Materials containing porous microspheres based on actinide oxides and their use in the manufacture of nuclear fuel

Abstract

The present invention relates to a material comprising porous microspheres based on actinide oxides, obtained by an internal gelation method. The present invention also relates to the use of this material as a sintering additive in the production of nuclear fuel, particularly in the production of MOX (mixed oxide) type nuclear fuel pellets, and more specifically as a pore-forming agent to replace conventionally used pore-forming agents in said production, namely azodicarbonamides.

Description

[Technical Field]

[0001] This invention relates to the field of the nuclear fuel cycle.

[0002] More specifically, the present invention relates to a material comprising porous microspheres based on actinide oxides, obtained by an internal gelation method.

[0003] The present invention also relates to the use of this material as a sintering additive in the production of nuclear fuel, particularly in the production of MOX (mixed oxide fuel) type nuclear fuel pellets, and more specifically, as a porosity-forming agent that has been conventionally used in this production, namely azodicarbonamide. [Background technology]

[0004] Currently, the method used in France to produce MOX nuclear fuel for conventional water reactors is a unique method called "MIMAS" (Micronized Master Blend), which was developed for use at the MELOX plant in France, and is now, in general terms... - A process to create a primary mixture of powder having a plutonium concentration higher than the plutonium concentration required for the fuel, by co-grinding UO2 powder, PuO2 powder, and optionally refractory clay until a finely ground and completely mixed powder is obtained. - A process to create a final mixture of powders having the required plutonium concentration for the fuel. - The process of pressing the final mixture in the form of "unprocessed" pellets. - A process of sintering unprocessed pellets at high temperature, and - A process of adjusting the dimensions of sintered pellets to meet specifications. This is called the "MELOX process," which consists of these elements.

[0005] Sintered pellets deemed compliant after modification are introduced into metal sheaths to form fuel rods, which are then bundled together to form fuel assemblies. Pellets deemed non-compliant are used to manufacture refractory clay.

[0006] All operations for the method of producing MOX fuel are described in detail in the monograph published in 2008 by the Nuclear Energy Section of the CEA entitled "Le traitement-recyclage du combustible nucleaire use - La separation des actinides - Application a la gestion des dechets" (Editions Le Moniteur, ISBN 978-2-281-11376-1), hereafter referenced [1].

[0007] In the final powder mixture, it is common to introduce sintering additives, particularly porosity-forming agents, whose function is to create a specific porosity in the pellets during sintering.

[0008] Currently, the standard porosity-forming agent is an organic compound also known by the acronym AZB or ADCA, namely an azodicarbonamide having the formula: NH2-C(O)-N=NC(O)-NH2.

[0009] In the future, particularly in the context of implementing methods for producing MOX fuel from nuclear materials capable of radioactive decomposition of azodicarbonamides, induced by the presence of higher proportions of plutonium-238 and americium-241 isotopes, such as materials reused multiple times, the decomposition of azodicarbonamides leads to swelling of the raw pellets, which is detrimental to sintering. This phenomenon is observed during the storage of raw pellets awaiting sintering.

[0010] As a result, using azodicarbonamide as a porosity-forming agent in the production of MOX fuel generates a significant amount of unprocessed pellets that are not discarded.

[0011] Therefore, it is desirable to have a porosity-forming agent available that allows for the creation of a satisfactory porosity in the MOX fuel pellets during sintering that is at least as good as that produced by azodicarbonamide, while not exhibiting the drawbacks of the latter, for future manufacturing prospects.

[0012] Accordingly, the inventors set the objective of providing such a pore-forming agent and, to do so, being able to produce porous microspheres that are resistant to radiolysis and thermal decomposition and are compatible with methods for producing nuclear fuel, particularly the MIMAS method, and especially in terms of composition, size, and resistance to crushing and shear effects induced during mixing operations performed as part of the implementation of these methods. In this context, the inventors chose to produce microspheres having a size between 20 μm and 1000 μm and made from actinide oxide, and containing calibrated sized closed pores dispersed within this matrix.

[0013] In a completely different context from proposing sintering additives, or more specifically, porosity-forming agents, for the production of MOX fuel, prior art has proposed producing solid or porous microspheres of uranium dioxide (UO2) or triuranium octoxide (U3O8) by internal gelation.

