Patent Number: 044302916
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a structure to surround the fusion plasma of a fusion reactor, or other fusion plasma containing device, said structure commonly called a blanket. Fusion reactors of all varieties produce energetic neutrons which can advantageously be captured in a blanket region or structure substantially or completely surrounding the reactor core. In a fusion-fission hybrid reactor, the blanket contains a fertile fuel intended to breed fissile fuel and to produce energy by neutron induced fission. In a pure fusion reactor, the blanket contains fertile species which capture neutrons to form valuable isotopes. In both reactor types, the blanket is cooled by a coolant by which means heat is transferred away to cool the reactor, and perhaps generate useful power. While many nuclear transformations are possibly of interest in a fusion reactor blanket, the generation of tritium from a neutron-induced nuclear reaction of lithium nuclei is especially important since tritium is a fusion reactor fuel. The design of a suitable blanket is considered a significant obstacle to the development of a practical fusion power reactor. Several concepts have been proposed encompassing liquid lithium or solid lithium compounds for tritium breeding, solid fertile fuel for fissile element breeding, and gas, liquid, and even pebble bed coolants. All have technical handicaps. A solid blanket must be replaceable for isotope recovery and for blanket repair which, it develops, is a difficult task. A liquid lithium blanket can be continually or intermittently processed for tritium recovery and can readily be used to recover heat deposited therein by conventional means. However, liquid lithium and other liquid metals in the presence of the strong magnetic fields (in magnetic confinement fusion reactors) experience magnetohydrodynamic forces which limit the serviceability of that type of blanket. Gas cooled blankets have intrinsically lower material densities which reduce the efficiencies of heat transfer. Pebble bed blankets generally require high coolant pumping power. Consequently, it is desired to provide a blanket for a fusion reactor which can be refueled on-line, has high density, and is well adapted for power and isotope production. SUMMARY OF THE INVENTION The invented blanket is a bed of solid fuel particles which serve to absorb radiation from the fusion plasma. The bed is a packed bed during reactor operation but is fluidized intermittently for fuel particle removal when desired. The particles are chosen to be UO.sub.2, UC, ThO.sub.2, Th-ZR, Li.sub.7 Pb.sub.2, L.sub.2 O or other "fuel" material as desired depending partially upon the nuclear transformation in the blanket to be achieved. The particles may be spheres, which if sized between 40 to 300 microns in diameter are termed microspheres. In some cases, it may be desired to enclose each fuel particle with a metal cladding. The bed is contained in each of a series of pressure tubes sufficient in number to substantially or completely surround the plasma region, the assembly of all such pressure tubes constituting the blanket for the reactor. During operation of the reactor, coolant, perhaps helium, flows radially through the bed of solid particles and removes blanket heat. The same coolant can be used to generate power by various processes, including the generation of steam in an external heat exchanger/steam generator. The arrangement of coolant flow through the particle bed is considered best if it is radially outward since the radial flow path is short compared to the axial length of the pressure tube and consequently minimizes coolant pumping power requirements and coolant pressure. For blanket fueling/refueling, (particle replacement), coolant flow is stopped and a second fluid, most likely of composition identical to the coolant, is passed axially upward through the particle bed. This "fluidizing" stream serves to transport the bed fuel particles out of the pressure tube for replacement and/or reprocessing. Replacement is performed intermittently during shutdown of the reactor or even during operation. On-line refueling capability is of significant advantage in a fusion-fission hybrid reactor because the rate of fission reactions in the fuel particles caused by the buildup of fissionable nuclei due to neutron reactions in fertile nuclei can be controlled by approprite removal of the enriched particles and replacement with new fertile particles. Also, in the case of a pure fusion reactor, the net yield from the reactor of radioactively decaying tritium can be increased by continuous or frequent removal and early use of the isotope. The blanket is provided with a series of staged fluidization flow distributors to allow the fluidization and/or removal of particles in stages so as to minimize the required fluidization flow pumping power.