Patent GB-927601-A (GB)
Title: Improvements in or relating to thermoelectric cells
IPC: G21C3/40
Filed: 1959-09-08
Abstract: 927,601. Thermionic converters; fuel elements; utilizing energy of radiations. UNITED STATES ATOMIC ENERGY COMMISSION. Sept. 8, 1959 [Oct. 1, 1958; March 3, 1959; June 18, 1959], No. 30610/59. Classes 39 (1) and 39 (4). A thermionic converter comprises an electron emitter containing a fissionable or radioactive nuclide, and operating at a temperature of between 1500‹ C. and 3000‹ C. The electrons are collected by an electrode spaced 10<SP>-2</SP> to 10 cms. from the emitter. The device contains an alkali metal vapour at a density of 10<SP>17</SP> to 10<SP>12</SP> molecules per cc. As shown, the emitter 1, Fig. 1, is mounted on metal envelope part 4 and is additionally heated by flue gases, chemical combustion within container 7, or alpha particles, fission products or other radiations impinging on part 4. The collector 3, of zirconium, tantalum, tungsten, copper or stainless steel, is cooled by oil circulated through pipe 15. The metal container 5, insulated from pipe 15 by bushing 20, is arranged in a silicone oil bath maintained by heater 17 at a temperature of about 250‹ C. to obtain the desired operating pressure, e.g. 1 mm. Hg, of the caesium or other alkali metal vapour e.g. sodium potassium or rubidium. The electrode spacing may be 0.635 cm. In an alternative arrangement the electrodes are cylinders joined at their ends by annular insulating discs to form an annular enclosure. If the inner electrode is the emitter it may be heated by hot gases flowing therethrough, baffles being provided to restrict the flow for greater heat extraction. If the outer electrode is the emitter it may be located at the focus of a reflector and heated by solar energy. The collector temperature may be approximately half the emitter temperature to minimize the plasma resistance, or, to obtain the same effect, the area of the collector relative to that of the emitter may be increased. Separate means for ionising the vapour may be provided. The emitter is also self-heating, being made of intensely radioactive materials available as fission reactor waste products, of polonium, phosphorus 31 and strontium 90, converted to refractory compounds such as oxides or carbides. In particular the emitter may comprise 80 atomic per cent zirconium carbide and 20 atomic per cent uranium carbide enriched with uranium 235, either as a fused coating <SP>1</SP>/ 32  inch thick on a tantalum base formed by arc melting the carbides in an argon atmosphere after forming a tantalum carbide coating by heating the surface in contact with outgassed graphite powder, or as a sintered rod. The latter may be prepared by hot pressing the two carbides in powder form with 5 weight per cent cobalt and later distilling out the cobalt before finally reheating the carbides. Other carbides which may be used are those of thorium protactinium, plutonium, neptunium and americium. A thermionic cell comprising a carbide rod emitter within a cylindrical collector surrounded by cooling fins may be mounted in a dewar flask and operated in a nuclear fission reactor. A fuel rod Fig. 6 may comprise a plurality of such cells in a single envelope, the collector 53 of one cell being connected to the emitter of the next. Each emitter comprises a tantalum or zirconium base 47, a section 54 of ZrC and fuel section 55 of ZrC and UC. Insulating sleeves 56 of BeO or Al 2 O 3  separate adjacent collectors and a sleeve 58 of BeO insulates the collectors from envelope 52, a gap 69 being provided between sleeve 58 and envelope 52 to allow condensed vapour from the top cell to return to the bottom cell. Gaps 69, 70, the latter allowing for expansion of the collectors, are provided with corrugated beryllium copper spacers for good thermal conduction between the collectors and the envelope. The outside of the envelope is provided with an insulating coating since it will be immersed in cooling fluid. Continuous withdrawal of gaseous fission products which collect in space 75 is effected through passage 77. Otherwise the fuel rod is sealed at the top by alumina rings 82 and annular springs 83, 84, electrical connections being made to top collector 78 via rod 81 and to bottom emitter base 44, via envelope 52. To reduce heat radiation from the emitter, electrically floating mesh shields may be interposed between it and the collector.