Abstract: A thermochemical method for storing and releasing thermal energy by means of a compound in solid form of formula AOxBy.zH2O, in which: A is an element selected from uranium (U) and thorium (Th); O is the element oxygen; B is an anion or an oxoanion; x is a number comprised between 0 and 4; y is a number comprised between 0 and 2; z is a number greater than 0 and less than 10; it being understood that at least one of x and y is different from 0 and that the compound of formula Th(SO4)2.xH2O is excluded.

Abstract: A method to activate U3O8 for conversion of this uranium oxide to hydrated UO4 via reaction with hydrogen peroxide H2O2, wherein the following successive steps are performed: a) an aqueous suspension is prepared containing a powder of U3O8 and hydrogen peroxide; b) the aqueous suspension containing a powder of U3O8 and hydrogen peroxide is contacted with ozone, whereby an aqueous suspension is obtained of a powder of activated U3O8; c) optionally the powder of activated U3O8 is separated from the aqueous suspension. A method to convert U3O8 to hydrated UO4 of formula UO4, nH2O where n is 2 or 4, comprising at least one step at which hydrogen peroxide H2O2 is added to the aqueous suspension of a powder of activated U3O8 obtained at the end of step b) of the activation method or to an aqueous suspension prepared by placing in suspension in water the powder of activated U3O8 obtained at the end of step c) of the activation method.

Abstract: A process is described for synthesizing a mixed peroxide or hydroxo-peroxide of an actinyl and at least one cation X1, wherein the actinyl is a uranyl or neptunyl and the at least one cation X1 is a di-, tri- or tetra-charged metal cation. This process includes the reaction in a solvent of a salt of the at least one cation X1 with a compound C2 selected from mixed peroxides and hydroxo-peroxides of the actinyl and of at least one singly charged cation X2, whereby compound C2 is converted to the peroxide or hydroxo-peroxide by replacement of the at least one cation X2 by said at least one cation X1. Also disclosed is a process for synthesizing a mixed oxide of an actinide selected from uranium and neptunium, and of at least one metal able to form a di-, tri- or tetra-charged cation, which implements the preceding synthesis process.

Abstract: A method to activate U3O8 for conversion of this uranium oxide to hydrated UO4 via reaction with hydrogen peroxide H2O2, wherein the following successive steps are performed: a) an aqueous suspension is prepared containing a powder of U3O8 and hydrogen peroxide; b) the aqueous suspension containing a powder of U3O8 and hydrogen peroxide is contacted with ozone, whereby an aqueous suspension is obtained of a powder of activated U3O8; c) optionally the powder of activated U3O8 is separated from the aqueous suspension. A method to convert U3O8 to hydrated UO4 of formula UO4, nH2O where n is 2 or 4, comprising at least one step at which hydrogen peroxide H2O2 is added to the aqueous suspension of a powder of activated U3O8 obtained at the end of step b) of the activation method or to an aqueous suspension prepared by placing in suspension in water the powder of activated U3O8 obtained at the end of step c) of the activation method.

Abstract: A process is described for synthesizing a mixed peroxide or hydroxo-peroxide of an actinyl and at least one cation X1, wherein the actinyl is a uranyl or neptunyl and the at least one cation X1 is a di-, tri- or tetra-charged metal cation. This process includes the reaction in a solvent of a salt of the at least one cation X1 with a compound C2 selected from mixed peroxides and hydroxo-peroxides of the actinyl and of at least one singly charged cation X2, whereby compound C2 is converted to the peroxide or hydroxo-peroxide by replacement of the at least one cation X2 by said at least one cation X1. Also disclosed is a process for synthesizing a mixed oxide of an actinide selected from uranium and neptunium, and of at least one metal able to form a di-, tri- or tetra-charged cation, which implements the preceding synthesis process.

Abstract: A method for converting UO3 and/or U3O8 into hydrated UO4 of formula UO4.nH2O wherein n is 2 or 4, comprising the following successive steps: a) preparing an aqueous suspension of a UO3 powder and/or a U3O8 powder; b) adding hydrogen peroxide H2O2 to the aqueous suspension of a UO3 and/or U3O8 powder, converting the UO3 and/or U3O8 into hydrated UO4 and precipitating, crystallizing the hydrated UO4 in the suspension; c) recovering the precipitate, crystals of UO4 hydrate; d) optionally, washing the recovered UO4 hydrate precipitate, crystal(s); e) optionally, repeating step d); f) optionally, drying the precipitate, the crystals; wherein the addition of H2O2 to the aqueous suspension is carried out so that the suspension contains a stoichiometric excess of H2O2 relatively to the stoichiometry of the reaction from UO3: UO3+nH2O+H2O2?UO4.nH2O+H2O??(1) or of the reaction from U3O8 UO2.67+1.33H2O2+nH2O?UO4.nH2O+1.

Abstract: A method for converting UO3 and/or U3O8 into hydrated UO4 of formula UO4.nH2O wherein n is 2 or 4, comprising the following successive steps: a) preparing an aqueous suspension of a UO3 powder and/or a U3O8 powder; b) adding hydrogen peroxide H2O2 to the aqueous suspension of a UO3 and/or U3O8 powder, converting the UO3 and/or U3O8 into hydrated UO4 and precipitating, crystallizing the hydrated UO4 in the suspension; g) recovering the precipitate, crystals of UO4 hydrate; h) optionally, washing the recovered UO4 hydrate precipitate, crystal(s); i) optionally, repeating step d); j) optionally, drying the precipitate, the crystals; wherein the addition of H2O2 to the aqueous suspension is carried out so that the suspension contains a stoichiometric excess of H2O2 relatively to the stoichiometry of the reaction from UO3: UO3+nH2O+H2O2?UO4.nH2O+H2O??(1) or of the reaction from U3O8 UO2.67+1.33H2O2+nH2O?UO4.