Patent Number: 047553502
Section: description

DETAILED DESCRIPTION Referring now to FIGS. 1 and 1A of the drawings, there is shown a perspective view of an assembly 10 of four semi-circular thermionic converters 12 wired in parallel, and a wedge section of assembly 10 removed to show more detail. Each thermionic converter 12 comprises an outer emitter electrode 14, separated at a fixed distance from an inner collector electrode 16 by insulated spacers and expansion joints 18. Spacers 18 include the connections for the parallel wiring of thermionic converters 12. Assembly 10 includes an outer layer of nuclear fuel 20, and an enclosed container 22 containing a core of heat sink material 24. Container 22 is electrically insulated from collectors 16 by an insulating coating 26. Voids 28 are allowed to form inside the core of heat sink material 24 to provide for expansion. Heat stored in heat sink material 24 is removed by an enclosed heat pipe 30. FIG. 2 is a perspective view of a thermionic reactor module 32 comprising approximately 100 thermionic converter assemblies 10 stacked in series. Each assembly 10 is electrically distinct from other assemblies 10 on either side. Container 22 and the core of heat sink material 24 extend unbrokenly through module 10 and to form a thermal reservoir for the reactor module. Heat from nuclear fuel 20 is applied to the outside of the emitters 14 to produce an electric current from emitters 14 to collectors 16. Waste heat is stored in the thermal reservoir comprising container 22 and the core of heat sink material 24 and is slowly removed by heat pipe 28. FIG. 3 is a representative perspective view of a space based nuclear reactor system 34 comprising an array of ninety-one modules 32. Heat pipes 28, as shown in FIGS. 1 and 2, but hidden in this figure, connect to an lithium hydride (LiH) radiation shield 36, which serves as a further heat sink and is connected to radiator 38 which ultimately transfers the waste heat to space. Control rods 40 are variously located between individual modules 32 of the array to moderate the fission producing neutron flux between modules 32. Bus bar 42 provides a common current path for the connected upper terminals of modules 32. In a space based system, emitter electrodes 14 are preferably made of a material such as molybdenum. The collector electrodes 16 are also preferably made of molybdenum, or similar material such as niobium. Spacers 18 are preferably made of materials such as aluminum oxide or yttrium oxide. Nuclear fuel 20 will typically be uranium carbide. Container 22 preferably is made of stainless steel or titanium and is insulated from collectors 16 by insulating coating 26 preferably made of Al.sub.2 O.sub.3 or TiO.sub.x. The possible heat sink materials 24 include ice, lithium or similar materials that will absorb a large amount of energy through a phase change, and preferably are a lithium salt, particularly lithium hydride. These materials are particularly suitable in a nuclear fission powered reactor because they act as neutron moderators (the lithium when enriched in the Li-7 isotope), moderating the energy of neutrons passing through them, and increasing the number of neutrons of lower energy levels able to create new fission chain reactions. Calculations indicate that as much as 8 megajoules/Kg of thermal energy may be stored in LiH. This permits a hexagonal array of ninety-one modules 32, as shown in FIG. 3, to produce 25 megawatts of electric power for 450 seconds with a total system weight of approximately 20,000 kilograms. The reactor will regain about 50 percent of its capacity in one orbit of 90 to 120 minutes. Each container 22 of heat sink material 24 is approximately 3.0 m in length with an outer diameter of 20 cm. The inner diameter of thermionic converters 12 is a corresponding 20 cm. Each electrode 14 and 16 is 2.6 mm thick with an interelectrode gap distance of 0.3 mm. The neutron absorbing property of the lithium hydride heat sink material 24 permits the thickness of the uranium fuel layer 20 to be 1 mm or less for a ninety-one module array. The overall length of nuclear reactor 34 is 7-8 m and the diameter 2.5-3.5 m. The disclosed method of making the nuclear powered thermionic reactor successfully demonstrates the use and advantages of placing the emitter electrode on the outside and containing a heat sink material inside the thermionic converter. Though the disclosed use is specialized, it will find application in other areas of energy generation where the advantages of a self-contained system are required. The disclosed embodiment of a nuclear energy powered thermionic reactor uses an array of modules of stacked thermionic assemblies. Those with skill in the art will see that individual assemblies made with single instead of multiple thermionic converters are generally equivalent structures and may provide different desired operating characteristics. Similarly, the individual assemblies are shown as generally cylindrical concentric structures. Those with skill in the art will see that any other structure wherein the heat sink is generally enclosed by, respectively, a collector and an emitter, such as concentric spheres or other shapes, is equivalent. In a spherical design, the heat pipe for removal of heat from heat sink material 24 becomes a more critical element. Those with skill in the art will see also that the use of a combination heat source and emitter, such as an alloy of metal and uranium carbide, will result in an equivalent structure. Also, reactor modules 32 are described as a stacked series of discrete assemblies, similar to cells in a battery, with heat sink material 24 and nuclear fuel 20 being continous from assembly to assembly in an assembled module. Other advantages may be found in modules having a different and more continous arrangement of emitters and collectors. It is understood that other modifications to the invention as described may be made, as might occur to one with skill in the field of the invention, within the intended scope of the claims. Therefore, all embodiments contemplated have not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the claims.