Patent Number: 
Section: claims

1. A material consisting of uranium (U), gadolinium (Gd) and oxygen (O) exhibiting a crystalline phase with a crystallographic structure of cubic type, with a Gd/[Gd+U] atomic ratio between 0.6 and 0.93, the uranium being present therein in the +IV and/or +V oxidation state. 2. The material as claimed in claim 1, exhibiting a crystalline phase referred to as cubic 1 phase, the Gd/[Gd+U] atomic ratio of which is between 0.79 and 0.93. 3. The material as claimed in claim 2, in which the crystallographic structure of cubic type exhibits a unit cell parameter between 10.8 and 10.9 Å. 4. The material as claimed in claim 1, exhibiting a crystalline phase referred to as cubic 2 phase, the Gd/[Gd+U] atomic ratio of which is between 0.6 and 0.71. 5. The material as claimed in claim 4, in which the crystallographic structure of cubic type exhibits a unit cell parameter between 5.3 and 5.5 Å. 6. The material as claimed in claim 1 of two-phase type, exhibiting (i) a cubic 1 phase, the Gd/[Gd+U] atomic ratio of which is between 0.79 and 0.93, and (ii) a cubic 2 phase, the Gd/[Gd+U] atomic ratio of which is between 0.6 and 0.71. 7. The material as claimed in claim 1, in which the uranium is uranium isotopically enriched in 235U, uranium isotopically depleted in 235U or natural uranium. 8. The material as claimed in claim 1, in which the gadolinium is natural gadolinium or gadolinium isotopically modified in its 155Gd/Gdtotal and/or 157Gd/Gdtotal ratio. 9. A process for the preparation of a material defined according to claim 1, comprising a stage of sintering, at a temperature ranging from 1200 to 2200° C. and under a reducing atmosphere, a powder formed of a mixture of uranium oxide and gadolinium oxide Gd2O3 in proportions such that the gadolinium is present in a Gd/[Gd+U] atomic ratio ranging from 0.6 to 0.93. 10. The process as claimed in claim 9, in which the sintering is carried out under an argon atmosphere to which 5 mol % of hydrogen has been added. 11. The process as claimed in claim 9, in which the sintering is carried out for a period of time of greater than or equal to 1 hour. 12. A burnable neutron poison of a nuclear fuel element, which comprises the material as claimed in claim 1. 13. A nuclear fuel pellet, comprising a material as defined according to claim 1. 14. A nuclear fuel rod comprising at least one fuel pellet as defined according to claim 13. 15. A nuclear fuel assembly comprising at least one fuel rod as defined in claim 14. 16. A heterogeneous nuclear fuel pellet formed of at least an internal part comprising at least one fissile material, the internal part being coated with an annular external part that is formed in whole or part of a material as defined according to claim 1. 17. The pellet as claimed in claim 16, in which said annular external part exhibits a thickness ranging from 0.05 to 7.5% of the radius of said pellet. 18. The pellet as claimed in claim 16, in which said internal part is formed in whole or part of uranium oxide, plutonium oxide, thorium oxide or their mixtures. 19. A process for manufacturing a heterogeneous nuclear fuel pellet defined according to claim 16, comprising at least the following steps:(i) providing a powder comprising a material based on uranium (U), gadolinium (Gd) and oxygen (O) exhibiting a crystalline phase with a crystallographic structure of cubic type, with a Gd/[Gd+U] atomic ratio between 0.6 and 0.93, the uranium being present therein in the +IV and/or +V oxidation state;or providing a powder formed of a mixture of uranium oxide and gadolinium oxide Gd2O3 in proportions such that the gadolinium is present in a Gd/[Gd+U] atomic ratio ranging from 0.6 to 0.93;(ii) preparing a slip from the powder of stage (i);(iii) depositing the powder in the slip form on the surface of a pellet comprising at least one fissile material; and(iv) sintering the pellet obtained on conclusion of stage (iii) under a reducing atmosphere and at a temperature between 1200° C. and 2200° C. 20. The process as claimed in claim 19, in which stage (iii) includes the drying of the slip layer deposited at the surface of the pellet. 21. A nuclear fuel element of plate-type geometry comprising one or more fissile regions covered, at least in part, with a material as defined according to claim 1.