The topic of this thesis is the accident that occurred on April 26, 1986 at the Chernobyl nuclear power plant, caused by flaws in the design of the reactor, staff’s lack of information about the mentioned flaws and the ignoring of safety measures when conducting the safety test on reactor No. 4. The fission process that occurs in nuclear power plant reactors that use enriched uranium as fuel is explained. The history of nuclear power plants and the development of reactors are presented, with special emphasis on the RBMK-1000 reactor, which was used in nuclear power plants in the former Soviet Union, including in the Chernobyl nuclear power plant. The working principle of the RBMK-1000 reactor is described and shown schematically, and its most important characteristics are explained - the positive void coefficient, which is especially important at low reactor powers, and the poor design of the control rods, the end of which is made of graphite, which does not slow down the chain reaction but, on the contrary, it promotes it. The accident at the Chernobyl nuclear power plant is presented in paragraphs chronologically, with schematic representations of the course of the accident. The human efforts and sacrifices during the remediation of the consequences of the accident and the health consequences of the inhabitants of the surrounding area, especially of children, where the number of thyroid cancers has increased a hundredfold, are shown.