The temperature-initiated passive cooling system (TIPACS) is a new reactor containment cooling system that is applicable to multiple reactor types. TIPACS, which transfers heat from a hot, insulated system to a cooler, external environment, has five defining characteristics: It has efficient heat transfer, is passive (i.e., no moving mechanical components), has a thermal switch mechanism that allows heat transfer only above a preset temperature, has one-way (heat diode) heat transfer from the internal warm system to ambient, and is suitable to use with any size power reactor. TIPACS consists of two subsystems: a heat transfer system (HTS) and a temperature control system (TCS). The HTS in full operation is a single-phase, natural-circulation system that uses carbon dioxide (CO2) above its vapor-liquid critical point (T > 31°C; P > 72.85 atm) as the heat transfer fluid. The TCS is a passive device that blocks the flow of CO2 if the interior containment temperature drops below a preset temperature, which is between the vapor-liquid critical point and ∼15°C below the vapor-liquid critical temperature of CO2. The preset temperature is determined by the system hardware design. The control mechanism is driven only by the change of fluid properties near the critical point (i.e., there are no active mechanical components)