Patent Number: 
Section: claims

1. A containment enclosure for shielding a nuclear waste cask comprising:a lower base portion at least partially embedded in a concrete pad, the base portion comprising a baseplate supporting a plurality of coaxially aligned shells defining a lower cavity;an upper radiation shielding portion coupled to and supported by the lower base portion, the shielding portion defining an upper cavity;the shielding portion comprising a radiation shielding material configured to block gamma and neutron radiation;the lower and upper cavities collectively defining a contiguous containment space configured for holding the cask;wherein the base and shielding portions enclose the cask. 2. The system according to claim 1, wherein the radiation shielding material comprises boron. 3. The system according to claim 2, wherein the shielding portion has a composite wall construction comprising an outer wall, an inner wall, and an intermediate layer sandwiched therebetween which comprises the boron. 4. The system according to claim 3, wherein the outer and inner walls are formed of steel. 5. The system according to claim 3, wherein the shielding portion comprises a straight cylindrical sidewall section and a frustoconical top wall section terminating in a top opening. 6. The system according to claim 1, wherein the shielding portion is separated from and sealed to the lower base portion at a horizontal joint by an annular gasket compressed between an annular upper seal plate of the lower base portion and an annular lower seal plate of the shielding portion. 7. The system according to claim 6, wherein the annular gasket is spaced radially inwards from an exterior surface of the cask containment enclosure. 8. The system according to claim 1, further comprising an ambient cooling air ventilation system comprising an annular downcomer formed in the lower base portion, and an annular riser collectively formed in the lower base and shielding portions between the cask and the base and shielding portions. 9. The system according to claim 8, wherein the ventilation system further comprises:a plurality of upper air inlets circumferentially spaced apart at a circumferential joint between the base portion and the shielding portion;an annular upper air inlet plenum in fluid communication with the upper air inlets and the downcomer;a plurality of air exchange passageways circumferentially spaced apart at a bottom of the base portion, the passageways in fluid communication with the downcomer and riser; anda top discharge opening formed in the shielding portion in fluid communication with the riser. 10. The system according to claim 9, wherein the ventilation system defines a natural convection-driven ventilation air pathway in which air enters the upper air inlets in a laterally inward direction, flows downwards in the downcomer to the air exchange passageways, reverses direction and flows upwards in the riser to the top discharge opening. 11. The system according to claim 9, wherein a bottom end of the shielding portion has a castellated configuration which defines the upper air inlets. 12. The system according to claim 11, further comprising a castellated annular air inlet skirt welded to a bottom end of shielding portion which comprises a plurality of circumferentially spaced castellations. 13. The system according to claim 12, wherein bottom ends of the castellations are welded to a top end of the base portion. 14. The system according to claim 9, wherein the annular upper air inlet plenum is located at the circumferential joint and recessed inside the shielding portion. 15. The system according to claim 14, wherein the circumferential joint comprises an annular gasket located inboard of the annular air inlet plenum. 16. The system according to claim 9, further comprising a frustoconical shaped discharge plenum located in the upper shielding portion above the cask. 17. The system according to claim 9, wherein no straight line of sight exists through the upper air inlets to outside to prevent straight line radiation streaming. 18. The system according to claim 1, wherein the containment space is configured for holding no more than a single one of the cask. 19. The system according to claim 1, wherein the cask contains a spent nuclear fuel canister therein which contains the nuclear waste. 20. The system according to claim 3, wherein the shells of the base portion comprises an outer shell, and a pair of conjugate shells spaced radially inwards therefrom by a distance greater than a radial gap formed between the conjugate shells. 21. The system according to claim 20, wherein a bottom end of the shielding portion comprises an inwardly recessed stepped portion including a pair of conjugate walls spaced radially apart by a radial gap the same as the radial gap between the conjugate shells of the base portion. 22. The system according to claim 21, further comprising a first annular seal plate attached to the conjugate walls and a second annular seal plate attached to the conjugate shells, and an annular gasket compressed between the first and second annular seal plates. 23. A nuclear waste storage facility comprising a plurality of the containment enclosures according to claim 1. 24. A nuclear waste storage system comprising:a concrete pad;a plurality of cask containment enclosures arranged on the concrete pad, each containment enclosure housing a storage cask containing a canister holding radioactive nuclear waste;each containment enclosure comprising:a cylindrical lower base portion at least partially embedded in the concrete pad, the lower base portion comprising a baseplate supporting a plurality of coaxially aligned shells defining a lower cavity;a separate upper shield jacket coupled to and supported by the lower base portion, the shield jacket having a wall construction comprising boron-containing radiation shielding materials, the upper shield jacket defining an upper cavity;a plurality of ambient cooling air inlets formed in the shield jacket, the air inlets circumferentially spaced apart at a circumferential joint between the base portion and the shield jacket;an annular air inlet plenum formed at a bottom of the shield jacket by an inwardly recessed stepped portion of the shield jacket, air inlet plenum located at the circumferential joint and in direct fluid communication with the air inlets and a downcomer formed in the lower base portion;a plurality of air exchange passageways circumferentially spaced apart at a bottom of the base portion, the passageways in fluid communication with the downcomer and a riser which extends vertically between the cask and innermost surfaces of the lower base portion and shield jacket; anda top discharge opening formed in the shield jacket in fluid communication with the riser and ambient air;wherein the storage cask containing nuclear waste is positioned partially in both the upper and lower cavities of the lower base portion and shield jacket. 25. The system according to claim 24, further comprising a cap attached to a top of the shield jacket, the cap defining a plurality of laterally open air discharge openings for venting air heated in the riser by the cask to atmosphere. 26. The system according to claim 24, wherein the cooling air inlets are formed by an annular castellated skirt attached to a bottom end of the shield jacket. 27. The system according to claim 24, wherein the circumferential joint is located radially inboard of the cooling air inlets, the circumferential joint including an annular gasket compressed between the lower base portion and the shield jacket. 28. The system according to claim 24, wherein the upper and lower cavities are collectively configured to hold no more than a single storage cask. 29. The system according to claim 24, wherein ambient cooling air flow is driven by natural convection circulation, the cooling air flowing through the cooling air inlets into the inlet plenum, downward through the downcomer, inwards through the air exchange passageways into a bottom of the riser, and upwards through the riser to the discharge opening in the shield jacket. 30. The system according to claim 24, wherein the shield jacket comprises an upper frustoconical section, a lower cylindrical section, and an annular air inlet skirt attached to the lower cylindrical section which defines openings for the cooling air inlets.