Patent Number: 
Section: claims

1. Process for the preparation of a product comprising a phosphate of at least one element M(IV) chosen from thorium(IV) and actinide(IV)s, wherein it comprises the following stages:a) mixing a solution comprising thorium(IV) and/or at least one actinide(IV) with a phosphoric acid solution in amounts such that the molar ratio      PO    4        M    ⁢                  ⁢          (      IV      )       where M(IV) represents the total concentration of thorium(IV) and/or actinide(IV)s, is from 1.4 to 2,b) heating the mixture of the solutions in a closed container at a temperature of 50 to 250° C. to precipitate a product comprising a phosphate of at least one element M chosen from thorium(IV) and actinide(IV)s having a P/M molar ratio of 1.5, andc) separating the precipitated product from the solution. 2. Process according to claim 1, which additionally comprises the following stages:d) washing with water the precipitated product thus separated, ande) drying the washed product. 3. Process according to claim 1, in which the thorium solution is a solution of ThCl4 in hydrochloric acid. 4. Process according to claim 1, in which the actinide is uranium(IV) and the uranium(IV) solution is a solution of UCl4 in hydrochloric acid. 5. Process according to claim 1, in which the actinide is neptunium(IV) and the neptunium(IV) solution is a solution of neptunium in nitric acid. 6. Process according to claim 1, in which the actinide is plutonium(IV) and the plutonium(IV) solution is a solution of plutonium in nitric acid. 7. Process according to claim 1, in which, in stage a), at least one element chosen from trivalent actinides and trivalent lanthanides is additionally added to the mixture in order to include trivalent actinide(s) and/or trivalent lanthanide(s) in the product precipitated in stage b). 8. Process for the preparation of thorium phosphate/diphosphate of formula Th4(PO4)4P2O7, wherein a product based on thorium(IV) phosphate is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising thorium and phosphoric acid and wherein the product based on thorium(IV) phosphate is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C. 9. Process according to claim 8, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h. 10. Process for the preparation of uranium phosphate, wherein a product based on uranium(IV) phosphate is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising uranium and phosphoric acid and wherein the product based on uranium(IV) phosphate is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C. 11. Process according to claim 10, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h. 12. Process for the preparation of a solid solution of phosphates of thorium and of at least one tetravalent actinide, wherein a product comprising phosphates of Th and of at least one tetravalent actinide is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising thorium and at least one actinide and phosphoric acid and wherein the product based on phosphates of thorium(IV) and of actinide(IV) is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C. 13. Process according to claim 12, in which the solid solution of phosphates corresponds to the formula:Th4-xMx(PO4)4P2O7in which M is an element chosen from Pa(IV), U(IV), Np(IV) and Pu(IV), and x satisfies the following conditions:x≦3.75 for Pa(IV)x≦3 for U(IV)x≦2.14 for Np(IV)x≦1.67 for Pu(IV). 14. Process according to claim 12, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h. 15. Process for the preparation of a composite material including at least one actinide(III) and/or at least one lanthanide(III), wherein the precipitate comprising a phosphate of Th(IV) and/or of actinide(IV)(s) is prepared by carrying out the process according to claim 1 and wherein a powder comprising at least one actinide(III) and/or at least one lanthanide(III) in the phosphate form is dispersed in the precipitate and wherein the combination is subsequently subjected to a heat treatment, optionally preceded by a compacting, carried out at least partially at a temperature of 700 to 1300° C. 16. Process for the separation of uranium(VI), in the form of the uranyl ion UO22+, present in a solution with other cations, including thorium, wherein phosphoric acid is added to the solution in an amount such that the molar ratio      phosphoric    ⁢                  ⁢    acid        other    ⁢                  ⁢    cations  is from 1.4 to 2, wherein the solution thus obtained is heated at a temperature of 50 to 250° C. in a closed container, in order to precipitate a product comprising the cations other than uranium, and wherein the solution comprising uranium(VI) is recovered. 17. Process for the decontamination of radioactive aqueous effluent comprising contaminating radioactive cations, the process comprising:adding thorium and then phosphoric acid to the effluent in amounts such that the P/Th molar ratio is from 1.4 to 2;heating the effluent in a closed container at a temperature of 50 to 250° C.; andprecipitating a product from the effluent comprising thorium phosphate and entrained contaminating radioactive cations.