METHOD FOR ACTIVATING U3O8 WITH A VIEW TO CONVERTING SAME INTO HYDRATED UO4

Abstract

A method to activate U.sub.3O.sub.8 for conversion of this uranium oxide to hydrated UO.sub.4 via reaction with hydrogen peroxide H.sub.2O.sub.2, wherein the following successive steps are performed: a) an aqueous suspension is prepared containing a powder of U.sub.3O.sub.8 and hydrogen peroxide; b) the aqueous suspension containing a powder of U.sub.3O.sub.8 and hydrogen peroxide is contacted with ozone, whereby an aqueous suspension is obtained of a powder of activated U.sub.3O.sub.8; c) optionally the powder of activated U.sub.3O.sub.8 is separated from the aqueous suspension.

A method to convert U.sub.3O.sub.8 to hydrated UO.sub.4 of formula UO.sub.4, nH.sub.2O where n is 2 or 4, comprising at least one step at which hydrogen peroxide H.sub.2O.sub.2 is added to the aqueous suspension of a powder of activated U.sub.3O.sub.8 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 U.sub.3O.sub.8 obtained at the end of step c) of the activation method.

Claims

1. A method to activate U.sub.3O.sub.8 for conversion of this uranium oxide to hydrated UO.sub.4 via reaction with hydrogen peroxide H.sub.2O.sub.2, comprising the following successive steps: a) preparing an aqueous suspension is prepared containing a powder of U.sub.3O.sub.8and hydrogen peroxide; and b) contacting the aqueous suspension containing a powder of U.sub.3O.sub.8 and hydrogen peroxide is contacted with ozone, to obtain an aqueous suspension of a powder of activated U.sub.3O.sub.8.

2. The method according to claim 1 wherein during step b) a gas stream containing ozone is introduced and bubbled into the aqueous suspension containing a powder of U.sub.3O.sub.8 and hydrogen peroxide.

3. The method according to claim 2, wherein ozone is bubbled in the aqueous suspension for a time of 1 to 10 hours.

4. The method according to claim 2, wherein during step a) a U.sub.3O.sub.8 powder is added to a mixture of water and of an aqueous solution of hydrogen peroxide H.sub.2O.sub.2.

5. The method according to claim 2 wherein the gas stream containing ozone is a stream of air containing ozone, or a stream of oxygen containing ozone, at an ozone concentration of 1 to 500 g/m.sup.3.

6. The method according to claim 4, wherein the concentration of the aqueous solution of hydrogen peroxide is 30% to 70% by weight.

7. The method according to claim 1, wherein the uranium concentration of the aqueous suspension prepared in step a) is 10 to 500 g/L.

8. The method according to claim 1, wherein during step b) a molar ratio between ozone and U.sub.3O.sub.8 in the aqueous suspension is between 1 and 5.

9. The method according to claim 1, wherein during step b) an amount of hydrogen peroxide in the aqueous suspension is at least equal to an amount of ozone introduced into the aqueous suspension.

10. The method according to claim 1, wherein said contacting is conducted under agitation.

11. The method according to claim 1, wherein the U.sub.3O.sub.8 oxide is in the form of a uranium-containing concentrate called Yellow Cake, or the U.sub.3O.sub.8 oxide is derived from the drying and then calcining of an uranium concentrate, based or uranium tricarbonatebased uranium concentrate, obtained by precipitation in a reactor from a uranium-containing solution.

12. The method according to claim 1, further comprising: converting U.sub.3O.sub.8 to hydrated UO.sub.4 of formula UO.sub.4, nH.sub.2O where n is 2 or 4, by adding hydrogen peroxide H.sub.2O.sub.2 to the aqueous suspension of a powder of activated U.sub.3O.sub.8 obtained at the end of step b).

13. The method according to claim 12, further comprising the following successive steps: a1) adding hydrogen peroxide H.sub.2O.sub.2 to the aqueous suspension of a powder of activated U.sub.3O.sub.8 obtained at the end of step b), converting U.sub.3O.sub.8 to hydrated UO.sub.4, and precipitating, crystallizing the hydrated UO.sub.4 in the suspension; and b1) recovering the precipitate, crystals, of hydrated UO.sub.4, wherein the addition of H.sub.2O.sub.2 to the aqueous suspension is conducted so that the suspension contains a stoichiometric excess of H.sub.2O.sub.2 relative to the stoichiometry of the reaction starting from U.sub.3O.sub.8:
UO.sub.2.67+1.33H.sub.2O.sub.2+nH.sub.2O.fwdarw.UO.sub.4,nH.sub.2O+H.sub.2O(1) and the pH of the suspension in steps a) and b) is maintained at a value between 1 and 3.

