Abstract Conceptual design studies of advanced power reactors are summarized. The concept of power reactors cooled by supercritical water was developed. The coolant system is once through type like a supercritical fossil-fired power plant. It attains higher thermal efficiency and simpler reactor system than the light water reactors. Breeding is possible in the tight lattice core. The coolant void reactivity of the fast reactors becomes negative by placing a thin zirconium hydride layer between seed and blanket. The basic safety principles follows those of LWRs. Heterogeneous cores of the liquid metal cooled fast breeder reactor was designed for assessing the effectiveness of coolant orificing. Overpower transients after withdrawing a peripheral control rod were analysed by a three-dimensional space-time kinetics code which was developed for the study. The space-time power variations were more influential on boiling behavior in the heterogeneous cores. Conceptual design of the fusion-fission hybrid reactors and the inertial confinement fusion reactor were described for comparing with the fission power systems. The differences in heat generation profiles and technical features are discussed.