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Release and spread of radioactive materials

Although it is difficult to compare the Chernobyl accident with a deliberate air burst nuclear detonation, it is estimated that Chernobyl released about 400 times more radioactive material than the combined atomic bombings of Hiroshima and Nagasaki. However, the Chernobyl disaster released only about one-hundredth to one-thousandth of the total radioactivity released during nuclear weapons testing at the height of the Cold War, due to varying isotope abundances.

Approximately 100,000 square kilometres (39,000 sq mi) of land was significantly contaminated, with the worst-affected areas in Belarus, Ukraine, and Russia. Lower contamination levels were detected across Europe, except for the Iberian Peninsula. On 28 April, workers at the Forsmark Nuclear Power Plant, 1,100 km (680 mi) from Chernobyl, were found with radioactive particles on their clothing. Sweden's elevated radioactivity levels, detected at noon on 28 April, were traced back to the western Soviet Union. Meanwhile, Finland also detected rising radiation levels, but a civil service strike delayed the response and publication.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions, much of it deposited on mountainous regions such as the Alps, the Welsh mountains and the Scottish Highlands, where adiabatic cooling caused radioactive rainfall. The resulting patches of contamination were often highly localized, and localized water-flows contributed to large variations in radioactivity over small areas. Sweden and Norway also received heavy fallout when the contaminated air collided with a cold front, bringing rain. There was also groundwater contamination.

Rain was deliberately seeded over 10,000 square kilometres (3,900 sq mi) of Belarus by the Soviet Air Force to remove radioactive particles from clouds heading toward highly populated areas. Heavy, black-coloured rain fell on the city of Gomel. Reports from Soviet and Western scientists indicate that the Belarusian SSR received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that up to half of the volatile particles had actually landed outside the former USSR area currently making up Ukraine, Belarus, and Russia. An unconnected large area in Russian SFSR south of Bryansk was also contaminated, as were parts of northwestern Ukrainian SSR. Studies in surrounding countries indicate that more than one million people could have been affected by radiation. 2016 data from a long-term monitoring program showed a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel.

In Western Europe, precautionary measures taken in response to the radiation included banning the importation of certain foods. A 2006 study found contamination was "relatively limited, diminishing from west to east", such that a hunter consuming 40 kilograms of contaminated wild boar in 1997 would be exposed to about one millisievert.