A state of a nuclear fission reaction in a nuclear reactor in which the fission rate gradually decreases, e.g. due to a high concentration of neutron absorbers. The subcritical state occurs when the reactor power is being reduced or when the reactor is shut down. It is induced by insertion of the control rods. The amount of neutrons absorbed is higher than the amount of neutrons produced in the fission reactions, so the number of fission reactions taking place gradually decreases.