[0014] In general terms, internal gelation involves preparing a solution called a "sol" containing a uranyl salt and a polymerization precursor, dispersing the sol in the form of droplets in a heated liquid that is immiscible with the sol, and obtaining spheroidization by internal gelation of the droplets, particularly by the formation of a three-dimensional polymer network in which uranyl ions complex. After various aging, washing, and drying operations, uranyl hydroxide is formed. The resulting gelled microspheres are subjected to a compaction heat treatment, during which the three-dimensional polymer network is removed, and the uranyl hydroxide is converted to UO2 if the heat treatment is performed in a reducing atmosphere, or to U3O8 if the heat treatment is performed in an oxidizing atmosphere.

[0015] Two solutions have been proposed to obtain porous microspheres.

[0016] The first solution described in Patent US 4,218,430 and the following reference [2] consists of impregnating microspheres in a gelled state with an organic compound such as polyethylene glycol, glycerol, or mannitol, and then removing the organic compound by decomposing it into a volatile compound during the heat treatment for densification of the microspheres. Thus, microspheres with an open porosity, i.e., pores that communicate with each other through channels that allow fluid circulation, are obtained.

[0017] The second solution described by G. Colak et al. in Journal of Nuclear Materials 2022, 562, 153587 and Journal of Nuclear Materials 2023, 577(1), 154319, and the following references [3] and [4] respectively, consists of incorporating starch or graphite particles into a sol, and then dispersing the sol in a heated liquid medium. Here too, the particles are then removed by decomposing them into a volatile compound during the heat treatment for densification of the microspheres. In these references, it has been shown that using starch particles (more effective than using graphite particles in creating porosity) results in a decrease in porosity with an increase in the densification temperature, and microspheres with a porosity that is substantially accessible to the intrusion of an aqueous solution, and thus an open porosity, are obtained.

[0018] Therefore, none of the solutions proposed in references [1] to [3] make it possible to obtain microspheres of UO2 and / or U3O8 with a calibrated closed porosity.

Prior Art Documents

Patent Documents

[0019]

Patent Document 1

Non-Patent Documents

[0020]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0021] The present invention first provides a material comprising porous microspheres, - each porous microsphere comprising a matrix containing an actinide oxide and one or more closed pores dispersed in the matrix, - the microspheres having a size between 20 μm and 1,000 μm, - the size of the pores being calibrated, a) preparing an aqueous suspension or sol by mixing an aqueous solution A1 containing first and second prepolymers capable of forming a polymer gel together and an aqueous suspension A2 containing actinide salts and beads of a sacrificial material; b) converting the sol into gelated microspheres by dispersing the sol in the form of droplets in a bath of an organic liquid immiscible with water, thereby obtaining microspheres formed by a polymer gel surrounding the beads of the sacrificial material c) A step of separating the gelled microspheres from the organic liquid bath, d) A step of washing and drying the gelled microspheres. e) A step of selectively chemically dissolving the sacrificial material beads present in the gelled microspheres, and f) A process of firing gelled microspheres, Obtained by a method including, In step b), the objective is precisely to address the shortcomings of the prior art by proposing a material in which the droplet size is controlled by an automatic distribution system. [Means for solving the problem]

[0022] In the above and below, the term "size" applied to microspheres is understood to correspond to the diameter of a microsphere if it is a perfect sphere, or to the diameter (or equivalent diameter) of a circle having the same surface area if the microsphere is not a perfect sphere. As is known, the size of a microsphere can be determined by optical diffraction particle size analysis using a laser particle size analyzer, such as the one sold by Malvern Panalytical under the reference name Mastersizer® 3000, or by particle size and shape analysis using a particle size and shape analyzer, such as the one sold by Malvern Panalytical under the reference name Morphologi® G3.

[0023] Furthermore, - A "closed stoma" refers to one or more stomata that do not communicate with the surface of the microsphere in which they are located, and if there are two or more stomata on the same microsphere, they do not communicate with each other. - Pore size is considered to be calibrated by the diameter of the spherical indentations left on the microspheres by the sacrificial material beads or the beads themselves after selective dissolution (step e) and subsequent firing of the microspheres (step f). The sacrificial bead indentations that constitute the closed porosity of the microspheres retain their spherical shape during firing, but undergo a partial reduction in size depending on the temperature and duration at which the firing is performed.

