Release of radioactive material into the environment
The destroyed reactor released a very large amount of radioactive material into the environment: 1019 international units of activity, termed becquerels. Although the discharge included many radioactive chemical elements, just two of them - iodine (in the short term) and caesium (in the long term) - were particularly significant from a radiological point of view. About 1018 becquerels of iodine-131 were released by the accident. Iodine is mainly absorbed by a person's thyroid gland after inhalation or after consumption of contaminated foodstuffs such as milk products; its short range beta particles irradiate the gland from the inside. Uptake of iodine by the thyroid is very easy to prevent, for example by banning consumption of contaminated food for a few weeks until the iodine-131 decays sufficiently or by administering small amounts of non-radioactive iodine prophylactically to block the thyroid gland.
About 1017 becquerels of radioactive caesiums were released, and precipitated over a vast area (see map on page 5). Exposure to caesium is difficult to prevent. Once it is deposited in the soil, its long range gamma rays can expose anybody in the area. To clean the surfaces is difficult and, if the concentration of caesium is high, often the only feasible countermeasure is to evacuate the inhabitants. Caesium in the soil can also be transferred into agricultural products and grazing animals.
For iodine-131, there is no clear information on where the release went, who was exposed to it and to which levels, or whether iodine uptake was effectively prevented. Indirect estimations gave firm indication that very high thyroid doses were incurred by some population groups. Children, who are particularly sensitive because of their normally high ingestion of milk products and their small thyroids, received higher doses. In 1990 the International Chernobyl Project (see box) had predicted that, with high doses, a significant increase in the incidence of the relatively rare cancer of the thyroid would occur in affected children after a few years.