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 (CO{sub 2}) 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 CO{sub 2} if the interior containment temperature drops below a present temperature, which is between the vapor-liquid critical point and {approximately}15 C below the vapor-liquid critical temperature of CO{sub 2}. The preset temperature is determined by the system hardware design. The control mechanism is driven only by the change of fluid propertiesmore » near the critical point (i.e., there are no active mechanical components).« less