On 26 April 1986, the most serious accident in the history of the nuclear industry occurred at Unit 4 of the Chernobyl nuclear power plant in the former Ukrainian Republic of the Soviet Union. The explosions that ruptured the Chernobyl reactor vessel and the consequent fire that continued for 10 days or so resulted in large amounts of radioactive materials being released into the environment.
The cloud from the burning reactor spread numerous types of radioactive materials, especially iodine and caesium radionuclides, over much of Europe. Radioactive iodine-131, most significant in contributing to thyroid doses, has a short half-life (8 days) and largely disintegrated within the first few weeks of the accident. Radioactive caesium-137, which contributes to both external and internal doses, has a much longer half-life (30 years) and is still measurable in soils and some foods in many parts of Europe, see Fig. 1. The greatest deposits of radionuclides occurred over large areas of the Soviet Union surrounding the reactor in what are now the countries of Belarus, the Russian Federation and Ukraine.
FIG. 1. Surface-ground deposition of 137 Cs throughout Europe as a result of the Chernobyl accident (De Cort et al. 1998).
An estimated 350 000 emergency and recovery operation workers, including army, power plant staff, local police and fire services, were initially involved in containing and cleaning up the accident in 1986-1987. Among them, about 240 000 recovery operation workers took part in major mitigation activities at the reactor and within the 30-km zone surrounding the reactor. Later, the number of registered "liquidators" rose to 600 000, although only a small fraction of these were exposed to high levels of radiation.
More than five million people live in areas of Belarus, Russia and Ukraine that are classified as "contaminated" with radionuclides due to the Chernobyl accident (above 37 kBq m-2 of 137Cs) 2 [Becquerel (Bq) is the international unit of radioactivity equal to one nuclear decay per second]. Amongst them, about 400 000 people lived in more contaminated areas - classified by Soviet authorities as areas of strict radiation control (above 555 kBq m-2 of 137Cs). Of this population, 116 000 people were evacuated in the spring and summer of 1986 from the area surrounding the Chernobyl power plant (designated the "Exclusion Zone") to non-contaminated areas. Another 220 000 people were relocated in subsequent years.
Unfortunately, reliable information about the accident and the resulting dispersion of radioactive material was initially unavailable to the affected people in what was then the Soviet Union and remained inadequate for years following the accident. This failure and delay led to widespread distrust of official information and the mistaken attribution of many ill health conditions to radiation exposure.