Patent Number: 
Section: claims

1. A natural circulation nuclear reactor system comprising:a reactor pressure vessel having a body comprising an internal cavity containing a primary coolant, a first portion of the reactor pressure vessel located above a ground level and a second portion of the reactor pressure vessel located below the ground level within a reactor well comprising vertical walls and a floor;a partition dividing the internal cavity of the reactor pressure vessel into a primary coolant riser passageway and a primary coolant downcomer passageway, wherein the partition is configured to prohibit cross-flow of the primary coolant between the primary coolant downcomer passageway and the primary coolant riser passageway at a top portion of the reactor pressure vessel;a reactor core comprising nuclear fuel disposed within the primary coolant riser passageway of the internal cavity at a bottom portion of the reactor pressure vessel;the reactor pressure vessel comprising a primary coolant outlet port in fluid communication with a top portion of the primary coolant riser passageway and a primary coolant inlet port in fluid communication with a top portion of the primary coolant downcomer passageway;a heat exchange sub-system located outside of the reactor pressure vessel, the primary coolant outlet port fluidly coupled to the heat exchange sub-system to form an incoming hot leg of the heat exchange sub-system and the primary coolant inlet port fluidly coupled to the heat exchange sub-system to form an outgoing cold leg of the heat exchange sub-system;a closed-loop primary coolant circuit that flows the primary coolant through the reactor pressure vessel to cool the reactor core and through the heat exchange sub-system to transfer heat to a secondary coolant, wherein the reactor core heats the primary coolant which flows via natural circulation through the closed-loop primary coolant circuit in a single phase;a first annular reactor flange welded to the first portion of the reactor pressure vessel and seated on top of a concrete slab which at least partially covers the reactor well and defines the ground level;the first reactor flange supporting the entire weight of the reactor pressure vessel in a vertically oriented cantilevered manner in which the reactor pressure vessel is spaced apart from vertical wall surfaces and a bottom end of the reactor pressure vessel receives no support from the floor of a reactor well, wherein the reactor pressure vessel is free to radially and axially expand due to thermal expansion as the reactor core heats up during operation; anda second annular reactor flange welded to the first portion of the reactor pressure vessel above ground level and located above the first flange, and a plurality of welded lugs extending vertically between the first and second flanges, and wherein the primary coolant inlet and outlet ports are located between the first and second flanges on the first portion of the reactor pressure vessel. 2. The natural circulation nuclear reactor system according to claim 1 wherein the partition comprises a first portion having a first end that is coupled to an inner surface of the reactor pressure vessel and a second portion that extends from the first portion to a terminal end of the partition, the terminal end of the partition spaced from a floor of the reactor pressure vessel, a bottom plenum formed between the terminal end of the partition and the floor of the reactor pressure vessel that allows cross-flow of the primary coolant from the primary coolant downcomer passageway to the primary coolant riser passageway, wherein operation of the reactor core causes the primary coolant to rise within the primary coolant riser passageway. 3. The natural circulation nuclear reactor system according to claim 2 wherein the primary coolant within the primary coolant riser passageway is fluidly coupled to the primary coolant within the primary coolant downcomer passageway only at the bottom plenum. 4. The natural circulation nuclear reactor system according to claim 2 wherein the primary coolant outlet port is located at an elevation above the first end of the first portion of the partition and wherein the primary coolant inlet port is located at an elevation below the first end of the first portion of the partition. 5. The natural circulation nuclear reactor system according to claim 1 wherein the partition is a multi-layer construction comprising an insulating layer that is sandwiched between two outer layers, wherein the insulating layer comprises a refractory material and the two outer layers comprise stainless steel. 6. The natural circulation nuclear reactor system according to claim 1 wherein the partition has an effective coefficient of thermal conductivity measured from the primary coolant riser passageway to the primary coolant downcomer passageway that is less than an effective coefficient of thermal conductivity of the primary coolant. 7. The natural circulation nuclear reactor system according to claim 1 wherein the partition comprises a first wall portion that is parallel and spaced apart from an inner surface of the reactor pressure vessel and a second wall portion that extends transversely from the first wall portion to the inner surface of the reactor pressure vessel, the first wall portion having an inner surface and an outer surface, the inner surface of the first wall portion forming the primary coolant riser passageway, and the primary coolant downcomer passageway being formed between the outer surface of first wall portion and the inner surface of the reactor pressure vessel with the second wall portion having an annular structure which encloses a top of the primary coolant downcomer passageway, the primary coolant downcomer passageway being an annular passageway that circumferentially surrounds the primary coolant riser passageway. 8. The natural circulation nuclear reactor system according to claim 1 wherein the reactor pressure vessel extends along a substantially vertical axis, wherein the second portion of the reactor pressure vessel comprises a major portion of an axial length of the reactor pressure vessel, wherein the reactor core is located below the ground level, and wherein the primary coolant outlet port, the primary coolant inlet port, and the heat exchange sub-system are located above the ground level. 9. The natural circulation nuclear reactor system according to claim 8 wherein the major portion of the reactor pressure vessel is at least 75% of the axial length of the reactor pressure vessel and is free of penetrations. 10. The natural circulation nuclear reactor system according to claim 1 wherein the natural circulation nuclear reactor system is free of active equipment for forcing circulation of the primary coolant through the closed-loop primary coolant circuit.