A passive cooling system for a spent fuel Pool (5,5') in a nuclear power plant, to provide cooling in the absence of onsite and offsite power, the passive cooling system comprising, in use: a gap having a first side and a second side formed at least partially along a periphery of the spent fuel pool (5,5'); the passive cooling system further comprising: a heat sink selected from the group consisting of a mass of earth, a mass of concrete or other material used in foundations or floors of spent fuel Pool structures and combinations thereof; one or more thermal conductive members having a first end connected to the second side of the gap and a second end connected to the heat sink, said one or more members structured to transport heat from the gap to the heat sink; and a water supply system (25', 25"), comprising: a water source; a discharge header having a first end connected to the water source and a second end connected to the gap; and a thermal switch mechanism having an activate position and a deactivate position, structured to deliver water from the water supply system (25', 25") into the gap when in the activate position and structured to inhibit the release of water from the water system and into the gap when in the deactivate position, the passive cooling system being configured such, that when the thermal switch mechanism is in the activate position, heat generated in the spent fuel Pool (5,5') is conducted to the gap, transported through the one or more conductive members and to the heat sink. a gap having a first side and a second side formed at least partially along a periphery of the spent fuel pool (5,5'); the passive cooling system further comprising: a heat sink selected from the group consisting of a mass of earth, a mass of concrete or other material used in foundations or floors of spent fuel Pool structures and combinations thereof; one or more thermal conductive members having a first end connected to the second side of the gap and a second end connected to the heat sink, said one or more members structured to transport heat from the gap to the heat sink; and a water supply system (25', 25"), comprising: a water source; a discharge header having a first end connected to the water source and a second end connected to the gap; and a thermal switch mechanism having an activate position and a deactivate position, structured to deliver water from the water supply system (25', 25") into the gap when in the activate position and structured to inhibit the release of water from the water system and into the gap when in the deactivate position, the passive cooling system being configured such, that when the thermal switch mechanism is in the activate position, heat generated in the spent fuel Pool (5,5') is conducted to the gap, transported through the one or more conductive members and to the heat sink. a heat sink selected from the group consisting of a mass of earth, a mass of concrete or other material used in foundations or floors of spent fuel Pool structures and combinations thereof; one or more thermal conductive members having a first end connected to the second side of the gap and a second end connected to the heat sink, said one or more members structured to transport heat from the gap to the heat sink; and a water supply system (25', 25"), comprising: a water source; a discharge header having a first end connected to the water source and a second end connected to the gap; and a thermal switch mechanism having an activate position and a deactivate position, structured to deliver water from the water supply system (25', 25") into the gap when in the activate position and structured to inhibit the release of water from the water system and into the gap when in the deactivate position, the passive cooling system being configured such, that when the thermal switch mechanism is in the activate position, heat generated in the spent fuel Pool (5,5') is conducted to the gap, transported through the one or more conductive members and to the heat sink. a water source; a discharge header having a first end connected to the water source and a second end connected to the gap; and a thermal switch mechanism having an activate position and a deactivate position, structured to deliver water from the water supply system (25', 25") into the gap when in the activate position and structured to inhibit the release of water from the water system and into the gap when in the deactivate position, the passive cooling system being configured such, that when the thermal switch mechanism is in the activate position, heat generated in the spent fuel Pool (5,5') is conducted to the gap, transported through the one or more conductive members and to the heat sink.