Patent Number: 
Section: claims

1. An accident tolerant fuel combination for light water and lead fast reactors comprising:a multi-layered ceramic cladding; andfuel in pellet form positioned within the cladding such that a gap sized to prevent pellet-cladding mechanical interaction and centerline melt during high power excursions is defined between the cladding and the pellet, the fuel comprising U15N intermixed with a boron-containing integral fuel burnable absorber selected from the group consisting of UB2 and ZrB2 and having a 1310 content at 19% to 80% of the boron. 2. The fuel combination recited in claim 1 wherein the ceramic cladding comprises at least one monolith layer and at least one composite layer. 3. The fuel combination recited in claim 2 wherein the ceramic cladding is made of a SiC monolith layer and a SiC ceramic composite layer. 4. The fuel combination recited in claim 1 wherein the ceramic cladding has a total wall thickness between 0.4 mm and 1.4 mm. 5. The fuel combination recited in claim 1 wherein the U15N fuel has a N15 isotope content between 75% and 99.9%. 6. The fuel combination recited in claim 1 wherein the U15N fuel has a UN purity greater than 90%. 7. The fuel combination recited in claim 1 wherein the boron-containing integral fuel burnable absorber is UB2. 8. The fuel combination recited in claim 1 wherein the boron-containing integral fuel burnable absorber content in the U15N pellet is between 100 ppm and 10000 ppm. 9. The fuel combination recited in claim 1 wherein the U15N fuel has a density between 80% and 99% of theoretical density. 10. An accident tolerant nuclear fuel combination comprising:a ceramic cladding comprised of at least a monolith layer and a composite layer;a plurality of fuel pellets stacked in the cladding such that a gap sized to prevent pellet-cladding mechanical interaction and centerline melt during high power excursions is defined between the cladding and the fuel pellets, the fuel pellets formed from U15N mixed with from 100 to 10000 ppm of a boron-containing integral fuel burnable absorber selected from the group consisting of UB2 and ZrB2, having a B10 content at 19% to 80% of the boron. 11. The nuclear fuel combination recited in claim 10 wherein the ceramic cladding is formed from a SiC monolith wrapped in SiC composite fibers. 12. The nuclear fuel combination recited in claim 10 wherein the U15N fuel has a N15 isotope content between 75% and 99.9% and a UN purity greater than 90%. 13. The fuel combination recited in claim 1 wherein the gap is between about 0.01 to 0.3 mm. 14. The nuclear fuel combination recited in claim 10 wherein the gap is between about 0.01 to 0.3 mm.