[0024] According to the present invention, the step of preparing a sol, or step a), is advantageously, - Prepare aqueous solution A1 by dissolving the first and second polymerization precursors in preferably desalted water with stirring, and cooling this solution between 0°C and 4°C to prevent these precursors from beginning to form a polymer gel during sol preparation (when mixed with solution A2). - The aqueous suspension A2 is prepared by dissolving the actinide salt in preferably desalted water with stirring, and adding to the resulting solution an aqueous suspension containing sacrificial beads and optionally, also preferably, a nonionic surfactant in desalted water (e.g., Triton® X-100), wherein the presence of the nonionic surfactant promotes the dispersion of the beads in the aqueous medium. - Mix aqueous solution A1 with aqueous suspension A2, and maintain the resulting mixture at a temperature between 0°C and 4°C to prevent the first and second polymerization precursors from beginning to form a polymer gel. Includes.

[0025] The first and second polymerization precursors are preferably hexamethylenetetramine or HMTA and urea, which together form a polymer gel by the decomposition of HMTA into ammonia and formaldehyde (or methanal), and the reaction of the latter with urea to form a urea-formaldehyde resin.

[0026] The actinide salt may, a priori, be an actinide salt that is soluble in water, i.e., has a solubility value greater than 1 mol / L at 20°C.

[0027] However, this is preferably an actinide nitrate, sulfate, citrate, or chloride, and the actinide may be uranium, thorium, plutonium, neptunium, or americium, depending on whether the matrix of microspheres is intended to contain uranium oxide, thorium oxide, plutonium oxide, neptunium oxide, or americium oxide.

[0028] Among these salts, nitrates, particularly uranyl nitrate, are preferred.

[0029] Sacrificial material beads typically have a diameter between 20 nm and 100 μm, but for the preparation of a batch of microspheres, it is understood that, advantageously, the sacrificial material beads all have the same diameter. The beads are preferably made from an organic polymer having the following characteristics: hydrophobic, stable, i.e., no phase transition between 0°C and 90°C, and soluble in an organic solvent that does not dissolve the polymer gel that forms the gelled microspheres in step e).

[0030] Therefore, these may be polystyrene beads soluble in dimethylformamide, poly(methyl methacrylate) beads soluble in chloroform, or polyethylene beads soluble in xylene (at high temperatures) or dimethylformamide.

[0031] Among these, polystyrene beads are preferred, and polystyrene beads with carboxyl groups functionalized on the surface, such as those available from AlphaNanotech in sizes ranging from 20 nm to 100 μm, are even more preferred.

[0032] It goes without saying that the size and number of sacrificial material beads present in the sol should be carefully selected according to the size of the microspheres to be produced and the degree of pore closure to be imparted to these microspheres. However, as a prerequisite, the "shrinkage" phenomenon of the microspheres, i.e., the decrease in size that occurs from the start of gelation to the end of firing, and consequently the decrease in the size of the pores of the microspheres during firing, must be taken into consideration, as shown in the experimental tests reported below. The degree of this shrinkage, which depends on the operating conditions carried out in the production of the microspheres, can be experimentally determined before production.

[0033] In this regard, the closed porosity of microspheres is the overall porosity, that is, defined on a batch scale of prepared microspheres, and is specified to correspond to the ratio (total volume of pores / total volume of microspheres) multiplied by 100. Therefore, the closed porosity of microspheres can be adjusted on a batch scale of microspheres by acting on the size and number of sacrificial material beads introduced into the sol.

[0034] According to the present invention, the molar concentrations of HMTA and urea in aqueous solution A1, the molar concentration of actinide salt in aqueous suspension A2, and the volumes of aqueous solution A1 and aqueous suspension A2 mixed together to form a sol are preferably - On the one hand, the molar ratio of HMTA to urea present in the sol is between 0.1 / 2 and 1, more preferably equal to 2 / 3. - On the other hand, the molar ratio of HMTA to actinide ions present in the sol is between 1 / 2 and 4 / 3, and more preferably equal to 1. It will be selected in that way.

[0035] In step b), the dispersion of the sol into a bath of an organic liquid that is immiscible with water can be carried out by any automated device capable of distributing the aqueous suspension into the liquid medium in the form of homogeneous sized droplets, without the droplets coming into contact with each other during distribution.

[0036] Therefore, this device may be an automated syringe equipped with a needle with an inner diameter of 0.2 mm to 0.8 mm, or a nanodispenser having a piezoelectric actuator, such as one available from BioFluidix under the reference name Pipejet®, when small microspheres are manufactured.

[0037] In any case, the bath of organic liquid immiscible with water is preferably a bath of silicone oil, which is advantageously heated to a temperature between 80°C and 95°C, ideally equal to 90°C ± 2°C, to obtain a gelation time (by polymerization of the first and second precursors) and to allow the shells of the microspheres to gel before reaching the bottom of the reactor (where step b is performed), the objective being to obtain microspheres that do not deform and do not fuse with each other. In any case, the temperature of the bath of organic liquid should not be higher than the boiling point of water in order to avoid the rupture of the microspheres in the bath of organic liquid.