14. The method according to claim 1, further comprising separating the powder of activated U.sub.3O.sub.8 from the aqueous suspension.

15. The method according to claim 3, wherein ozone is bubbled in the aqueous suspension for a time of 8 hours.

16. The method according to claim 5, wherein the gas stream containing ozone is a stream of air containing ozone, or a stream of oxygen containing ozone, at an ozone concentration of 1 to 225 g/m.sup.3.

17. The method according to claim 16, wherein the gas stream containing ozone is a stream of air containing ozone, or a stream of oxygen containing ozone, at an ozone concentration of 75 to 225 g/m.sup.3.

18. The method according to claim 7, wherein the uranium concentration of the aqueous suspension prepared in step a) is 200 to 300 g/L.

19. The method according to claim 18, wherein the uranium concentration of the aqueous suspension prepared in step a) is 250 g/L.

20. The method according to claim 11, wherein the U.sub.3O.sub.8 oxide is an hydrated UO.sub.4-based, ammonium diuranate, and wherein said reactor is a fluidised bed reactor.

21. The method according to claim 1, further comprising: converting U.sub.3O.sub.8 to hydrated UO.sub.4 of formula UO.sub.4, nH.sub.2O where n is 2 or 4, by adding hydrogen peroxide H.sub.2O.sub.2 to the aqueous suspension prepared by placing in suspension in water the powder of activated U.sub.3O.sub.8 obtained at the end of step c).

22. The method according to claim 21, further comprising the following successive steps: a1) adding hydrogen peroxide H.sub.2O.sub.2 to the aqueous suspension prepared by placing in suspension in water the powder of activated U.sub.3O.sub.8 obtained at the end of step c), converting U.sub.3O.sub.8 to hydrated UO.sub.4, and precipitating, crystallizing the hydrated UO.sub.4 in the suspension; and b1) recovering the precipitate, crystals, of hydrated UO.sub.4, wherein the addition of H.sub.2O.sub.2 to the aqueous suspension is conducted so that the suspension contains a stoichiometric excess of H.sub.2O.sub.2 relative to the stoichiometry of the reaction starting from U.sub.3O.sub.8:
UO.sub.2.67+1.33H.sub.2O.sub.2+nH.sub.2O.fwdarw.UO.sub.4,nH.sub.2O+H.sub.2O(1) and the pH of the suspension in steps a) and b) is maintained at a value between 1 and 3.

23. The method according to claim 13, further comprising washing the recovered precipitate, crystals of hydrated UO.sub.4.

24. The method according to claim 23, further comprising repeating step c1).

25. The method according to claim 23, further comprising drying the precipitate, crystals.

26. The method according to claim 22, further comprising washing the recovered precipitate, crystals of hydrated UO.sub.4.

27. The method according to claim 26, further comprising repeating step c1).

28. The method according to claim 26, further comprising drying the precipitate, crystals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0105] FIG. 1 is a graph showing the diffractogram obtained upon X-ray diffraction analysis of the activated solid obtained in Example 2 via action, according to the activation method of the invention, of a gas stream containing ozone on an aqueous suspension of U.sub.3O.sub.8 oxide further containing hydrogen peroxide.

[0106] FIG. 2 is a graph giving the specific surface area (in m.sup.2/g) of a reference U.sub.3O.sub.8 oxide not having undergone any activation treatment (on the left), of the powder of activated U.sub.3O.sub.8 obtained in Example 2 via action, according to the activation method of the invention, of a gas stream containing ozone on an aqueous suspension of U.sub.3O.sub.8 oxide further containing hydrogen peroxide (in the centre), and finally of the hydrated UO.sub.4 obtained in Example 2 (on the right).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0107] In the first step of the activation method of the invention (step a)), an aqueous suspension is prepared containing a powder of uranium octoxide U.sub.3O.sub.8, and hydrogen peroxide.

[0108] This suspension can be prepared by adding a powder of U.sub.3O.sub.8 to a solution containing hydrogen peroxide.

[0109] This solution can be prepared by mixing water and oxygenated wateri.e an aqueous solution of hydrogen peroxideto obtain an aqueous solution having a hydrogen peroxide concentration of 0.5 to 10% by weight. Oxygenated waters generally have a hydrogen peroxide concentration of 30% to 70% by weight.

