Patent Number: 050376063
Section: claims

1. A nuclear fuel particle comprising a core of generally spheroidal shape containing fissile or fertile nuclear fuel material and having a diameter not greater than about 1,000 microns,  a buffer layer of low density pyrocarbon in surrounding location to said core,  fission-product-retentive means surrounding said buffer layer designed to retain fission products therewithin and having an exterior surface formed of a material having a density equal to at least about 80% of theoretical density, and  a protective overcoating disposed exterior of said fission-product-retentive means and in surrounding relation thereto and having a density not greater than about sixty percent of its theoretical maximum density, said diameter of the nuclear fuel particle including said protective overcoating being not greater than about 5 millimeters, whereby said protective overcoating mechanically protects said relatively fragile fission-product-retentive means from stress encountered during fabricating solid nuclear fuel compacts from said overcoated particles by the uniting of such overcoated particles into integral masses using a hardenable flowable binder.  forming fission-product-retentive nuclear fuel particles by coating spheroidal cores of fissile or fertile nuclear fuel material with a plurality of surrounding layers which constitute fission-product-retention means that will retain substantially all fission products generated therewithin throughout burnup up to about 30 percent of the fissile atoms present in said cores, a region at the outer surface of said fission-product-retention means having a density of at least about 80% of its theoretical maximum density,  overcoating said fission-product-retention means with a layer of relatively porous material by depositing onto said dense outer surface at least about 20 microns of a protective material having a density of not greater than about 60% of its maximum theoretical density,  combining precise amounts of said overcoated nuclear fuel particles and a flowable, hardenable binder under pressure in a mold of desired shape, said overcoated nuclear fuel particles being loaded into said mold and then subjected to pressure to pre-compact them prior to said binder being injected into the interstices of said pre-compacted overcoated nuclear fuel particles under pressure, and  hardening said binder which is a mixture of petroleum pitch and graphite flour by heating said combination of overcoated particles and binder to a temperature of at least about 1000.degree. C. to create a nuclear fuel compact of desired fuel loading wherein substantially all of said fission-product-retention means remain intact and unfractured. 2. A nuclear fuel particle according to claim 1 wherein said protective overcoating is made of isotropic pyrocarbon having a density between about 1.0 and about 1.3 g/cm.sup.3. 3. A nuclear fuel particle in accordance with claim 1 wherein said spheroidal core contains uranium oxide, uranium carbide or a mixture thereof and has a diameter not greater than about 550 microns and wherein the outer diameter of said protective overcoating is not greater than about 1200 microns. 4. A nuclear fuel particle in accordance with claim 2 wherein the thickness of said overcoating is between about 20 microns and about 70 microns. 5. A nuclear fuel particle comprising a core of generally spheroidal shape containing fissile or fertile nuclear fuel material, a buffer layer of low density pyrocarbon in surrounding location to said core, fission-product-retentive means surrounding said buffer layer designed to retain fission products therewithin, and a protective overcoating of relatively porous aluminum oxide disposed exterior of said fission-product-retentive means and in surrounding relation thereto and having a density not greater than about sixty percent of its theoretical maximum density whereby said protective overcoating mechanically protects said relatively fragile fission-product-retentive means from stress encountered during fabricating solid nuclear fuel compacts from said overcoated particles by the uniting of such overcoated particles into integral masses using a hardenable flowable binder. 6. A nuclear fuel particle in accordance with claim 5 wherein said aluminum oxide has a density between about 1.5 and about 2.0 g/cm.sup.3. 7. A nuclear fuel particle in accordance with claim 1 wherein said spheroidal core contains thorium oxide, or thorium carbide, or a mixture of thorium carbide and thorium oxide, or a mixture of thorium oxide and uranium oxide and has a diameter not greater than 650 microns and wherein the outer diameter of said overcoating is not greater than about 1300 microns. 8. A method of making nuclear fuel compacts, which method comprises 9. A method in accordance with claim 8 wherein said heating is carried out at a temperature of about 2100.degree. C. or below. 10. A method in accordance with claim 8 wherein said pre-compacting is carried out at a pressure of between about 100 psig and about 600 psig. 11. A method in accordance with claim 8 wherein said flowable binder is injected under a pressure of at least about 600 psig.