Release, dispersion and deposition of radionuclides

The reactor released radioactive materials continuously in the next ten days after its explosion. The composition of radioactive materials released was complex, but the isotopes of iodine and caesium were of the greatest radiological significance because they accounted for the most radiation dose on the public. The iodine isotopes, with their short radioactive half-lives, had greater radiological impact in the short term whereas the caesium isotopes, with half-lives of the order of tens of years, had a longer-term impact.

Materials released to the atmosphere were widely dispersed and eventually deposited onto the earth's surface. The dispersion of the radioactive particles was related to their size and the prevailing meteorological conditions. Larger particles subsided within 100 kilometres of the reactor but smaller particles could spread farther, and became deposited on the ground mainly by the mechanism of rainfall. Deposition eventually encompassed the whole Northern Hemisphere, though the farther from the reactor, the lower was the level of contamination. The most highly contaminated areas were in Russia, Belarus and Ukraine and practically no pollutant was detected in the Southern Hemisphere. Twenty years have passed. The radiological influence of those radionuclides having short life-span has become negligible. Caesium-137 with a half-life of 30-year, is now the main radionuclide in the contaminated areas that is of concern. Its radiological impact can last for hundreds of years. The radiation caused by the release in the accident affects people's health through direct irradiation, inhalation and ingestion.