Patent Number: 050826174
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS The preferred embodiment of the invention, shown schematically in FIG. 1, comprises an isotopic heat source 10. For purposes of illustration, one fuel stack 12 is shown adjacent to one heat pipe 14 which extends from the heat source 10 to a heat exchanger 16. The heat pipe 14 contains a working fluid 18 that transfers heat from the heat source 10 to the heat exchanger 16. The working fluid 18 flows along an inner surface 20 of the heat Pipe 14 which comprises means for capillary action. The heat pipe working fluid 18 can be restricted by the pressure of a single phase gas 22, the source of which is a gas reservoir 24. FIG. 2 is a vertical cross-section of a preferred embodiment of the heat source 10. FIG. 3 is another view of the embodiment shown in FIG. 2 along line 3--3. FIG. 2 illustrates a plurality of fuel stacks 12. The fuel stacks 12 comprise a refractory fuel 26 and diluent 28. The fuel 26 is neutron activated to form a relatively short-lived isotope that produces heat. The preferred embodiment for the fuel 26 is thulium-169 in the form of thulium oxide (Tm.sub.2 O.sub.3). The diluent 28 is a refractory, heat conductive, and low atomic weight material. The preferred embodiment for the diluent 28 is graphite. In the preferred embodiment, the fuel stacks 12 are formed of a plurality of thin individual layers of thulium fuel 26 and graphite 28. The thulium layers 26 and graphite layers 28 are stacked in an alternating pattern. The fuel stacks 12 are irradiated in a conventional manner with thermal neutrons, converting thulium-169 to thulium-170 (and thulium-171, etc.). After irradiation, one or more of the fuel stacks 12 are mounted in one or more holes 32 in a heat block 34, preferably made of graphite. In the preferred embodiment, the fuel stacks 12 are cylindrical and fit snugly into the heat block 34. A plurality of heat pipes 14 for heat removal are arranged in a plurality of holes 36 in the heat block 34. In the preferred embodiment, the heat pipes 14 are enclosed at both ends and may be oversized in length, extending beyond the heat exchanger 16 to provide additional heat rejection area. The heat block 34 is surrounded by a sealed structural container 38, which is surrounded by an insulation layer 40. The heat block 34 is also encased in at least one layer of radiation shielding 42,44, made from a suitable structural material such as iron or tantalum. In the preferred embodiment, an inner layer of the shielding 42 surrounds the insulation layer 40 and an outer layer of the shielding 44 surrounds the inner layer of the shielding 42. Free convection space fills the cavity 46 defined by the two layers of the shielding 42,44. Holes 48 defined by the outer layer of the shielding 44 are located along the inside perimeter of the outer layer of the shielding 44. The holes 48 are present at both the top 50 and bottom 52 ends of the heat source apparatus 10. In the preferred embodiment, the neutron activated fuel 26 is thulium in the form of thulium oxide. However, thulium in the form of thulium hydride or thulium carbide, as well as an altogether different radionuclide, might be used. In the preferred embodiment, the diluent 28 is graphite. Alternative embodiments for the low atomic weight diluent 28 are possible, including: zirconium hydride (hydrogen), beryllium oxide (beryllium), boron, lithium, and beryllium. Graphite is advantageous as a diluent 28 for several reasons. Graphite is highly refractory, which allows the heat source 10 to operate at high temperatures. Graphite and thulium oxide do not react appreciably at high temperatures. Also, graphite is readily available and inexpensive. Diluting thulium layers 26 with intervening graphite layers 28 may enhance the production of thulium-170 in the irradiation reactor and reduce the shielding needed around the fuel stack 12. The production of thulium-170 is increased because graphite acts as a moderator during irradiation. Shielding of the fuel stack 12 is reduced because graphite, being a low atomic weight material, produces less bremsstrahlung radiation than high atomic weight materials. Graphite also stops the beta particles and secondary electrons produced in radioactive decay. In the preferred embodiment, the fuel stack 12 comprises alternating layers of fuel 26 and diluent 28. The layers of thulium fuel 26 and graphite diluent 28 may be thin, flat, circular individual disks or wafers. The layers of