Patent Number: 044302916
Section: description

DETAILED DESCRIPTION Refer to FIG. 1 which is a schematic section of a Tokamak fusion reactor utilizing a packed-fluidized bed blanket. In FIG. 1, a section of the doughnut-like reactor is shown in which a core region 1 is surrounded by a plurality of pressure tubes 2, several of which are shown in FIG. 1, the rest being omitted for clarity. In practice, sufficient tubes 2 are installed such that most if not all radiation 3 emitted by the plasma in core region 1 is interdicted by tubes 2. The overall blanket is a grouping of inner pressure tubes 4 and outer pressure tubes 5. Tubes 2 contain the fuel particles of the packed fluidized bed. FIG. 2 is a schematical plan view of a typical Tokamak hybrid reactor. FIG. 2 shows field coil 6 and shield 7 which surround and restrict access to the blanket of the reactor. Vacuum vessel 8 contains core region 1. Inner pressure tubes 4 and outer pressure tubes 5 have coolant inlets 9 and coolant outlets 10 served by inlet manifolds 11 and outlet manifolds 12, which manifolds 11 and 12 are used to distribute (or collect) coolant flow to (or from) all pressure tubes 4 or 5. Refer to FIG. 3 which is a schematic of one pressure tube 2 from FIG. 1 or 2, which could be either an inner pressure tube 4 or an outer pressure tube 5. Tube 2 has tube inlet 13 and tube outlet 14 for routing of coolant flow to and from the manifolds 11 and 12 shown in FIG. 2. Pressure tube 2 is shown to have a fuel outlet and inlet port 15 for fuel particle 17 replacement during refueling. Port 15 may be arranged to distribute particles to many tubes 2 via a manifold or may be provided individually to each tube as shown in FIG. 3. All pressure tubes 2 have fluidization rakes 18 at various elevations. These rakes 18 have a source of fluidization flow (not shown) which may be individual to each rake 18 or which may have a grouping such as, for example, all rakes 18 of all tubes 4 and 5 at a common elevation. Refer to FIG. 4. This Figure is a section as indicated from FIG. 3. Arrow 19 shows the direction of coolant flow during reactor operation. Support walls 20 function to form coolant inlet and coolant outlet volume manifolds 21 and 22. Walls 20 have holes 23 to permit coolant flow passage. Screens 24, supported by walls 20 as shown, prevent carryover of particles 17 in the coolant flow. Screens 24 have a mesh, or hole size, considerably smaller than the particle diameter such that the screens 24 do not become plugged or blocked but do function to prevent particle passage. The pressure tubes need not have the circular cross-section as indicated in FIG. 4 but could be square, rectangular, or of other geometry. Refer to FIG. 5. This schematic section through a typical rake in FIG. 3 shows one possible geometric shape of this component which serves to distribute fluidization flow over the cross-sectional area of the tube which contains particles and to direct the flow upward. Refer to FIG. 6. Because of the geometric shape of the reactor, the perimeter of a circle drawn on the inside surface of the reactor (L.sub.1) is less than the perimeter of a circle drawn on the outside surface (L.sub.2). In order for sufficient pressure tubes 2 to be installed to interdict all radiation from the reactor, an overlap 25 of tubes must occur in certain regions (core top and bottom) as illustrated in FIG. 6. In other regions, tubes 2 abut without overlap (see region 29). An operational cycle of the reactor will be described to illustrate the packed fluidized bed blanket concept. Refer again to FIG. 3. Prior to reactor operation, all pressure tubes 2 are filled with particles 17 using port 15. Packing of the bed to a high density is desirable since a high density enhances neutron absorption rates which produce useful nuclear transformations, permits efficient use of blanket space, and improves the efficiency of energy deposition in the bed. Isolation valves in the port 15 piping (not shown) are then closed. The flow of the main coolant is begun, entering via inlet tube 13, passing through inlet plenum 21, (in FIG. 4) through screen 24 and into the packed bed 26 of particles 17. The coolant flow is in the radial direction through the packed bed 26, as shown by arrow 19, and exits via outlet plenum 22 and outlet tube 14 (in FIG. 3). External systems may be used to recover heat from this coolant flow. During reactor shutdown, coolant flow is stopped and fluidization flow is initiated. This flow enters via rakes 18, passes