[0038] According to the present invention, step b) advantageously includes, in addition to the dispersion of sols into droplets in a bath of organic liquid immiscible with water, maturation of microspheres resulting from the gelation of these droplets by maintaining the microspheres in this bath for 30 minutes to 2 hours.

[0039] The separation of gelled microspheres from a bath of organic liquid immiscible with water, or step c), may be carried out by any technique that allows the gelled particles to be recovered from the liquid phase without altering them, for example, by pouring the bath into a container through a sieve capable of holding the gelled microspheres.

[0040] Step d), which involves washing and drying the gelled microspheres recovered in step c), may be performed immediately after this step. However, a step of aging the gelled microspheres in the air may be included between steps c) and d), and this step may simply consist of bringing the gelled microspheres recovered in step c) into contact with air and letting them stand for several hours, for example, 6 to 24 hours, ideally 12 hours.

[0041] Step d), which involves washing and drying the gelled microspheres obtained after step c) or in either the aging step, is intended in particular to remove any residue left from these microspheres by the organic liquid from which the microspheres were separated in step c).

[0042] This process may include, for example, if the organic liquid is silicone oil, one or more operations of washing the microspheres by immersing them in a bath such as a petroleum ether bath followed by an ammonium hydroxide bath, and at least after the last washing operation, an operation of drying the gelled microspheres in an oven heated to, for example, 50°C to 60°C.

[0043] According to the present invention, the selective chemical dissolution of sacrificial material beads, or step e), preferably includes immersing the gelled microspheres obtained after step d) in a bath of organic solvent, the organic solvent entering these microspheres by osmosis and dissolving the sacrificial material beads, but not the polymer gel that forms the gelled microspheres.

[0044] As shown above, the organic solvent may be, for example, dimethylformamide if the sacrificial beads are polystyrene or polyethylene beads, xylene (high temperature) if the sacrificial beads are polyethylene beads, or chloroform if the sacrificial beads are poly(methyl methacrylate) beads.

[0045] In either case, the gelled microspheres are kept in an organic solvent bath for a sufficient amount of time to ensure that the entire sacrificial material beads are completely dissolved, and are optionally heated. This period may be several hours, for example, 24 hours.

[0046] The process involves decomposing the polymer gel that forms the microspheres, converting the actinides present in the gel in the form of hydroxides into actinide oxides, and firing the gelled microspheres, which has the function of compacting the microspheres. Alternatively, step f) preferably includes heat treatment at a temperature between 600°C and 1500°C in a reducing, neutral, or oxidizing atmosphere, depending on the type of microspheres that are desired to be obtained.

[0047] Therefore, for example, to obtain microspheres in which the matrix contains uranium dioxide (UO2), calcination is carried out under a reducing atmosphere such as an atmosphere composed of a mixture of argon and dihydrogen, for example, with an Ar / H2 volume ratio of 95 / 5. On the other hand, to obtain microspheres in which the matrix contains triuranium octoxide (U3O8), calcination is carried out under an oxidizing atmosphere such as air, followed by cooling from a specific temperature, for example below 600°C, under a neutral atmosphere, for example under argon.

[0048] According to the present invention, the firing may include two consecutive heat treatments in a reducing, neutral, or oxidizing atmosphere, namely a first treatment at a temperature between 350°C and 700°C followed by a second treatment at a temperature between 1300°C and 1600°C (each treatment accompanied by a preceding temperature increase).

[0049] Another objective of the present invention is the use of the materials defined above in the production of nuclear fuel.

[0050] In particular, an object of the present invention is the use of this material as a sintering additive, and more specifically as a porosity-forming agent, in the production of MOX nuclear fuel pellets.

[0051] In this case, the material preferably comprises microspheres whose matrix contains uranium oxide, more specifically, microspheres whose average size (i.e., mode of particle size distribution) is between 20 μm and 100 μm, and more preferably between 20 μm and 50 μm.

[0052] In this type of use, as is known, the material is advantageously introduced into a final mixture of uranium dioxide powder and plutonium dioxide powder, which is then pressed in the form of raw pellets.

[0053] Alternatively, the material can also be used to produce TRISO fuel particles for high-temperature reactors or HTRs, in which case the size of the microspheres is preferably between 400 μm and 1,000 μm. The principle of producing this type of fuel is described in particular in the monograph by the Nuclear Energy Section of the CEA, entitled "Les combustibles nucleaires" (Editions Le Moniteur, ISBN 978-2-281-11325-9), published in 2008, hereafter referred to as [5].

