The importance of passive safety for commercial nuclear power plants has been emphasized after the nuclear accidents that occurred at Three Mile Island and Fukushima. A combination of unexpected human errors, severe natural disasters, and defects of system designs led to the accidents, thereby highlighting the vulnerability of established safety systems of commercial nuclear power plants. Various passive safety systems are under development to mitigate design-basis accidents. However, several uncertainties and problems have been pointed out. As a solution to the problems, this paper proposes various designs for a passive in-core cooling system (PINCs) based on hybrid heat pipes. The feasibility and coolability of the PINCs as a passive safety system for commercial pressurized water reactors was investigated using experimental works and numerical analyses. The PINCs showed sufficient coolability to mitigate station blackout conditions by delaying core uncovery. Additionally, several PINCs concepts for advanced nuclear power plants such as a small modular reactor and Generation IV reactors are suggested.