[en] In the history of the world nuclear industry there were four major accidents of operating nuclear reactors, i.e., at plutonium production facility in Windscale, UK, 1957; at NPP Three Mile Island, USA, 1979; at Chernobyl NPP, USSR, 1986; and at Fukushima-1 NPP, Japan, in 2011. The Chernobyl accident was the most severe, causing a huge release and deposition of radionuclides over large areas of Europe. Only after this accident there were real health effects caused by radiation, including carcinogenic effect among the population of the adjacent areas of Belarus, Russia and Ukraine. In this paper, same-type basic radiological characteristics are presented for four reactor accidents with more detail presented for the Chernobyl accident. The latter include mean radiation doses incurred by various groups of inhabitants of the three more affected countries, Belarus, Russia and Ukraine. Those who were children at the time and drank milk with high levels of radioactive iodine received high thyroid doses. Since early 1990s there was dramatic increase in thyroid cancer incidence among those exposed to radioiodine at a young age. Apart from this kind of health effects there was no clearly demonstrated increase in the somatic diseases due to radiation. The paper discusses the reasons why the Chernobyl accident had severe radiological consequences. (author)