The paper considers the possibility of generating energy in a hybrid fission and fusion reactor in which deuterium is used as the thermonuclear fuel. It studies the energy balance of a plasma for the case of injection and non-injection of a beam of high-energy deuterons (hundreds of keV). The authors determine the conditions under which the requirements in respect of the plasma parameters are least rigorous. They show that the D-D nuclear fusion reaction can be used in a hybrid reactor with a deeply subcritical blanket.