Patent Number: 
Section: claims

1. A device for granulating powders by cryogenic atomisation, comprising:a device for mixing powders by a cryogenic fluid, comprising at least one chamber for mixing powders, comprising the cryogenic fluid, wherein the cryogenic fluid is a liquified gas and chemically inert; anda device for atomising a suspension of powders mixed by the device for mixing powders in order to allow for a granulation of the powders, wherein the device fractionates the suspension of powders making it possible to adjust the size of droplets of the powders and adjusts the moisture of the mixed powders and/or moisture of the atomisation atmosphere and wherein for fractionating the suspension of powders, the device is configured to allow for the adjusting of the diameter of the droplets of powders to be atomised, in such a way that the diameter of the droplets of powders is defined according to the following relationship:            d      o        D    =      G    ⁢          {                        fD          v                ,                  1          We                ,                  1          Re                ,                  A          D                    }      with We=ρν2·(do)/σ and Re=ρdoν/μ,where:f represents the vibration frequency of the device for atomising,ν represents the speed of the suspension of powders,ρ represents the density of the suspension of powders to be fractionated,μ represents the viscosity of the suspension of powders to be fractionated,σ represents the surface tension of the suspension of powders to be fractionated,A represents the oscillation amplitude of an atomisation nozzle of the device for atomising,d0 represents the diameter of the droplets, andD represents the diameter of an atomisation nozzle of the device for atomising. 2. The device according to claim 1, wherein the powders to be mixed are actinide powders. 3. The device according to claim 1, wherein the cryogenic fluid comprises a slightly hydrogenated liquid, which is a liquid comprising at most one hydrogen atom per molecule of liquid, having a boiling temperature less than that of water, and wherein the cryogenic fluid is a gas at room temperature and a liquid at a temperature lower than room temperature. 4. The device as claimed in claim 1, wherein the device for mixing further comprises:a chamber for supplying powders in order to allow the powders to be introduced into the at least one chamber for mixing,means for agitation in the mixing chamber so as to allow the mixing of the powders placed in suspension in the cryogenic fluid. 5. The device according to claim 4, wherein the device for mixing comprises means for mixing of the at least one chamber for mixing according to a gyroscopic movement. 6. The device according to claim 4, wherein the device for mixing comprises:a plurality of mixing chambers of the powders, arranged successively in series one after the other, the chamber for supplying powders allowing for the introduction of powders into at least the first mixing chamber,a plurality of systems for restricting passage of the powders, with each system for restricting the passage being located between two successive mixing chamber, in order to constrain the distribution of powders from one mixing chamber to the next, with each mixing chamber comprising the cryogenic fluid and means for agitation so as to allow the mixing of the powders placed in suspension in the cryogenic fluid. 7. The device according to claim 6, wherein the systems for restricting the passage comprise screens and/or diaphragms. 8. The device according to claim 4, wherein the means for agitation comprise mobile mixing devices. 9. The device according to claim 4, wherein the means for agitation comprise means for generating vibrations. 10. The device according to claim 4, wherein the device for mixing comprises a system of electrostatic charge of the powders intended to be introduced into the mixing chamber or chambers. 11. The device according to claim 10, wherein a portion of the powders is put into contact with a portion of the electrostatic charge system in order to be positively electrostatically charged and wherein the other portion of the powders is put into contact with the other portion of the electrostatic charge system in order to be negatively electrostatically charged, in order to allow for a differentiated local agglomeration. 12. The device as claimed in claim 1, wherein the cryogenic fluid is liquefied nitrogen. 