A state of a nuclear fission reaction in a nuclear reactor in which the fission intensity gradually increases, e.g. due to a low concentration of neutron absorbers. The supercritical state occurs when the reactor power is being increased. It is induced by withdrawing a suitable number of control rods. The amount of neutrons absorbed is then lower than the amount of neutrons produced in the fission reactions. In each generation the number of neutrons involved in fission increases and the number of fission reactions taking place increases.