[0054] Other features and advantages of the present invention will become apparent from the following further description.

[0055] This further explanation is provided merely as an example of the purposes of the present invention and should not be construed in any way as a limitation of those purposes. [Brief explanation of the drawing]

[0056] [Figure 1] The images correspond to images of mechanically destroyed 1000 μm microspheres prepared from ammonium and cerium(IV) double nitrates, taken using a scanning electron microscope (SEM) in secondary electron mode at 150x magnification. The images were taken before the polystyrene beads present in these microspheres dissolved. [Figure 2] Similar to Figure 1, but corresponding to an image taken at 750x magnification, it shows the spherical crown of polystyrene microbeads (20 μm) protruding from the polymer gel. [Figure 3] The fractured microspheres shown in Figures 1 and 2 correspond to SEM images in the secondary electron mode at a magnification of 1400x after the compaction of these microspheres by chemical dissolution of polystyrene beads and firing at 1200°C. [Figure 4] The image corresponds to a SEM image in secondary electron mode at 800x magnification of a polished cross-section of a microsphere prepared from uranyl nitrate. The cross-section was performed after compaction of the microsphere by chemical dissolution of polystyrene beads and firing at 1400°C. [Modes for carrying out the invention]

[0057] The following tests were conducted to verify the following: - On the one hand, by using piezoelectric nanodispensers for sol dispersion, there is a possibility of preparing microspheres based on actinide oxides by internal gelation, and - On the other hand, internal gelation may be used to prepare microspheres based on actinide oxides with calibrated closed porosity.

[0058] I - Preparation of microspheres by sol dispersion using a piezoelectric nanodispenser: This experiment was conducted using a double salt of ammonium and cerium(IV) instead of actinide salts. Cerium has traditionally been used as a dummy material for uranium(VI) and plutonium(IV) in the development and validation of experimental protocols for these actinides, particularly for the purpose of radiation protection for experimenters.

[0059] In this study, microspheres were prepared by following the operating protocol below.

[0060] Preparation of the sol: A series of sols with the same composition were prepared by performing the following for each sol: - In the first beaker, dissolve hexamethylene-tetramine (HMTA) and urea in 500 μL of demineralized water with stirring to obtain an aqueous solution containing 3 mol / L HMTA and 3 mol / L urea, and cool the resulting solution in an ice bath for 30 minutes. - In a second beaker, dissolve the ammonium and cerium(IV) double nitrate ((NH4)2Ce(NO3)6) and ammonium hydroxide (NH4OH) in 500 μL of demineralized water with stirring to obtain an aqueous solution containing 2 mol / L (NH4)2Ce(NO3)6 and 1 mol / L NH4OH. The addition of NH4OH can raise the pH and promote polymerization, then - The two aqueous solutions thus obtained, HMTA / Ce 4+ and OH- / Ce 4+ The two components are mixed while stirring in proportions such that their molar ratios are equal to 3 / 2 and 1 / 2, respectively, and the resulting sol is cooled in an ice bath for 15 minutes.

[0061] Conversion of sols into gelled microspheres: The sol obtained above was dispersed in the form of spherical droplets in a silicone oil bath heated to 90°C using a BioFluidix Pipejet™ piezoelectric nanodispenser set to a frequency of 4 Hz, with a dispensing tube having an inner diameter in the range of 125 μm to 500 μm and capable of delivering droplets in the range of 6 nL to 63 nL.

[0062] The gelled microspheres obtained by gelling the sol upon contact with heated silicone oil were left in the oil for 2 hours to mature, then collected and aged for 12 hours. They were then washed three times for 1 hour each with 20 mL of petroleum ether, followed by three washes of 15 minutes each with 10 mL of 0.5 M aqueous solution of NH4OH. After the final wash, they were dried in a 60°C oven.

[0063] Compaction of gelled microspheres by firing: For compaction, gelled microspheres are used. - Heating from 40°C to 1400°C at a rate of 5°C / min. - Plateau at 1400℃ for 2 hours, and - Cooling from 1400℃ to 40℃ It was subjected to a heat treatment at 1400°C in air accompanied by [unclear].

[0064] Images of microspheres were captured using a scanning electron microscope (SEM) to determine the size of droplets corresponding to the volume injected into the silicone oil bath at three different stages of preparation: after maturation in the oil, after the final washing with NH4OH, and after calcination.

