Patent Number: 
Section: claims

1. A nuclear fuel composition comprising:a nuclear fissile material; anda neutron-absorption material adjoining the nuclear fissile material, the neutron-absorption material including 25 wt %-75 wt % samarium as a partial substitute for a remainder amount of a rare earth element to limit destroying a negative temperature coefficient of reactivity of the nuclear fissile material by the rare earth element, wherein the neutron-absorption material comprises ≦0.5 wt % of a combined weight of the nuclear fissile material and the neutron-absorption material. 2. The nuclear fuel composition as recited in claim 1, wherein the rare earth element is gadolinium. 3. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material includes 30 wt % -40 wt % of the samarium. 4. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material includes 35 wt % -38 wt % of the samarium. 5. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material comprises ≦0.1 wt % of the combined weight. 6. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material comprises ≦0.05 wt % of the combined weight. 7. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material is dispersed within the nuclear fissile material. 8. The nuclear fuel composition as recited in claim 1, wherein the neutron-absorption material is a coating disposed on pellets of the nuclear fissile material. 9. The nuclear fuel composition as recited in claim 1, wherein the nuclear fissile material comprises uranium-zirconium-hydride (UZrHx). 10. The nuclear fuel composition as recited in claim 1, wherein the samarium has a neutron-absorption energy peak (cross-section) that at least partially overlaps a thermal energy range of the neutrons from the nuclear fissile material. 11. A nuclear reactor comprising:a nuclear fissile material; anda neutron-absorption material adjoining the nuclear fissile material, the neutron-absorption material including 25 wt % -75 wt % samarium as a partial substitute for a remainder amount of a rare earth element to limit destroying a negative temperature coefficient of reactivity of the nuclear fissile material by the rare earth element, wherein the neutron-absorption material comprises ≦0.5 wt % of a combined weight of the nuclear fissile material and the neutron-absorption material. 12. The nuclear reactor as recited in claim 11, wherein the neutron-absorption material is dispersed within the nuclear fissile material. 13. The nuclear reactor as recited in claim 11, wherein the neutron-absorption material is a coating disposed on pellets of the nuclear fissile material. 14. The nuclear reactor as recited in claim 11, further comprising a hollow cladding containing the nuclear fissile material, and the neutron-absorption material is a coating disposed on an inside surface of the hollow cladding. 15. The nuclear reactor as recited in 11, wherein the rare earth element is gadolinium. 16. The nuclear reactor as recited in claim 11, wherein the neutron-absorption material includes 30 wt % -40 wt % of the samarium. 17. A method of rendering a nuclear fuel inherently subcritical, comprising:forming nuclear fuel from a nuclear fissile material and a neutron-absorption material adjoining the nuclear fissile material, and the neutron-absorption material includes 25 wt % -75 wt % samarium as a partial substitute for a remainder amount of a rare earth element to limit destroying a negative temperature coefficient of reactivity of the nuclear fissile material by the rare earth element, wherein the neutron-absorption material comprises ≦0.5 wt % of a combined weight of the nuclear fissile material and the neutron-absorption material, and the neutron-absorption material has a neutron absorption energy range that overlaps a thermal energy range of neutrons from the nuclear fissile material to thereby render the nuclear fuel inherently subcritical. 18. The method as recited in claim 17, including dispersing the neutron-absorption material within the nuclear fissile material. 19. The method as recited in claim 17, including depositing the neutron-absorption material as a coating. 20. The method as recited in claim 17, wherein the rare earth element is gadolinium. 21. The method as recited in claim 17, wherein the neutron-absorption material includes 30 wt % -40 wt % of the samarium.