Patent Number: 
Section: claims

1. A fuel channel assembly (FCA) reactor core, comprising:at least one FCA having a plurality of fuel elements;the at least one FCA comprising:a lower coolant inlet, a lower element plenum, at least one fuel element channel, an upper fuel element plenum, and a defueling chute;a plurality of coolant holes with diameters smaller than the fuel elements, the coolant holes positioned above the upper fuel element plenum to collect outlet flow of salt coolant to an exit plenum;a plurality of flow channels positioned above the fuel element exit plenum to collect outlet flow of salt coolant for transfer to a heat exchanger for removal of decay heat during loss of forced cooling; andone or more shutdown control channels disposed in the reactor core; said control channels being adjacent to and separate from said fuel element channel. 2. A FCA reactor core as recited in claim 1:wherein each fuel element comprises an inert, low-density graphite kernel, an annular fuel layer around the kernel that comprises TRISO particles and graphite binder, and a graphite outer shell around the annular fuel layer; andwherein the low-density graphite kernel is configured such that adjustment of the density of the kernel controls the total buoyancy of the pebble fuel element. 3. A FCA reactor core as recited in claim 1, further comprising at least one neutron absorber element positioned in each shut down control channel. 4. A FCA reactor core as recited in claim 3, wherein the neutron absorber elements comprise a mixture of graphite and a neutron poison. 5. A FCA reactor core as recited in claim 4, wherein the neutron poison comprises boron carbide. 6. A FCA reactor core as recited in claim 3:wherein quantity and density of the neutron absorber elements are selected to provide neutral buoyancy in salt coolant at a temperature between FCA inlet temperature and FCA outlet temperature; andwherein the neutron absorber elements are configured to passively sink into the FCA upon the temperature of the shut down control channel exceeding the neutral buoyancy temperature. 7. A FCA reactor core as recited in claim 3, further comprising:a control rod positioned above the neutron absorber elements;the control rod operable to force insertion of the neutron absorber elements into the FCA. 8. A liquid fluoride salt cooled, high temperature reactor, comprising:a reactor vessel;a reactor core contained in the reactor vessel;the reactor core comprising a plurality of parallel fuel channel assemblies (FCA's), each comprising;a lower coolant inlet, a lower fuel element plenum, a fuel element channel, an upper fuel element plenum, and an upper defueling chute;a plurality of moveable fuel elements;a plurality of coolant holes with diameters smaller than the moveable fuel elements, said coolant holes positioned above the upper fuel element plenum to collect outlet flow of salt coolant to an exit plenum;a plurality of flow channels positioned above the exit plenum to collect outlet flow of salt coolant for transfer to a heat exchanger for removal of decay heat during loss of forced cooling; andone or more shutdown control channels disposed in the reactor core; said one or more control channels being adjacent to and separate from said fuel element channel. 9. A reactor as recited in claim 8:wherein the moveable fuel elements comprise pebble fuel elements;wherein each pebble fuel element comprises an inert, low-density graphite kernel, an annular fuel layer around the kernel comprising TRISO particles and graphite binder, and a graphite outer shell around the annular fuel layer; andwherein the low-density graphite kernel is configured such that adjustment of the density of the kernel controls the buoyancy of the pebble fuel element. 10. A reactor as recited in claim 8, further comprising at least one neutron absorber element positioned in each shut down control channel. 11. A reactor as recited in claim 10, wherein each said neutron absorber element comprises a mixture of graphite and a neutron poison. 12. A reactor as recited in claim 11, wherein the neutron poison comprises boron carbide. 13. A reactor as recited in claim 10:wherein quantity and density of the neutron absorber elements are selected to provide neutral buoyancy in salt coolant at a temperature between FCA inlet temperature and FCA outlet temperature; andwherein the neutron absorber elements are configured to passively sink into the FCA upon the temperature of the shut down control channel exceeding the neutral buoyancy temperature. 14. A reactor as recited in claim 10, wherein salt coolant flows into the shut down control channel from an inlet plenum under forced circulation. 15. A reactor as recited in claim 10, further comprising:a control rod positioned above the at least one neutron absorber element;wherein the control rod is operable to force insertion of the neutron absorber elements into the FCA. 16. A reactor as recited in claim 10, further comprising:a graphite radial reflector connected to the reactor vessel near the top of the vessel;wherein the graphite radial reflector comprises a plurality of reflector blocks configured to float in said coolant; andmetal rods extending from the top of the reactor vessel to a metal reflector support structure below the graphite radial reflector, the metal rods maintaining the reflector blocks in compression during assembly, heating and filling of the reactor vessel. 17. A reactor as recited in claim 8, further comprising:a primary pump to circulate salt coolant through the at least one FCA and through an intermediate heat exchanger;wherein the primary pump is an overhung cantilever type pump;wherein the primary pump includes a suction pipe; andwherein the suction pipe includes an anti-siphon vent line to passively maintain salt coolant inventory.