011 Fukushima Daichi nuclear accident, which is the only other incident on record to be classified as an INES-7 event (a "major accident" on the International Nuclear and Radiological Event Scale), resulted in the release of just 538.1 petabecquerels.

The Chernobyl accident happened when a combination of operator error and flawed design led to the fission reaction in Reactor 4 accelerating out of control. This massive spike in energy super-heated the water that was used as both a neutron absorber and coolant, causing a massive steam explosion that blew the roof off the reactor building. The incredibly high temperatures generated by the uncooled and uncontrolled fuel then caused the reactor's graphite control rods to catch fire. This fire burned for almost 10 days before it was eventually extinguished.

During this time, the ongoing fission reaction caused the uranium fuel rods to partially break down, releasing radioactive elements known as "fission products" into the rising smoke plume. In the first few weeks after the initial accident, the most damaging fission products were Iodine-131 and Tellurium-132, both of which were dispersed as microscopic particles that settled on food or were breathed in by those near the plant.

Iodine and Tellurium both have relatively short half lives, however, meaning that they had mostly decayed into other, more stable elements within a few weeks. Unfortunately, the other major fission product in the plume, Caesium-137, is much more persistent. It is estimated that around 26.4 kg (58 lb) of radioactive caesium dust mixed into the smoke plume and was carried all over Europe. It is this element that is responsible for the bulk of the contamination in the Chernobyl Exclusion Zone today.

Of the roughly 400 workers who were on the site at the time of the accident, 237 were hospitalized with acute radiation sickness. It has been estimated that workers received radiation doses as high as 16 sievert (just 1 Sv is enough to cause bleeding, vomiting and to dramatically increase a person's long-term cancer risk). There is only fragmentary data on the doses received by the 600,000 "liquidators" bought in to assist with the clean-up operations, and by the approximately 116,000 civilians that lived within the 30-km (18.6-mile) exclusion zone. It has been estimated that those who worked in the exclusion zone during the first year of clean-up operations received an average dose of 170 millisieverts (mSv); a dose of more than 100 mSv has been shown to increase an individual's cancer risk.

In the first few weeks after the accident, 31 people died – either as a result of radiation poisoning or injuries sustained in the initial explosion. The long-term effects of exposure are much harder to gauge, but studies have shown a statistically significant increase in cancer rates for people living in the contaminated regions. Some studies have suggested that by 2065 there will have been roughly 30,000 premature deaths attributable to the effects of the radiation released, while others have suggested the death toll may already be as high as 50,000.