Patent Number: 050826174
Section: claims

1. An isotopic heat source comprising: at least one isotopic fuel stack, comprising alternating layers of:  thulium oxide; and  a low atomic weight diluent for thulium oxide;  a heat block defining holes into which the fuel stacks can be placed;  at least one heat pipe for heat removal, with said heat pipe being positioned in the heat block in thermal connection with the fuel stack; and  a structural container surrounding the heat block. PG,19  at least one layer of insulation surrounding the heat block.  at least one layer of radiation shielding surrounding the heat block.  two layers of shielding surrounding the heat block and defining a free convection space that separates the two layers of the shielding.  a working fluid contained in the heat pipe, said working fluid being suitable for conversion from liquid to vapor phases in the heat source area.  means for producing capillary action along the inner surface of the heat pipes so that condensed heat pipe working fluid can flow back along the inner surface of the heat pipes to the heat source area.  a single phase gas reservoir connected to the heat pipe to supply a single phase gas suitable for restricting the flow of the heat pipe working fluid and thus the heat rejection surface of the heat pipe. 2. A heat source in accordance with claim 1 further comprising, 3. A heat source in accordance with claim 1 further comprising, 4. A heat source in accordance with claim 1 further comprising, 5. A heat source in accordance with claim 1 wherein the heat pipes are oversized in length so as to extend beyond the heat source and the heat exchanger. 6. A heat source in accordance with claim 5 further comprising, 7. A heat source in accordance with claim 6 wherein the heat pipe has an inner surface comprising, 8. A heat source in accordance with claim 7 further comprising, 9. The isotopic heat source of claim 1 wherein the thulium oxide is thulium-169 oxide which is neutron activated to produce thulium-170 l oxide. 10. The isotopic heat source of claim 1 wherein the diluent is graphite. 11. The isotopic heat source of claim 1 wherein the thulium oxide layers do not exceed 1 cm in thickness.