Patent Number: 
Section: claims

1. A confinement matrix for the storage or incineration of at least one long-life radioactive element, comprising:at least one crystalline boron compound of a rhombohedral structure comprising said at least one long-life radioactive element. 2. The matrix according to claim 1, wherein said at least one long-life radioactive element is inserted in the crystalline network of the boron compound. 3. The matrix according to claim 1, wherein said at least one long-life radioactive element is dispersed in oxide form in the rhombohedral structured boron compound. 4. The matrix according to claim 3, wherein the boron compound is B3Si. 5. The matrix according to claim 3, wherein the boron compound is B6O. 6. The matrix according to claim 3, wherein the boron compound is B4C. 7. The matrix according to claim 1 for the incineration of at least one radioactive element, wherein the boron of the boron compound is enriched with 11B. 8. A method for preparing a confinement matrix for at least one long life radioactive element, comprising:mixing a powder of said at least one long-life radioactive element or a powder of at least one compound of said at least one long-life element with a boron powder or a boron precursor, to obtain a powder mixture, andthen producing a hot reaction of the powder mixture at a temperature of 800 to 1500° C. and sintering the powders obtained;thereby obtaining said confinement matrix which comprises at least one crystalline compound of a rhombohedral structure in the crystalline network into which said at least one long-life radioactive element is inserted. 9. The method according to claim 8, wherein the powder mixture also comprises one or more additives selected from the group consisting of metals, catalysts, metal oxides, and the adjuvants required to form the matrix or improve its properties. 10. The method according to claim 8, wherein the boron precursor is selected from the group consisting of B2O3, H3BO3, B3Si, B6O and B4C. 11. The method according to claim 8, wherein the powders of the mixture are powders of boron, a metal oxide and at least one radioactive element,wherein the powders are first reacted at a temperature of 1000 to 1500° C., under an inert gas stream, andwherein the sintering is then carried out at a temperature of 1200 to 1800° C., at a pressure of 30 to 200 MPa. 12. A method for preparing a confinement matrix for at least one long-life radioactive element, comprising:mixing a powder of said at least one long-life radioactive element or a powder of at least one compound of said at least one long-life element with a boron powder or a boron precursor, to obtain a powder mixture; andthen a hot reaction and sintering are performed at the same time by reactive sintering at a temperature of 1000 to 1800° C., at a pressure of 30 to 200 MPa;thereby obtaining said confinement matrix which comprises at least one crystalline compound of a rhombohedral structure in the crystalline network into which said at least one long-life radioactive element is inserted. 13. The method according to claim 9, wherein the powders of the mixture are powders of boron, a metal oxide and at least one radioactive element, andwherein the reactive sintering is performed at a temperature of 1300 to 1400° C., at a pressure of 30 to 200 MPa. 14. A method for preparing a confinement matrix in the form of a composite material, comprising:dispersing at least one long-life radioactive element in a crystalline boron compound of a rhombohedral structure by a method comprising:mixing of a powder of the crystalline boron compound having said rhombohedral structure with a powder of the radioactive element or a compound of said element selected from the group consisting of oxides, to obtain a mixture; andpressurised sintering of the mixture at a temperature of 1000 to 1800° C., and at a pressure of 30 to 200 MPa. 15. The method according to claim 14, wherein the boron compound is B4C, B6O or B3Si.