Patent Number: 
Section: claims

1. 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;a spent fuel pool recessed in the base mat below the operating deck and having a first depth, the spent 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 spent fuel pool, the cask pit located adjacent the spent fuel pool and having a second depth;a fluid passageway formed through the base mat between the cask pit and spent fuel pool, the cask pit in fluid communication through the passageway with the spent fuel pool wherein liquid coolant is exchangeable between the spent fuel pool and cask pit; andan isolation gate movably disposed in the passageway and operable to fluidly isolate the cask pit from the spent fuel pool, the gate movable between an open position in which liquid coolant can flow between the spent fuel pool and cask pit, and a closed position in which liquid coolant cannot flow between the spent fuel pool and cask pit;a passive cooling system for cooling the water in the spent fuel pool, the cooling system comprising an internal immersion heat exchanger submerged in the spent 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;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 wherein the spent 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, the silo attached to one of the plurality of substantially vertical perimeter walls of the facility and configured so that no part of the closed flow loop is exposed on an exterior of the building;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 silo includes 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; andwherein when the gate is in the open position, water from the spent fuel pool flows into the cask pit wherein the surface level of water in the cask pit and pool equalize. 2. The building according to claim 1, wherein the passageway has an open top which penetrates the operating deck, a closed bottom, and opposing vertical stub walls formed by the base mat. 3. The building according to claim 1, wherein the first depth of the pool is substantially the same as the second depth of the cask pit. 4. The building according to claim 2, wherein the passageway has a height greater than one-half of the second depth of the cask pit to allow a fuel assembly comprising a plurality of fuel rods to pass through the passageway from the cask pit into the spent fuel pool. 5. The building according to claim 1, wherein the cask pit and the spent fuel pool each include a horizontal slab defining an upward facing bottom surface, the bottom surfaces being at substantially the same elevation. 6. The building according to claim 1, further comprising an air lock formed between a pair of outer access doors and a pair of inner access doors spaced longitudinally apart from the outer access doors. 7. The building according to claim 6, further comprising a first cask containing a plurality of nuclear spent fuel assemblies located on the operating deck in an access bay formed between the inner access doors and the cask pit. 8. The building according to claim 7, further comprising a first crane configured and operable to lift and move the first cask from the access bay into the cask pit. 9. The building according to claim 8, wherein the cask pit is sized to hold only the first cask and no others at a same time. 10. The building according to claim 9, further comprising a second cask pit sized to hold only a single cask similar in configuration to the first cask. 11. The building according to claim 7, wherein the passageway has a height sufficient to allow a single fuel assembly removed from the first cask by a second crane so that the single fuel assembly remains underwater while moving the single fuel assembly through the passageway into the spent fuel pool. 12. The building according to claim 1, further comprising an air handling system configured to maintain a negative atmospheric pressure inside the building. 13. The building according to claim 1, wherein the roof has a domed shape. 14. The building according to claim 1, wherein the perimeter walls and roof are formed of steel reinforced concrete. 15. The facility according to claim 1, wherein the one of the plurality of substantially vertical perimeter walls forms a second sidewall of the silo. 16. An autonomous impact resistant wet storage facility for spent nuclear fuel, the facility comprising:a longitudinal axis;a concrete base mat defining a substantially horizontal operating deck, the base mat disposed in soil having a grade and extending for a depth below grade;a plurality of substantially vertical concrete perimeter walls supported by the base mat, the perimeter walls extending upwards from the operating deck and rising above grade;a concrete roof spanning across the perimeter walls and collectively forming an enclosed building with the perimeter walls and base mat;a spent fuel pool disposed in the building and recessed in the base mat below the operating deck for storing nuclear fuel assemblies, the spent fuel pool containing water for cooling the fuel assemblies;a cask pit recessed in the base mat below the operating deck and formed integrally with the spent fuel pool, the cask pit located adjacent the spent fuel pool and having a second depth;a vertically elongated fluid passageway formed through the base mat between the cask pit and spent fuel pool, the passageway extending vertically through the operating deck, wherein the cask pit is in fluid communication through the passageway with the spent fuel pool wherein pool water is flowable between the spent fuel pool and cask pit;an isolation gate movably disposed in the passageway which operates to fluidly isolate the cask pit from the spent fuel pool, the gate movable between an open position in which the passageway is unobstructed so pool water can flow between the spent fuel pool and cask pit, and a closed position in which the passageway is obstructed so pool water cannot flow between the spent fuel pool and cask pit;a passive cooling system for cooling the water in the spent fuel pool, the cooling system comprising an internal immersion heat exchanger submerged in the spent 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;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 wherein the spent 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, the silo attached to one of the plurality of substantially vertical perimeter walls of the facility; andwherein the silo includes 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. 17. The facility according to claim 16, 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. 18. The facility according to claim 16, wherein the one of the plurality of substantially vertical perimeter walls forms a second sidewall of the silo.