Patent Number: 
Section: claims

1. A small nuclear fission reactor designed to operate for a decade or longer without refueling, which reactor comprises:a reactor vessel,a central core within said vessel for creating heat via fission reactions in said core, which core includes a plurality of initial fissile sections located in a plurality of vertically spaced apart horizontal regions, and flanking conversion sections, said a plurality of initial fissile sections remaining an active, integral part of a critical core region throughout the lifetime of the central core,a helium circulation system for extracting heat from said core by the circulation of helium into and out of said vessel to maintain the core temperature between about 700° C. and 1000° C. and to generate power from said heated helium exterior of said vessel,said a plurality of initial fissile sections of said core comprising fuel elements in the form of silicon carbide containers which contain sintered fuel bodies comprising carbide fissile and fertile nuclides, the silicon carbide containers having a thickness of at least about 1 mm, anda system for continuously withdrawing volatile fission products from said fuel elements during normal operation, the fission products being removed in a flow path separate from the helium circulation system. 2. The reactor of claim 1 wherein said a plurality of initial fissile sections comprise two spaced apart horizontal regions with each comprising a generally annular area of said fissile fuel bodies and wherein said flanking conversion sections comprise horizontal regions of fertile fuel bodies located above, between and below said two horizontal regions containing said fissile fuel bodies. 3. The reactor of claim 2 wherein said core further comprises fertile fuel bodies located in the center of and about the periphery of both of said generally annular areas of said fissile fuel bodies in their respective horizontal regions that comprise said a plurality of initial fissile sections. 4. The reactor of claim 3 wherein said horizontal regions which comprise said a plurality of initial fissile sections and said flanking conversion sections each comprise a plurality of fuel element assemblies, each assembly comprising a holder with multiple fuel elements arranged therewithin, which fuel elements contain said sintered fuel bodies and are aligned within said core to facilitate helium coolant flow vertically through said assemblies in passageways adjacent each said fuel element. 5. The reactor of claim 4 wherein said assemblies of vertically aligned fuel elements in said conversion sections and in said initial fissile section are arranged to create a plurality of juxtaposed vertical columns extending through said central core. 6. The reactor of claim 4 wherein a plurality of said fuel elements within each said holder are manifolded to a common connector to facilitate the withdrawal of volatile fission products as a composite stream from said fuel elements therewithin. 7. The reactor of claim 6 wherein said assemblies each comprise at least one of a plurality of fissile and fertile fuel elements in the form of containers formed of silicon carbide cladding that each enclose an interior fuel region in the form of a flat plate comprising at least one of sintered carbide fissile and fertile nuclides. 8. The reactor of claim 7 wherein said central core is surrounded by a plurality of blocks of BeO or Be2C reflector material to provide a surrounding reflector region that has a right circular cylindrical exterior surface. 9. The reactor of claim 8 wherein core reactivity control mechanisms in the form of vertically aligned, right circular cylindrical control drums are disposed in recesses in said reflector region to control the neutron population within said core. 10. The reactor of claim 8 wherein said reflector region is surrounded by an annular graphite outer reflector, which is in turn surrounded by an annular neutron shield containing a neutron capture material that is located in juxtaposition with a tubular core barrel that is spaced from an interior surface of said reactor vessel to provide coolant flow passageways therebetween. 11. The reactor of claim 1 wherein said sintered carbide fuel in said fuel elements occupies the interior of each said container to a packing density of about 50 to 80 volume percent in order to provide space for the accumulation of nonvolatile fission products therewithin and assure sufficient interconnected porosity for volatile fission product migration and exit therefrom. 12. The reactor of claim 11 wherein said fuel elements contain sintered near-monocarbides which comprise at least about 5% excess carbon in the immediate fuel body region to provide carbon for potential chemical reaction with fission products. 13. The reactor of claim 12 wherein said initial fissile section fuel bodies comprises UC1.05-UC1.3 with an enrichment of between about 4% and 18%. 14. A small nuclear fission waste conversion reactor designed to operate for a decade or longer without refueling, which reactor comprises:a reactor vessel,a central core within said vessel for creating heat via fission reactions in said core, which core includes one or more initial fissile sections and flanking conversion sections, which one or more initial fissile sections remain a part of the critical central core throughout reactor lifetime,a helium circulation system for extracting heat from said core by the circulation of helium into and out of said vessel to maintain the core temperature between about 700° C. and 1000° C. and to generate power from said heated helium at a location exterior of said vessel,said core including a plurality of fuel elements in the form of silicon carbide containers that enclose sintered bodies of at least one of carbide fissile and fertile nuclides, the silicon carbide containers having a thickness of at least about 1 mm, the sintered bodies each include a central hole, anda system for withdrawing volatile fission products from said plurality of fuel elements during normal operation the fission products being removed in a flow path separate from the helium circulation system via the central hole in the sintered bodies. 15. The reactor of claim 14 wherein said core further comprises additional sintered fertile fuel bodies in two initial fissile horizontal regions, which additional fertile bodies are located centrally within and about the periphery of an annular area of each fissile fuel element. 16. The reactor of claim 14 wherein the materials present within the core are selected so that a majority of the fission reactions within the core occur using neutrons that have not yet slowed to thermal energy levels. 17. The reactor of claim 14 wherein the amounts of fertile and fissile fuel in said sintered bodies within said initial core are such that, after 10 years of essentially continuous operation, the major portion of the energy being produced in the reactor results from fissioning of nuclides that were present in the initial reactor core as fertile nuclides and were subsequently converted into fissile nuclides. 18. The reactor of claim 14 wherein silicon carbide fuel element containers enclose the fissile and fertile fuel in said central core, which containers have the ability to anneal radiation-induced displacements within the temperature range of 700°-1000° C. so as to allow the central core to operate at high total fluence levels. 19. The reactor of claim 18 wherein said fuel element containers comprise woven silicon carbide material impregnated with vapor-deposited n-SiC.