[0065] The results of these analyses, performed on 50 microspheres per injection volume, are shown in the table below.

[0066] [Table 1]

[0067] This table shows that, regardless of the size of the sol droplets dispersed in the silicone oil bath, the size of the microspheres resulting from the gelation of these droplets decreases rapidly during maturation, washing, and firing in this bath. This phenomenon, more simply called "shrinkage," must be taken into consideration when producing microspheres of the desired diameter.

[0068] Furthermore, it is shown that by dispersing a sol in the form of droplets with a volume of less than 6 nL in a heated silicone oil bath, which is possible with the piezoelectric nanodispenser used in this test, it is possible to obtain microspheres with an average size (i.e., mode of particle size distribution) between 20 μm and 50 μm after firing, which is a particularly preferred average size range for microspheres intended to be used as pore-forming agents in the production of MOX nuclear fuel.

[0069] II - Preparation of microspheres with calibrated closed porosity: Two tests were performed to verify the possibility of creating calibrated closed porosity in microspheres prepared by internal gelation: - The first method uses a double salt of ammonium and cerium(IV) for the same reasons specified in point I above. - The second method uses uranium(VI) salt.

[0070] II.1 - Tests using double salts of ammonium and cerium(IV): In this study, microspheres were prepared by following the operating protocol below.

[0071] Preparation of the sol: In the first beaker, hexamethylenetetramine and urea were dissolved in 500 μL of demineralized water with stirring to obtain an aqueous solution containing 3.18 mol / L HMTA and 3.18 mol / L urea. The solution thus obtained was cooled in an ice bath for 30 minutes.

[0072] In a second beaker, ammonium and cerium(IV) double nitrate and ammonium hydroxide were dissolved in 500 μL of demineralized water with stirring to obtain aqueous solutions containing 1.67 mol / L (NH4)2Ce(NO3)6 and 0.8 mol / L NH4OH, respectively. To these solutions, 500 μL of aqueous suspension containing 20 mg / mL of carboxylated polystyrene beads (AlphaNanotech) with a diameter of 20 μm was added.

[0073] The contents of the two beakers were mixed, and the resulting sol was cooled in an ice bath for 15 minutes.

[0074] Conversion of sols into gelled microspheres: The sol obtained above was dispersed in droplet form in a silicone oil bath heated to 90°C using a syringe equipped with a needle with an inner diameter of 0.8 mm.

[0075] Similar to point I above, the microspheres were left in this oil for 2 hours to mature, then collected and aged for 12 hours. They were then washed three times for 1 hour each with 20 mL of petroleum ether, followed by three washes of 15 minutes each with 10 mL of 0.5 M aqueous solution of NH4OH. After the final wash, they were dried in a 60°C oven.

[0076] Chemical dissolution of polystyrene beads: To dissolve the polystyrene beads, the microspheres were immersed in 10 mL of N,N-dimethylformamide (DMF), left in the solvent for 24 hours, then collected and dried in a 60°C oven.

[0077] Compaction of microspheres by firing: For compaction, microspheres are used. - Heating from 40°C to 1200°C at a rate of 1°C / min. - Plateau at 1200°C for 2 hours, and - Cooling from 1200℃ to 40℃ It was subjected to a heat treatment at 1200°C in air accompanied by [unclear].

[0078] SEM images of the cross-sections of the resulting microspheres were taken before the polystyrene beads were dissolved by DMF (Figures 1 and 2), and after the microspheres were compacted by the dissolution and calcination of the polystyrene beads by DMF (Figure 3).

[0079] In Figures 1 and 2, the polystyrene beads are clearly visible, whereas in Figure 3, the beads are no longer visible, but one of the pores, indicated by arrow f1, which remained due to the chemical dissolution of the polystyrene beads, is visible.

[0080] In addition to confirming that simply immersing gelled microspheres in a suitable solvent can chemically dissolve the sacrificial material beads present within the microspheres, thereby creating a calibrated closed porosity within these microspheres, Figure 3 shows that this porosity can be maintained by compacting the gelled microspheres, for example, by firing at 1200°C.

[0081] II.2 - Tests using uranium(VI) salts: In this study, microspheres were prepared by following the operating protocol below.

[0082] Preparation of the sol: In the first beaker, hexamethylenetetramine and urea were dissolved in 500 μL of demineralized water with stirring to obtain an aqueous solution containing 2 mol / L HMTA and 3 mol / L urea. The solution thus obtained was cooled in an ice bath for 30 minutes.

