Patent Number: 043437613
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a device for removing heat from a neutron-producing plasma. Heat energy is transferred from the plasma by means of neutron radiation and absorbed within a circulating solid media that is cooled for extracting heat energy for use. The neutron radiation is produced in a plasma containing ions of such as deuterium and tritium that react to produce helium and energetic neutrons. The plasma containing such reactions is fully described in U.S. Pat. No. 3,037,921 to Tuck, entitled "Method and Apparatus for Producing Neutrons and Other Radiations". This patent is expressly incorporated herein for the purpose of describing such a neutron-producing plasma. A neutron-producing plasma of this type can be produced not only by the reaction of tritium and deuterium to form helium ions and neutrons but also by various other reactions. For example, the reaction of deuterium with deuterium, helium isotopes with deuterium and helium with protons are contemplated. Reactions of these types also are suggested in the above patent as a source of neutron radiation. These neutron-producing reactions occur at extremely high temperatures and release very large quantities of energy. Previous coolant systems thus have been severely tested in regard to strength of materials and heat transfer rates due to the high temperatures and energetic output of these reactions. In addition, the problem of breeding additional fuel, particularly tritium, often is approached by combining this breeding function with that of heat transfer. One proposed system employs the gravity flow of solid lithium oxide microspheres for removing heat from the neutron-producing plasma as well as for breeding tritium through a neutron-lithium reaction. As is well known, both Li.sup.6 and Li.sup.7 react with neutrons to produce tritium and helium. However, the Li.sup.7 isotope has a greater propensity for reaction with energetic neutrons, which reaction additionally produces a secondary slow neutron. Other similar systems have proposed the use of molten lithium metal for this combined heat transfer and breeding function. The combination of heat removal and tritium breeding in a single media, although appealing from a functional and utilitarian viewpoint, has inherent and serious disadvantages. A major difficulty is that the heat transfer media becomes radioactive with the production of tritium which necessitates complicated and cumbersome maintenance techniques along with extended waiting periods for the decay of radioisotopes. The problem of tritium diffusion from the heat transfer system likewise must be considered. In addition, optimum breeding materials and conditions do not necessarily provide optimum characteristics for heat transfer such that a compromise as to desiderata in each of these functions may be required. Where lithium metal is selected its extremely high chemical reactivity and corrosiveness requires that it be kept scrupulously free of materials such as oxygen and nitrogen with which it reacts. PRIOR ART STATEMENT The following publications relate to but do not disclose the invention as claimed in the present application for patent. Miller et al., U.S. Pat. No. 3,976,888 discloses a device for reacting deuterium and tritium to produce 14 Mev neutrons and helium. The device is nested within the flux trap of a nuclear reactor and is cooled by a flow of deuterium-tritium gas. Goldstein et al., U.S. Pat. No. 3,899,676, discloses as in-core measuring device for power distribution and fuel breeding rates within a nuclear reactor. Beryllia balls containing uranium isotopes are fed into spindles located at desired positions within the reactor core and maintained there for a required period of irradiation. Subsequently the balls are driven from the spindles by sodium flow for radiation measurements. Winsche et al., U.S. Pat. No. 3,969,631, discloses a tritium breeding system in which lithium alloy targets are neutron-irradiated within gas coolant tubes. The product tritium is removed by the gas flow. Maniscalco and Meier, "Liquid-Lithium `Waterfall` Inertial Confinement Fusion Reactor Concept", Transactions of the American Nuclear Society, Vol. 26, page 62, June 1977. This report discloses a liquid-lithium waterfall which serves as a primary coolant, neutron moderator and fertile material for tritium breeding. Sze et al., "Gravity Circulated Solid Blanket Design for a Tokamak Fusion Reactor", Proc. at 2nd ANS Topical Meeting on Technology of Controlled Nuclear Fusion, 1976. This report discloses a falling bed of LiO.sub.2 microspheres for cooling and for breeding tritium in combination with a deuterium-tritium reactor. SUMMARY OF THE INVENTION Therefore, in view of the above, it is an object of the present invention to provide a system for removing energy from a neutron-producing plasma. It is also an object to provide a heat transport system for use with a neutron-producing plasma that is separate from the fuel breeding function. It is likewise an object to provide a heat transport system in which the heat transport media is a high-temperature, chemically inert material that does not activate to form long-lived radioisotopes and can be directly contacted by a secondary coolant fluid. It is also an object to provide a heat transport system in which the coolant media temperatures in regions near to the plasma are moderated in nearer accord with temperatures in regions away from the plasma. It is a further object to provide a heat transport system for use with a neutron-producing plasma in which containment walls between the plasma and media can be maintained at lower temperatures than that of the heat transport media. In accordance with the present invention, a heat transport system is disclosed for removing heat from a neutron-producing plasma. The system includes a vertical duct with its inlet above its outlet for passing a gravity flow of ceramic particles through its central region exposed to neutron radiation and thus energy transfer from the plasma. The ceramic particles are selected from alumina, magnesia, silica or a combination of these materials. A heat exchange vessel communicates with the outlet of the vertical duct and includes openings for passing a flow of coolant gas into direct heat exchange contact with the ceramic particles. The heated gas passes through circulatory means for maintaining its flow and for extracting heat energy for use. A conveyor is connected to the lower portion of the heat exchange vessel for upwardly transporting the ceramic particles to the inlet of the vertical duct and permitting the particles to gravitate through the central portion of the duct to the heat exchange vessel. In a more specific aspect of the invention, containers with lithium atoms in combined or elemental form are placed between the central region of the vertical chute and the neutron-producing plasma for breeding and recovering tritium by the reaction of lithium and neutron irradiation. In another important aspect, a lower portion of the vertical chute is constricted in open cross-sectional area to limit downward flow of particles and create a downwardly moving, packed bed. The duct and constriction are advantageously partitioned to provide a compartment, near to the neutron-producing plasma, which has a lower portion with a larger opening for discharge flow than the opening within the lower portion of a corresponding compartment disposed away from the neutron-producing plasma. This permits a greater linear flow of particles through the compartment near to the plasma and a lower maximum temperature than that of a falling bed that does not include this velocity partitioning. In another aspect of the invention, the ceramic particles are substantially free of material other than alumina, magnesia and silica. The particles are also generally globular in shape with a diameter of about 0.5 to 1.5 cm to facilitate solid flow within the bed. The invention also contemplates a method of removing energy from a neutron-producing plasma by passing a gravity flow of ceramic particles selected from magnesia, alumina, silica and combinations thereof through the neutron flux produced by the plasma to absorb energy and increase the particle temperature. The particles then flow outside the neutron flux to directly contact a coolant fluid which is subsequently employed as a source of heat energy. The cooled particles are recycled to again pass through the neutron flux.