axially upward through packed bed 26 and exits via outlet port 15. This flow can be used to remove blanket heat. Fluidization is a process technology which has been successfully utilized in a variety of applications. The basic principle is as follows: a coolant, usually a gas, is passed upward through a bed of granular particles at such a rate that the drag on the particles opposes gravity and causes the particle to be suspended. A further increase in gas flow causes the appearance of gas bubbles which rise through the bed causing vigorous circulation and mixing of the bed solids around the bubbles. The mixture of particles in fluidizing gas forms a dynamic froth of particles in gas defined here as a "fluidized" bed, which, besides having excellent thermal conductivity, also can be arranged to have a high density while still being easily transferable by pumping. When it is time to remove particles for processing and replacement, the fluidization flow is introduced via the staged fluidization rakes 18 at a sufficient flow rate to cause particle streaming out of the blanket via outlet 15 (FIG. 3). The containing walls 28 in FIG. 3 must sustain the pressure applied to pressure tube 2 by both the coolant and the fluidization flow streams. Naturally, the strength and thickness of walls 28 are determined by the greater of these two pressures. Since the radial path of the coolant flow is relatively short, as compared to the axial fluidization flow path, the tendency is for the required pressure to achieve the necessary fluidization flow rate to be greater than the required coolant flow pressure, potentially requiring thick walls 28. Consequently, a plurality of flow rakes 18 are provided to reduce the pressure needed to produce fluidization and removal of particles. In practice, the flow rakes 18 would be actuated in sequence from A to F (see FIG. 3) to remove the bed of particles in stages. Sufficient flow rakes 18 may be provided such that the necessary fluidization and coolant pressures are about the same, thereby minimizing the required wall 28 thickness. The fluidization flow can serve purposes in addition to fuel particle removal. This flow can serve for backup or emergency heat removal and can be used to mix the fuel particles when desired to achieve uniform radiation exposure. Since the particle bed expands during fluidization operation, staged fluidization reduces any void volume which must be maintained in the pressure tubes to accommodate this expansion, since only a fraction of the bed is fluidized at any one time. Table 1 is included to illustrate parameters presently considered pertinent to a preferred embodiment of the packed fluidized bed blanket as applied to a commercial Tokamak hybrid reactor. TABLE 1 ______________________________________ Packed Bed Operation Helium inlet pressure = 50 Atm Helium inlet temperature = 350.degree. C. Helium flowrate = 1800 kg/s Helium pressure drop through = .166 MPa the packed bed Total helium pumping power = 8.8 MWe (&lt;0.5 percent of blanket thermal power) Peak local power density =50 W/cm.sup.3 (assumed) Maximum film temperature drop =0.65.degree. C. (particle to coolant) Maximum particle temperature = 480.degree. C. Fluidized Bed Operation Stage height = 2 m No. of stages = 5 Pressure drop for fluidization = 0.141 MPa Fluid (helium) inlet pressure = 30 Atm Helium temperature = 77.degree. C. Fluidization velocity = 0.41 m/s Helium flowrate for full = 265 kg/s blanket Helium pumping power (refuel- = 7.8 MWe ing one blanket segment at a time, total of 32 seg- ments assumed) Blanket Auxiliary or Emergency Cooling Parameters Coolant path length = 10 meters (flow through the lowermost rake) Coolant pressure, average = 30 Atm Coolant inlet temperature = 100.degree. C. Coolant .DELTA.T = 300.degree. C. Helium Flowrate = 126 kg/s Decay Heat Level = 0.01 of full power Pressure drop through blanket = 0.209 MPa Total pressure drop = 0.251 MPa Coolant pumping power (85 = 0.66 MWe percent efficient com- pressor) ______________________________________ The above specification and the drawings are susceptible to various modifications without deviation from the true spirit and scope of the invention. For example, the fluidization flow and coolant flow described, if never commingled but rather separated by differing periods of operation, may be of different composition, perhaps helium and carbon dioxide respectively, or may even include water. Therefore, this disclosure should be interpreted as illustrative rather than limiting.