13. Method for granulating powders by cryogenic atomisation using the device claim 1, and comprising the following steps:a) introducing powders and cryogenic fluid into at least one chamber of the device for mixing powders by a cryogenic fluid in order to obtain a suspension of powders and of cryogenic fluid,b) atomising of the suspension of powders and of cryogenic fluid through the device for atomising in order to allow for a granulation of the powders, wherein the suspension of powders are fractionated making it possible to adjust the size of droplets of the powders and adjusts the moisture of the mixed powders and/or moisture of the atomisation atmosphere and wherein the diameter of the droplets of powders to be atomised are adjustable in such a way that the diameter of the droplets of powders is defined according to the following relationship:            d      o        D    =      G    ⁢          {                        fD          v                ,                  1          We                ,                  1          Re                ,                  A          D                    }      with We=ρν2·(do)/σ and Re=ρdoν/μ,where:f represents the vibration frequency of the device for atomising,ν represents the speed of the suspension of powders,ρ represents the density of the suspension of powders to be fractionated,μ represents the viscosity of the suspension of powders to be fractionated,α represents the surface tension of the suspension of powders to be fractionated,A represents the oscillation amplitude of an atomisation nozzle of the device for atomising,d0 represents the diameter of the droplets, andD represents the diameter of an atomisation nozzle of the device for atomising, andc) obtaining of granules formed from powders. 14. Method according to claim 13, wherein during the first step a), the powders are oppositely electrostatically charged in order to favour differentiated local agglomeration. 15. A device for granulating powders by cryogenic atomisation, comprising:a device for mixing powders by a cryogenic fluid, comprising at least one chamber for mixing powders, comprising the cryogenic fluid, wherein the cryogenic fluid is a liquified gas and chemically inert; anda device for atomising a suspension of powders mixed by the device for mixing powders in order to allow for a granulation of the powders, wherein the device fractionates the suspension of powders making it possible to adjust the size of droplets of the powders and adjusts the moisture of the mixed powders and/or moisture of the atomisation atmosphere and wherein for fractionating the suspension of powders, the device is configured to allow for the adjusting of the diameter of the droplets of powders to be atomised by modulation according to a reduction factor of the diameter between the diameter of the droplets of powders to be atomised and the diameter of the granules obtained after atomisation of the suspension of mixed powders and evaporation of the cryogenic fluid, in such a way that:            d      0              d      s        =      R    =                                        [            U            ]                    f                                      [            U            ]                    i                    3      where:d0 represents the diameter of the droplets,ds represents the diameter of the granules,[U]f represents the volume occupancy rate of powders in the agglomerate of granules formed after granulation, and[U]i represents the concentration in powders of the suspension of powders to be atomised. 16. A device for granulating powders by cryogenic atomisation, comprising:a device for mixing powders by a cryogenic fluid, comprising at least one chamber for mixing powders, comprising the cryogenic fluid, wherein the cryogenic fluid is a liquified gas and chemically inert; anda device for atomising a suspension of powders mixed by the device for mixing powders in order to allow for a granulation of the powders, wherein the device fractionates the suspension of powders making it possible to adjust the size of droplets of the powders and adjusts the moisture of the mixed powders and/or moisture of the atomisation atmosphere andwherein the device for mixing further comprises:the at least one chamber for mixing the powders, comprising the cryogenic fluid, provided with means for forming a fluidised powder bed,a chamber for supplying powders in order to allow the powders to be introduced into the mixing chamber,a chamber for supplying the cryogenic fluid in order to allow the cryogenic fluid to be introduced into the mixing chamber,a system for generating vibrations in the fluidised powder bed,a system for controlling the system for generating vibrations. 17. The device according to claim 16, wherein the device for mixing further comprises a system for analysing the concentration of the suspension of powders and of the cryogenic fluid in the at least one chamber for mixing. 18. The device according to claim 16, wherein the at least one chamber for mixing comprises a distribution system of the cryogenic fluid through the fluidised bed of powders in order to allow for a homogeneous distribution of the cryogenic fluid in the fluidised bed. 19. The device according to claim 16, wherein the system for generating vibrations is at least partially located in the fluidised bed of powders. 20. The device according to claim 19, wherein the system for generating vibrations comprises sonotrodes introduced into the fluidised bed of powders. 21. The device according to claim 20, wherein the sonotrodes are controlled independently by the controlling system in order to induce a periodic phase shift of the phases between the sonotrodes in order to introduce unsteady interferences that improve the mixture within the fluidised bed of powders. 22. The device according to claim 20, the sonotrodes are configured to generate pseudo-chaotic oscillations of the Van der Pol type.