thulium fuel 26 do not exceed one centimeter thickness in order to reduce flux depression. The thulium fuel layers 26 are placed with alternating layers of diluent 28 to form the fuel stack 12. In an alternate embodiment, the thulium fuel 26 can be flame sprayed or plated on graphite disks 28. Thulium oxide powder and graphite powder could also be mixed and heated to form a sintered body. After the fuel stacks 12 are irradiated, the stacks 12 may be placed directly into the heat block 34, eliminating post-activation handling. Alternatively, graphite layers 28, possibly of another thickness, may be substituted or inserted in the fuel stacks 12 to further minimize bremsstrahlung radiation. Excess graphite layers 28, of course, could be removed. The fuel stacks 12 are designed to maximize the opportunity for salvaging and recycling thulium fuel 26 and graphite diluent 28 from expended fuel stacks 12. The heat source 10 is designed to permit refueling for long term use. The heat pipes 14 provide means for heat removal. The heat pipes 14 contain a heat pipe working fluid 18, such as sodium, which is chosen according to the desired heat block 34 temperature. The working fluid 18 transfers heat from the heat source 10 to the heat exchanger 16. The heat pipes 14 are oversized in length to carry the working fluid 18 to the heat exchanger 16 and to permit passive cooling. In the preferred embodiment, the working fluid 18 transfers heat by repeated cycles of vaporization and condensation. The working fluid 18 vaporizes in the region of the fuel stack 12. The vapor expands and travels through the heat pipe 14 to the heat exchanger 16. The vapor cools, releases heat and condenses onto an inner surface 20 of the walls of the heat pipe 14 in the region of the heat exchanger 16. The inner surface 20 has means to allow capillary action. The condensed working fluid 18 flows back to the heat source 10 region by the capillary action means on the inner surface 20 to begin another cycle of vaporization and condensation. This heat transfer system can operate in a zero gravity environment or in a modest gravity field in any orientation. During the operation of the heat source 10 with the heat exchanger 16, the flow of the heat pipe working fluid 18 is restricted at an easily controlled interface by a single phase gas 22. The single phase gas 22, such as argon, is supplied from a sealed reservoir 24 attached to a heat pipe 14. The pressure of the single phase gas 22 restricts the flow of the working fluid 18 to direct heat to the heat exchanger 16 for maximum efficiency. Therefore, if the heat block 34 overheats, the vapor pressure of the working fluid 18 increases, causing displacement of the single phase gas 22, thereby expanding the heat rejection surface of the heat pipes 14 and permitting passive cooling. Conversely, if the pressure of the single phase gas 22 is increased, the working fluid 18 is displaced and the surface area of the heat pipes 14 for heat rejection is reduced (shortened). In an alternative embodiment, the heat pipes 14 need not extend linearly, but may be designed to fold back around toward the heat source 10 to reduce space requirements. Additionally, the number and arrangement of the heat pipes 14 and fuel stacks 12 are variable, depending on the power density and efficiency of heat removal required. The structural container 38, the insulation layer 40 and the radiation shielding 42,44 may be made of a variety of materials, depending on the particular use requirements. One embodiment for the structural container 38 is an x-ray absorbing material such as tantalum. The preferred embodiment for the insulation layer 40 is a material designed to fail at a high temperature that is below the failure temperature of the structural container 38. In the case of heat block 34 overheating, the insulation layer 40 would melt away, allowing thermal radiation to occur from the structural container 38 to the layer of inner shielding 42, thus providing containment and heat dissipation. Aerogel is one example of such an insulation material. The free convection space 46 between the layers of the shielding 42,44 provides yet another opportunity for passive cooling of the heat source 10 in the event of heat block 34 overheating. The description of the invention presented above is not intended to encompass all variations of the system but has attempted to present illustrative alternatives. The scope of the invention is intended to be limited only by the appended claims.