[0110] The concentration of oxide(s) in the aqueous suspension is generally 10 to 500 gU/L, preferably 200 to 300 gU/L.

[0111] The method of the invention can be implemented using all kinds of uranium octoxides U.sub.3O.sub.8, irrespective of their origin and the form thereof.

[0112] For example these oxides can be in the form of concentrates known as Yellow Cake.

[0113] These oxides may also derive from the drying, and then calcining of an uranium concentrate, for example an hydrated UO.sub.4, ammonium diuranate, or uranium tricarbonate based, concentrate, by precipitation in a reactor, in particular a fluidised bed reactor, from a uranium-containing solution.

[0114] One method to prepare a powder of U.sub.3O.sub.8 by drying and calcining a hydrated UO.sub.4, ammonium diuranateor uranium tricarbonate based uranium concentrate previously obtained by precipitation in a fluidised bed is described in document WO-A1-2010/051855 to the description of which reference can be made.

[0115] The U.sub.3O.sub.8 powders obtained in this document, on account of the preparation of the uranium concentrate in a fluidised bed, have particularly advantageous properties.

[0116] Oxides in the form of concentrates called Yellow Cake, or oxides derived from the drying and calcining of a uranium concentrate preferably obtained via precipitation in a fluidised bed are generally in the form of powders and can be used directly in the method of the invention and can be placed in suspension in water.

[0117] However it may be advantageous to carry out prior milling of the oxide powders to obtain a particularly fine particle size e.g. in the order of one micrometre.

[0118] The powders placed in suspension generally contain impurities and the conversion method of the invention following the activation method has the particular objective of reducing the content of these impurities in the hydrated uranium peroxide obtained.

[0119] Preferably, with the conversion method of the invention it is sought to obtain hydrated uranium peroxide having an impurity content compatible with conversion thereof to UF.sub.6 and having an impurity content meeting standard ASTM C-787.

[0120] The U.sub.3O.sub.8 powder may contain one or more of the following impurities, for example in the following contents expressed in ppm/U: [0121] As: 102 [0122] Ca: 1383 [0123] Si: 2312 [0124] Zr: 316 [0125] SO.sub.4: 29205 [0126] Mo: 1109 [0127] Na: 20

[0128] Obviously, the aqueous suspension containing a powder of uranium octoxide U.sub.3O.sub.8 and hydrogen peroxide may be prepared in other manners, for example it is possible to start by preparing an aqueous suspension containing a powder of uranium octoxide U.sub.3O.sub.8 by adding the powder(s) to non-oxygenated water, water not containing hydrogen peroxide (or vice versa), then by adding oxygenated water to this aqueous suspension.

[0129] During the second step of the conversion method of the invention (step b)) the aqueous suspension containing a powder of U.sub.3O.sub.8, and hydrogen peroxide, is contacted with ozone, whereby an aqueous suspension is obtained of a powder of activated U.sub.3O.sub.8.

[0130] The contacting of the aqueous suspension with ozone is generally conducted by introducing and bubbling a gas stream containing ozone into the aqueous suspension containing a U.sub.3O.sub.8 powder and hydrogen peroxide.

[0131] The ozone-containing gas stream is generally a stream of an oxygen-containing gas, such as air or oxygen, containing ozone, generally at a concentration of 1 to 500 g/m.sup.3.

[0132] Such a stream of gas containing ozone may be produced by an ozonator or ozone generator in which ozone is created by subjecting an oxygen-containing gas such as air, to a corona-effect discharge which converts part of the oxygen of the gas to ozone.

[0133] In general the ozone is bubbled through the aqueous suspension for a time of 1 to 10 hours, for example 8 hours.

[0134] In general during step b) the molar ratio between the ozone and U.sub.3O.sub.8 in the aqueous suspension is between 1 and 5.

[0135] In general, as indicated above, during step b) the amount of hydrogen peroxide in the aqueous suspension is at least equal to the amount of ozone introduced into the aqueous suspension (molar ratio of 1 or higher).

[0136] Steps a) and b) are generally performed under agitation.

[0137] Therefore the reactor used to implement the method of the invention and in particular to perform steps a) and b) is generally a perfectly agitated reactor generally equipped with a propeller agitator e.g. a three-blade propeller.

[0138] The reactor may also be provided with counter-blades or deflectors.

[0139] The volume of the reactor may be easily chosen by the man skilled in the art as a function of the volume of suspension it is desired to prepare.