[0083] In a second beaker, uranyl nitrate (UO2(NO3)2) was dissolved in 390 μL of desalted water with stirring to obtain an aqueous solution containing 2 mol / L of UO2(NO3)2. To this aqueous solution, 100 μL of an aqueous suspension containing 50 mg / mL of carboxylated polystyrene beads (AlphaNanotech) with a diameter of 30 μm, and 1 μL of the surfactant Triton® X-100 were added.

[0084] The contents of the two beakers were mixed, and the resulting sol was cooled in an ice bath for 15 minutes.

[0085] Conversion of sols into gelled microspheres: The sol obtained above was dispersed in the form of 50 nL droplets in a silicone oil bath heated to 90°C using the piezoelectric nanodispenser used at point I above, with the frequency set to 5 Hz.

[0086] The microspheres were left in this oil for 2 hours to mature, then successively collected on a sieve, left on the sieve in the air for 12 hours to age, rinsed with demineralized water in the sieve, washed three times for 1 hour each with 20 mL of petroleum ether, dried in a fume hood, washed three times for 15 minutes each with 10 mL of 0.5 M aqueous solution of NH4OH, and after the final wash, dried in a 50°C oven.

[0087] Chemical dissolution of polystyrene beads: To dissolve the polystyrene beads, the microspheres were immersed in 10 mL of DMF, left in the solvent for one day, then collected and dried in a 50°C oven.

[0088] Compaction of microspheres by firing: To compact the microspheres, - The first temperature increase from 40°C to 350°C takes 1 hour and 20 minutes. - First plateau at 350℃ for 30 minutes, - Second heating cycle from 350°C to 1400°C in 5 hours. - A second plateau at 1400°C for 2 hours, and - Cooling from 1400℃ to 40℃ It was subjected to heat treatment at 1400°C under an Ar / H2 (95 / 5, v / v) mixture.

[0089] The resulting microspheres were then subjected to analysis by infrared spectroscopy and scanning electron microscopy (SEM).

[0090] In the analysis by IR spectroscopy, it was confirmed that the urea-formaldehyde resin forming a three-dimensional network at the gel stage was completely decomposed during firing, and the densified microspheres now contain only uranium dioxide as the matrix.

[0091] The SEM image of a polished cross-section of one of the microspheres is shown in Fig. 4. As can be seen visually in this figure, this microsphere is completely spherical, with a diameter of about 100 μm, and the size of the closed pores is about 18 μm. The closed pores visible in Fig. 4 are also completely spherical.

[0092] III - Determination of the volume of the aqueous suspension of beads of the sacrificial material introduced into the sol to obtain the desired closed porosity: For the preparation of a batch of N microspheres having a selected average size, it is possible to determine the volume of the aqueous suspension of beads of the sacrificial material that must be introduced into the sol to obtain the desired closed porosity, at the batch scale.

[0093] Thus, for example, while using an aqueous suspension containing 50 mg / mL of polystyrene beads with a diameter of 30 μm and a density (hereinafter denoted as d ps equal to 1.05 g / cm 3 ), a batch of 20,000 UO2-based microspheres with an average size of 100 μm is prepared, and when aiming for a closed porosity of 5%, 1) The volume of the microspheres denoted as V 微小球 is equal to the following:

[0094]

Equation

[0095] 2) The total volume of the microspheres denoted as V 合計 is equal to the following: V 合計 = 20,000 × (5.23 × 10 -7 ) = 0.0104 cm 3 3) At the batch scale, V 気孔率The volume of the closed porosity of the microspheres shown is equal to the following:

[0096]

number

[0097] 4) Introduced into the sol, m psビーズ The total mass of the polystyrene beads indicated is equal to the following: m psビーズ =V 気孔率 ×d ps =0.0005×1.05=0.000525g=0.525mg.

[0098] Therefore, considering that an aqueous suspension of polystyrene beads contains 50 mg of beads per 1 mL, the volume of this suspension introduced into the sol is equal to:

[0099]

number

[0100] (References) [1] “Le traitement-recyclage du combustible nucleaire use - La separation des actinides - Application a la gestion des dechets”, 2008, Editions Le Moniteur, ISBN 978-2-281-11376-1 [2] Patent US 4 218 430 [3] G. Colak et al., Journal of Nuclear Materials 2022, 562, 153587 [4] G. Colak et al., Journal of Nuclear Materials 2023, 577(1), 154319 [5] “Nuclear Fuels”, 2008, Editions Le Moniteur, ISBN 978-2-281-11325-9

