The greatest radiation accident took place in Chernobyl Nuclear Power Plant(near Pripjat, Ukraine) on April 26, 1986. In clean-up works participated more than 600 000 people, among which 200 000 constituted Ukrainians. The works were carried out up to 1990. Radioactive contamination spread on the territory of Ukraine, Belorus and Russia. The density of contamination is shown on the map to the left.