[0140] The reactor may further be provided with sensors and devices to measure parameters values such as pH and temperature of the suspension.

[0141] The suspension may be prepared by introducing a known amount of powder of oxide(s) into the reactor.

[0142] To this known amount of oxide there is added the amount of water and oxygenated water (hydrogen peroxide solution) desired to obtain a suspension having the desired concentration.

[0143] The powder and the mixture of water and oxygenated water (hydrogen peroxide solution) having been introduced into the reactor, agitation is started to place the powder(s) in suspension in the mixture of water and oxygenated water (hydrogen peroxide solution).

[0144] Agitation speed is set to allow efficient placing in suspension of the powder. A gas stream containing ozone is then bubbled through the aqueous suspension thus obtained under the conditions detailed above.

[0145] Optionally, during an optional step c), the powder of activated U.sub.3O.sub.8 is separated from the aqueous suspension using any suitable solid/liquid separation method e.g. filtration.

[0146] The aqueous suspension of a powder of activated U.sub.3O.sub.8 obtained at the end of step b) of the activation method may be used directly in the method to convert U.sub.3O.sub.8 to hydrated UO.sub.4 of formula UO.sub.4, nH.sub.2O where n is 2 or 4, such as the method of document FR-A1-2 969 659. For example, this suspension also called a pulp may be conveyed directly from the reactor in which the activation method has been carried out, at the end of said activation process, to the reactor in which the conversion method is performed.

[0147] Or else, for this conversion method, such as the method of document FR-A1-2 969 659, an aqueous suspension may be used that is prepared by placing in suspension in water the powder of activated U.sub.3O.sub.8 obtained at the end of the separation step c) of the activation method of the invention.

[0148] The reactor used to implement the conversion method of the invention and to carry out steps a1) and b1) in particular is generally a perfectly agitated reactor generally equipped with a propeller agitator e.g. a three-blade propeller.

[0149] The reactor may also be provided with counter-blade or deflectors.

[0150] The volume of the reactor can easily be chosen by the man skilled in the art as a function of the volume of suspension it is desired to prepare.

[0151] The reactor may further by provided with sensors and devices to measure parameter values such as pH and temperature of the suspension.

[0152] As already indicated above, the aqueous suspension of a powder of activated U.sub.3O.sub.8 obtained at the end of step b) of the activation method may be used directly in the method to convert UO.sub.3 and/or U.sub.3O.sub.8 to hydrated UO.sub.4 of formula UO.sub.4, nH.sub.2O where n is 2 or 4, such as the method of document FR-A1-2 969 659 and, in this case, this suspension also called a pulp may be conveyed directly from the reactor in which the activation method was carried out, at the end of said activation method, to the reactor in which the conversion method is to be conducted.

[0153] Or else, in this conversion method, such as the method of document FR-A1-2 969 659, an aqueous suspension may be used that is prepared by placing in suspension in water the powder of activated U.sub.3O.sub.8 obtained at the end of the separation step c) of the activation method of the invention.

[0154] The suspension is generally prepared by adding a known amount of activated U.sub.3O.sub.8 powder to the reactor.

[0155] To this known amount of activated oxide is then added the amount of demineralised water desired to obtain a suspension having the desired concentration.

[0156] Obviously, it is also possible to start by introducing demineralised water in the reactor then adding the powder of activated oxide to the demineralised water.

[0157] The concentration of activated oxide in the suspension is generally such as defined above, namely a uranium concentration of 10 to 500 g/L, preferably 200 to 300 g/L, for example 250 g/L.

[0158] The pH of the demineralised water is adjusted to a value of 1 to 3 through the addition of an acid or of a mixture of acids. If, for the conversion method, the suspension obtained in step b) of the activation method is used directly, the acid is added to this suspension.

[0159] This or these acid(s) may be any mineral or organic acid.

[0160] As will be seen below, an acid having an anion additionally having a complexing action which improves reaction kinetics is preferred.

[0161] The preferred acids are sulfuric acid, oxalic acid and the mixtures thereof.

[0162] Other acids may be used to adjust pH but sulfuric acid has the advantage of not introducing any elements which may hamper the nuclear purity of UF.sub.6 since there are no ASTM specifications relating to sulfur.

[0163] In addition, the conversion rate to hydrated UO.sub.4 is limited by the formation of a reaction intermediate (uranyl ion UO.sub.2.sup.2+) but may be accelerated through the use of at least one complexing anion such as the sulfate anion or the oxalate anion or the citrate anion.

