The initiating events and propagating mechanisms of the Chernobyl accident are the subject of this analysis. The neutronics and thermohydraulics of RBMK reactors under different regimes are studied. It is found that the reactor response to a loss of pumping power was a reactivity trip that could not be fully overcome by the Doppler effect because of the neutronic importance of hydrogen captures under the conditions before the accident. This very high importance was induced by an incorrect hydraulic regime being established before the accident in order to conduct an electromechanical experiment. This experiment was responsible for the loss of pumping power that triggered the accident.