On April 26, 1986, a catastrophic accident occurred at the Chernobyl-4 reactor near the town of Pripyat, Ukraine, 100 kilometers northwest of Kiev. Figure 1 shows the reactor location and the regions of most intense radioactive contamination. The accident destroyed the reactor and released a large amount of radioactivity into the atmosphere, particularly radioactive iodine (I-131) and radioactive cesium (Cs-137), both of which have the potential to cause cancer. Thirty-one workers at the plant died within a few weeks, most of them from receiving lethal doses of radiation while putting out fires and responding to other emergencies.

Radiation fallout caused significant contamination in parts of Belarus, Russia, and the Ukraine, resulting in the resettlement of more than 350,000 people from 4,300 square kilometers. An approximate five- to ten-fold increase in thyroid cancer has been observed in children from Belarus, Russia, and Ukraine who received a large exposure to I-131. The economic impact has also...