[0164] It is therefore possible to add a compound providing this complexing anion during step a1) and/or step b1) of the conversion method of the invention. With regard to sulfur the optimal S/U ratio is lower than 0.125, and is in the order of 0.02 for example.

[0165] Preferably sulfuric acid is used as compound providing the complexing anion for suspending the uranium oxide in order to obtain fast conversion kinetics.

[0166] The powder and demineralised water having been introduced into the reactor, agitation is started to suspend the powder of activated oxide in the demineralised water.

[0167] The agitation speed is set to allow efficient suspending of the powder or to maintain the powder in suspension if, for the conversion method, use is made of the suspension obtained at the end of step b) of the activation method.

[0168] Agitation is continued throughout the entire duration of conversion to hydrated UO.sub.4 to allow complete crystallisation of the starting uranium.

[0169] It is then possible to start adding hydrogen peroxide into the suspension.

[0170] The adding of hydrogen peroxide may be carried out using any suitable device allowing to control the flow rate of hydrogen peroxide introduced into the reactor.

[0171] The addition of hydrogen peroxide is also preferably conducted under agitation.

[0172] The hydrogen peroxide is generally added in the form of an aqueous solution at a concentration of 30% to 70% by weight.

[0173] The total amount of hydrogen peroxide added, is, according to the invention, such that the stoichiometric excess of H.sub.2O.sub.2 relative to the starting uranium is more than 1.33 to 10 relative to the stoichiometry of following reaction (1):

UO.sub.2.67+1.33 H.sub.2O.sub.2+nH.sub.2O.fwdarw.UO.sub.4,nH.sub.2O+H.sub.2O(1)

[0174] The reaction between the oxide and hydrogen peroxide is exothermal and an increase in temperature of the bath is ascertained of about 10 C. for example.

[0175] In one embodiment, the following successive steps b2) and b3) are performed: [0176] b2) adding, as described above, hydrogen peroxide H.sub.2O.sub.2 to the aqueous suspension of U.sub.3O.sub.8 powder, preferably under agitation, then stopping the addition; [0177] b3) ripening of the suspension, preferably under agitation.

[0178] During step b2), it can be estimated that some conversion to hydrated uranium peroxide occurs but this conversion is not complete.

[0179] Step b2) can be termed a nucleation, crystallisation, formation of crystallites of hydrated uranium peroxide, step.

[0180] During step b3), conversion is continued until the conversion of UO.sub.3 and/or U.sub.3O.sub.8 to hydrated UO.sub.4 is complete or substantially complete, for example higher than 90%, even 99.9%.

[0181] Step b3) can be termed a ripening, growth of the crystallites obtained during step b2), step.

[0182] The duration of said step b2) may be 1 to 8 hours, preferably 1 to 3 hours, and the duration of step b3) may be 1 to 24 hours, preferably Ito 3 hours.

[0183] The total duration of steps b2) and b3) is such that the conversion to hydrated uranium peroxide is complete or substantially complete.

[0184] In another embodiment, no ripening step is performed after step b2), and step b3) is omitted.

[0185] It is to be noted that during the reaction of oxygenated water (hydrogen peroxide solution) with the oxides, the pH varies but globally remains stable, constant, at the value at which it had been adjusted through the addition of acid before the addition of oxygenated water, which means that it is generally not necessary to add more acid during step b) to control pH at the desired value.

[0186] It may be estimated that some regulation of pH is induced through the addition of H.sub.2O.sub.2 to U.sub.3O.sub.8.

[0187] At the end of the reaction, since conversion is complete or substantially complete, the pH generally stabilises at a value between 1 and 3.

[0188] At the end of step b1) the conversion to hydrated uranium peroxide being complete or substantially complete, the precipitate, the crystals of hydrated UO.sub.4, is recovered, generally in the form of tetrahydrated UO.sub.4, 4H.sub.2O or optionally in the form of dihydrated UO.sub.4, 2H.sub.2O.

[0189] In one variant of recovery step c1) it is possible to recover, collect the precipitate, crystals of hydrated UO.sub.4, by separating them from the suspension via a liquid-solid separation operation in the form of a wet solid, for example having an humidity of 30% to 80% by weight, also called a cake.

[0190] The crystals of hydrated UO.sub.4 recovered in this variant are generally crystals of UO.sub.4, 4H.sub.2O.

[0191] This liquid-solid separating operation may be a filtering operation of the suspension.

[0192] This filtering operation may be conducted in a vacuum or under centrifugal force.

[0193] The recovered wet solid may be collected and then washed with a washing liquid.

