Patent Number: 
Section: claims

1. A method for preparing particles of an oxalate of one or more actinides, comprising:precipitating the actinide(s) in a form of actinide(s) oxalate particles in a fluidized bed reactor by bringing a first aqueous solution comprising the actinide(s) into contact with a second aqueous solution of oxalic acid or of an oxalic acid salt in the fluidized bed reactor, and forming the actinide(s) oxalate particles having a spherical or quasi-spherical shape, andcollecting the so precipitated actinide(s) oxalate particles. 2. The method of claim 1, in which the first aqueous solution has a total concentration of actinide(s) of 0.01 to 300 g/L. 3. The method of claim 1, in which the second aqueous solution has a concentration of oxalic acid or oxalic acid salt of 0.05 to 1 mole/L. 4. The method of claim 2, in which the volume ratio of the first aqueous solution to the second aqueous solution is such that the oxalic acid or the oxalic acid salt is in excess compared to an amount of oxalic acid or oxalic acid salt required for precipitating the actinide(s) in stoichiometric conditions, the excess being from 0.01 to 0.5 mole/L. 5. The method of claim 1, in which the actinide(s) is (are) present in the first aqueous solution in a form of nitrate(s). 6. The method of claim 1, in which the first aqueous solution is an acid solution. 7. The method of claim 6, in which the first aqueous solution is an aqueous solution comprising from 0.1 to 4 moles/L of nitric acid. 8. The method of claim 1, in which at least one of the first and second aqueous solutions further comprises a monocharged cation constituted of atoms of oxygen, carbon, nitrogen and hydrogen. 9. The method of claim 8, in which the monocharged cation is a hydrazinium ion and is present in the first aqueous solution in a form of hydrazinium nitrate. 10. The method of claim 1, in which precipitating the actinide(s) oxalate particles comprises fluidizing, decanting and sedimenting the actinide(s) oxalate particles and the fluidized bed reactor has a vertical main axis and comprises:an intermediate part in which the actinide(s) oxalate particles are fluidized;an upper part in which the actinide(s) oxalate particles are decanted; anda lower part in which the actinide(s) oxalate particles are sedimented. 11. The method of claim 10, in which precipitating the actinide(s) oxaloate particles comprises:fluidizing the actinide(s) oxalate particles by introducing the first and second solutions into the fluidized bed reactor, at least one of the first and second solutions being introduced into the lower part of the fluidized bed reactor to create an ascending current of liquid and thereby to form a fluidized bed of actinide(s) oxalate particles in the intermediate part of the fluidized bed reactor;decanting the actinide(s) oxalate particles in the upper part of the fluidized bed reactor to form two phases, a solid phase constituted of the actinide(s) oxalate particles and a liquid phase resulting from a mixing and a depletion into actinide(s) and into oxalic acid or oxalic acid salt of the first and second aqueous solutions;sedimenting the actinide(s) oxalate particles in the lower part of the fluidized bed reactor; andwithdrawing the actinide(s) oxalate particles measuring less than 10 μm which are in the upper part of the fluidized bed reactor and transferring said particles into the lower part of the fluidized bed reactor. 12. The method of claim 1, in which the actinide(s) is (are) chosen from uranium, plutonium, neptunium, thorium, americium and curium. 13. The method of claim 1, in which the actinide(s) oxalate is an oxalate of uranium(IV) and plutonium(III), an oxalate of uranium(IV) and americium(III), an oxalate of uranium(IV) and curium(III), an oxalate of uranium(IV), plutonium(III) and neptunium(IV), an oxalate of uranium(IV), plutonium(III) and americium(III), an oxalate of uranium(IV), americium(III) and curium(III), an oxalate of uranium(IV), plutonium(III), americium(III) and curium(III), or an oxalate of uranium(IV), plutonium(III), neptunium(IV), americium(III) and curium(III). 14. A method for preparing a compound chosen from oxides, carbides and nitrides of one or more actinides, which comprises:preparing particles of an oxalate of one or more actinides by a method according to claim 1; andcalcining the actinide(s) oxalate particles. 15. The method of claim 14, in which the actinide(s) oxalate is an oxalate of uranium(IV) and plutonium(III), an oxalate of uranium(IV) and americium(III), an oxalate of uranium(IV) and curium(III), an oxalate of uranium(IV), plutonium(III) and neptunium(IV), an oxalate of uranium(IV), plutonium(III) and americium(III), an oxalate of uranium(IV), americium(III) and curium(III), an oxalate of uranium(IV), plutonium(III), americium(III) and curium(III), or an oxalate of uranium(IV), plutonium(III), neptunium(V), americium(III) and curium(III). 16. The method of claim 14, in which the compound is an oxide of uranium and plutonium, an oxide of uranium and americium, an oxide of uranium(IV) and curium(III), an oxide of uranium, plutonium and neptunium, an oxide of uranium, plutonium and americium, an oxide of uranium, americium and curium, an oxide of uranium, plutonium, americium and curium, or an oxide of uranium, plutonium, neptunium, americium and curium.