Claims

1. A material containing porous microspheres, - Each porous microsphere comprises a matrix containing actinide oxide and one or more closed spherical pores dispersed within the matrix. - The microspheres have an average size between 20 μm and 1,000 μm. - The size of the stomata is calibrated, a) A step of preparing a sol by mixing an aqueous solution A1 containing first and second polymerization precursors that can together form a polymer gel with an aqueous suspension A2 containing an actinide salt, optionally a nonionic surfactant, and sacrificial material beads. b) A step of dispersing the sol in the form of droplets in a bath of an organic liquid that is immiscible with water, thereby converting the sol into gelled microspheres, and thereby obtaining microspheres formed by a polymer gel surrounding sacrificial material beads. c) A step of separating the gelled microspheres from the organic liquid bath, d) A step of washing and drying the gelled microspheres. e) A step of selectively chemically dissolving the sacrificial material beads present in the gelled microspheres, and f) A process of firing gelled microspheres, Obtained by a method including, In step b), the size of the droplets is controlled by an automated dispensing system for the material.

2. Step a) is, - Prepare aqueous solution A1 by dissolving the first and second polymerization precursors in water while stirring, and maintaining this solution at a temperature between 0°C and 4°C. - Prepare aqueous suspension A2 by dissolving the actinide salt in water while stirring, and adding to the resulting solution an aqueous suspension containing sacrificial beads and optionally a nonionic surfactant in water, and - Mix aqueous solution A1 with aqueous suspension A2, and maintain the resulting mixture at a temperature between 0°C and 4°C. The material according to claim 1, including the material described in claim 1.

3. The material according to claim 1 or claim 2, wherein the first and second polymerization precursors are hexamethylenetetramine and urea.

4. The material according to any one of claims 1 to 3, wherein the actinide salt is a nitrate, sulfate, citrate, or chloride of uranium, thorium, plutonium, neptunium, or americium, preferably a nitrate, and more preferably uranium nitrate.

5. The material according to any one of claims 1 to 4, wherein the sacrificial material beads are made from a hydrophobic organic polymer, the hydrophobic organic polymer does not undergo a phase transition between 0°C and 90°C, and the organic solvent can dissolve the polymer gel forming gelled microspheres in step b) without dissolving it.

6. The material according to claim 5, wherein the organic polymer is polystyrene, poly(methyl methacrylate), or polyethylene, preferably polystyrene.

7. - The molar ratio of hexamethylenetetramine to urea present in the sol is between 1 / 2 and 1, preferably equal to 2 / 3. - The material according to claim 3, wherein the molar ratio of hexamethylenetetramine to actinide ions present in the sol is between 1 / 2 and 4 / 3, preferably equal to 1.

8. The material according to any one of claims 1 to 7, wherein the sol is dispersed in the form of droplets in a bath of organic liquid by a piezoelectric nanodispenser.

9. The material according to any one of claims 1 to 8, wherein the bath of organic liquid immiscible with water is a bath of silicone oil, and the temperature of the silicone oil bath is between 80°C and 95°C, preferably equal to 90°C ± 2°C.

10. The material according to any one of claims 1 to 9, wherein step b) comprises aging the gelled microspheres in a bath of organic liquid for 30 minutes to 2 hours.

11. The material according to any one of claims 1 to 10, wherein step e) comprises immersing the gelled microspheres obtained after step d) in a bath of an organic solvent capable of dissolving the sacrificial material beads without dissolving the polymer gel forming the gelled microspheres.

12. The material according to claim 11, wherein the organic solvent is dimethylformamide, xylene, or chloroform.

13. The material according to any one of claims 1 to 12, wherein step f) includes a heat treatment at a temperature between 600°C and 1500°C in a reducing, neutral, or oxidizing atmosphere.

14. The material according to any one of claims 1 to 12, wherein step f) comprises a first heat treatment at a temperature between 350°C and 700°C, followed by a second heat treatment at a temperature between 1300°C and 1600°C, each treatment being carried out in a reducing, neutral, or oxidizing atmosphere.

15. The material according to any one of claims 1 to 14, wherein the actinide oxide of the microsphere matrix is ​​uranium dioxide or triuranium octoxide.

16. Use of the material according to any one of claims 1 to 15 as a sintering additive in the manufacture of MOX-type nuclear fuel pellets.

17. The use according to claim 16, wherein the material is used as a porosity-forming agent.

18. The use according to claim 16 or claim 17, wherein the material comprises microspheres, and the matrix of the microspheres comprises uranium oxide, preferably uranium dioxide.

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