[0194] Said washing liquid may be demineralised water or an aqueous solution, acidified, preferably at a pH of 2 to 3, for example with sulfuric acid.

[0195] As washing liquid it is also possible to use an aqueous solution, preferably at a pH of 2 to 3, of an anion complexing the impurities contained in the wet solid such as those already mentioned above.

[0196] Sulfuric acid has the advantage of acting both as acidifying agent and as complexing agent due to sulfate anions.

[0197] The washing operation can be repeated 1 to 10 times depending on the desired impurity content of the uranium peroxide.

[0198] Advantageously the washing ratio defined as the ratio of mass of washing liquid (for the total number of washings) to the mass of wet solid is 1 to 30, preferably 1 to 10, to limit the volumes of water required for washing.

[0199] The method of the invention may optionally comprise a drying step of the recovered crystals of hydrated UO.sub.4.

[0200] This drying step is generally conducted at a temperature of 60 C. to 100 C. for a time of 1 to 24 hours.

[0201] During this step the recovered hydrated uranium peroxide is converted to UO.sub.4, 2H.sub.2O if it is UO.sub.4, 4H.sub.2O.

[0202] The hydrated uranium peroxide obtained has high reactivity for rapid conversion to UF.sub.4.

[0203] For example a conversion rate of uranium to UF.sub.4 of at least 90% is obtained within 800 seconds.

[0204] The conversion method of the invention allows hydrated UO.sub.4to be obtained having a high specific surface area possibly reaching 30 m.sup.2/g.

[0205] The impurity contents of the final peroxide are lower than that of the peroxides obtained using prior art methods and mostly conform to standard ASTM C-787.

[0206] The invention is now described with reference to the following examples that are non-limiting and given for illustration purposes.

EXAMPLES

[0207] In the following examples a description is given of the conversion of non-activated U.sub.3O.sub.8 to hydrated UO.sub.4 via reaction with hydrogen peroxide alone (Example 1, comparative); of the activation of U.sub.3O.sub.8 according to the invention using a mixture of ozone and oxygenated water (hydrogen peroxide solution); and finally of the conversion of the activated U.sub.3O.sub.8 obtained in Example 2 by applying the conversion method of the invention (Example 3).

Example 1 (Comparative Example)

[0208] 0.5 g of U.sub.3O.sub.8 are placed in suspension in a flask containing 10 mL of water. Hydrogen peroxide is added to the suspension of U.sub.3O.sub.8 to reach a molar concentration of 1 mol/L of H.sub.2O.sub.2. Agitation is continued for 24 h.

[0209] On completion of the test, the suspension is filtered and the solid obtained is analysed. A black solid is obtained.

[0210] The solid is analysed by X-ray diffraction analysis and the diffractogram obtained shows the conversion of U.sub.3O.sub.8 to hydrated UO.sub.4.

[0211] Under these conditions the conversion of U.sub.3O.sub.8 to hydrated UO.sub.4 is low, in the order of 10%.

Example 2

[0212] 0.5 mmol of U.sub.3O.sub.8 are placed in suspension in a flask containing 10 mL water and 0.15 mL 30% oxygenated water (hydrogen peroxide solution). A gas stream containing ozone is passed through the suspension of U.sub.3O.sub.8 for 8 hours. The ozone is generated by an OZ 1000-L ozonator. The average concentration of ozone measured is 4.5 g/m.sup.3.

[0213] At the end of the test the suspension is filtered.

[0214] A grey solid is obtained.

[0215] This solid is analysed by X-ray diffraction (FIG. 1). The diffractogram shows the presence of a mixture of U.sub.3O.sub.8 and hydrated UO.sub.4, a mixture containing in the order of 40% hydrated UO.sub.4.

[0216] Observations under scanning electron microscope SEM and measurement of the specific surface area of the powder show a modified grain morphology.

[0217] This treatment with O.sub.3/H.sub.2O.sub.2 allows an improvement in the reactivity of the starting U.sub.3O.sub.8 by increasing the specific surface area of the solid (FIG. 2).

Example 3

[0218] The solid obtained in Example 2 is then able to react is a second step with oxygenated water (hydrogen peroxide solution) according to the conversion method of the invention implemented in accordance with document FR-A1-2 969 659 to produce 99.9% of hydrated uranium peroxide within 8 hours.

[0219] Through the use of activated U.sub.3O.sub.8 according to the invention, the conversion to hydrated UO.sub.4 is therefore substantially complete.