A building for wet storage of spent nuclear fuel, the building comprising: a longitudinal axis; a concrete base mat defining a substantially horizontal operating deck; a plurality of substantially vertical perimeter walls supported by the base mat; a roof spanning across the perimeter walls (23, 24); a fuel pool recessed in the base mat below the operating deck and having a first depth, the fuel pool containing coolant water having a surface level; a cask pit recessed in the base mat below the operating deck and formed integrally with the fuel pool, the cask pit located adjacent the fuel pool and having a second depth; a fluid passageway formed through the base mat between the cask pit and fuel pool, the cask pit in fluid communication through the passageway with the fuel pool such that liquid coolant is exchangeable between the fuel pool and cask pit; and an isolation gate movably disposed in the passageway which is operable to fluidly isolate the cask pit from the fuel pool, the gate movable between an open position in which liquid coolant can flow between the fuel pool and cask pit, and a closed position in which liquid coolant cannot flow between the fuel pool and cask pit; wherein when the gate is in the open position, water from the fuel pool flows into the cask pit such that the surface level of water in the cask pit and pool equalize; characterized by a passive cooling system for cooling the water in the fuel pool, the cooling system comprising an internal immersion heat exchanger submerged in the fuel pool and an external air cooled heat exchanger located outside the perimeter walls, the immersion heat exchanger located below grade and the air cooled heat exchanger located above grade creating an elevation difference; and a closed flow loop formed between the immersion and air cooled heat exchangers, the flow loop containing a circulating heat exchange working fluid; wherein the working fluid circulates by natural gravity driven flow between the immersion and air cooled heat exchangers such that the fuel pool is cooled in the absence of electric power to the facility; wherein the air cooled heat exchanger is disposed inside a cavity of a vertically elongated steel reinforced concrete silo; wherein the silo includes an open top defining an air outlet and air intake vent in a sidewall of the silo defining an air inlet, the air inlet and outlet in fluid communication with the air cooled heat exchanger; wherein the sidewall of the silo is a vertically extending semicircular sidewall which extends below grade, and opposing ends of the semicircular sidewall are attached to the one of the plurality of substantially vertical perimeter walls. a longitudinal axis; a concrete base mat defining a substantially horizontal operating deck; a plurality of substantially vertical perimeter walls supported by the base mat; a roof spanning across the perimeter walls (23, 24); a fuel pool recessed in the base mat below the operating deck and having a first depth, the fuel pool containing coolant water having a surface level; a cask pit recessed in the base mat below the operating deck and formed integrally with the fuel pool, the cask pit located adjacent the fuel pool and having a second depth; a fluid passageway formed through the base mat between the cask pit and fuel pool, the cask pit in fluid communication through the passageway with the fuel pool such that liquid coolant is exchangeable between the fuel pool and cask pit; and an isolation gate movably disposed in the passageway which is operable to fluidly isolate the cask pit from the fuel pool, the gate movable between an open position in which liquid coolant can flow between the fuel pool and cask pit, and a closed position in which liquid coolant cannot flow between the fuel pool and cask pit; wherein when the gate is in the open position, water from the fuel pool flows into the cask pit such that the surface level of water in the cask pit and pool equalize; characterized by a passive cooling system for cooling the water in the fuel pool, the cooling system comprising an internal immersion heat exchanger submerged in the fuel pool and an external air cooled heat exchanger located outside the perimeter walls, the immersion heat exchanger located below grade and the air cooled heat exchanger located above grade creating an elevation difference; and a closed flow loop formed between the immersion and air cooled heat exchangers, the flow loop containing a circulating heat exchange working fluid; wherein the working fluid circulates by natural gravity driven flow between the immersion and air cooled heat exchangers such that the fuel pool is cooled in the absence of electric power to the facility; wherein the air cooled heat exchanger is disposed inside a cavity of a vertically elongated steel reinforced concrete silo; wherein the silo includes an open top defining an air outlet and air intake vent in a sidewall of the silo defining an air inlet, the air inlet and outlet in fluid communication with the air cooled heat exchanger; wherein the sidewall of the silo is a vertically extending semicircular sidewall which extends below grade, and opposing ends of the semicircular sidewall are attached to the one of the plurality of substantially vertical perimeter walls.