Patent Number: 
Section: claims

1. A process for sintering a compacted powder of at least one oxide of a metal M1 selected from an actinide and a lanthanide, this process comprising the following successive steps (a) to (c), carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water, of:(a) increasing the temperature from an initial temperature TI to a hold temperature TP, TP being between 1 400° C. and 1 800° C.,(b) maintaining the temperature at the hold temperature TP for a duration between 1 h and 10 h, and(c) decreasing the temperature from the hold temperature TP to a final temperature TF,wherein the ratio of the partial pressures P(H2)/P(H2O) in said atmosphere is such that:500<P(H2)/P(H2O)≤50 000, during step (a), from TI until a first intermediate temperature Ti1 between 1 000° C. and TP is reached, andP(H2)/P(H2O)≤500, at least during step (c), from a second intermediate temperature Ti2 between TP and 1 000° C., until TF is reached. 2. The process according to claim 1, wherein the first intermediate temperature Ti1 is between 1 300° C. and TP. 3. The process according to claim 1, wherein the second intermediate temperature Ti2 is between TP and 1 300° C. 4. The process according to claim 1, wherein the ratio P(H2)/P(H2O)≤500 is implemented as soon as the first intermediate temperature Ti1 is reached and maintained until TF is reached. 5. The process according to claim 1, which further comprises, between steps (a) and (b), the following successive steps (a1) to (a3) of:(a1) maintaining, for a duration between 1 h and 10 h, the temperature at the hold temperature TP,(a2) decreasing the temperature from the hold temperature TP to a third intermediate temperature Ti3,(a3) increasing the temperature from the third intermediate temperature Ti3 to the hold temperature TP,and wherein the ratio P(H2)/P(H2O) in said atmosphere is such that:500<P(H2)/P(H2O)≤50 000, during steps (a), (a1) and (a2), andP(H2)/P(H2O)<500, during steps (a3), (b) and (c). 6. The process according to claim 5, wherein the third intermediate temperature Ti3 is between 20° C. and 500° C. 7. The process according to claim 1, wherein TP is 1 700° C. 8. The process according to claim 1, wherein at least one from TI and TF is room temperature. 9. The process according to claim 1, wherein increasing the temperature in step (a) is performed at a rate between 30° C./h and 400° C./h. 10. The process according to claim 1, wherein decreasing the temperature in step (c) is performed at a rate between 100° C./h and 900° C./h. 11. The process according to claim 1, further comprising, after step (c), an oxidation heat treatment. 12. The process according to claim 1, wherein the inert gas is chosen from the group consisting of argon, helium and nitrogen. 13. The process according to claim 1, wherein the volume ratio between the inert gas and dihydrogen is between 90/10 and 98/2. 14. The process according claim 1, wherein M1 is an actinide. 15. The process according to claim 1, wherein M1 is a lanthanide. 16. The process according to claim 1, wherein the compacted powder further comprises at least one oxide of a metal M2 selected from Sc, Y, an actinide and a lanthanide, M2 being different from M1, whereby, at the end of step (c), a mixed oxide comprising M1 and M2 is obtained. 17. The process according to claim 16, wherein M2 is an actinide. 18. A process for manufacturing a pellet of at least one oxide of a metal M1 chosen from an actinide and a lanthanide, said process comprising the following successive steps of:preparing a powder of said at least one oxide of metal M1,compacting the powder prepared as a pellet, andheat treating the pellet of compacted powder by the sintering process according to claim 1. 19. The process according to claim 14, wherein M1 is selected from the group consisting of U, Pu and Th. 20. The process according to claim 15, wherein M1 is Ce. 21. The process according to claim 17, wherein M2 is selected from the group consisting of U, Pu and Th. 22. The process according to claim 18, wherein the pellet is a nuclear fuel pellet.