WO2024256781A1 - Material comprising porous microspheres on an actinide oxide basis, and uses thereof in the manufacture of nuclear fuels - Google Patents

Abstract

The invention relates to a material comprising porous microspheres on an actinide oxide basis, which is obtained by an internal gelation method. It also relates to the uses of this material in the manufacture of nuclear fuels, in particular as a sintering additive in the manufacture of pellets of an MOX (mixed oxide) type nuclear fuel and, more specifically, as a pore-forming agent replacing the pore-forming agent conventionally used in said manufacture, namely azodicarbonamide.

Description

MATERIAL COMPRISING POROUS MICROSPHERES BASED ON AN ACTINIDE OXIDE AND ITS USES IN THE MANUFACTURE OF NUCLEAR FUELS

DESCRIPTION

TECHNICAL AREA

The invention relates to the field of the nuclear fuel cycle.

More specifically, the invention relates to a material comprising porous microspheres based on an actinide oxide which is obtained by an internal gelation process.

It also relates to the uses of this material in the manufacture of nuclear fuels, in particular as a sintering additive in the manufacture of pellets of a MOX (Mixed OXide Fuel) type nuclear fuel and, more specifically, as a pore-forming agent replacing that conventionally used in this manufacture, namely azodicarbonamide.

STATE OF THE ART

Currently, the process implemented in France for the manufacture of MOX nuclear fuel, which is used in ordinary water reactors, is the original process called "MIMAS" (Mlcronized MASter Blend), which has been the subject of developments in its implementation in the French Melox plant and is now called "MELOX Process", which schematically consists of:

- produce a primary mixture of powders with a plutonium content higher than the plutonium content required for the fuel, by co-grinding a UO2 powder, a PuO2 powder and, optionally, chamotte until a micronized and intimately mixed powder is obtained;

- produce a final mixture of powders having the plutonium content required for the fuel;

- press the final mixture into the form of so-called “raw” pellets;

- sintering the raw pellets at high temperature; and - rectify the sintered pellets to adjust their dimensions to specifications.

The sintered pellets deemed to be compliant after rectification are then introduced into metal sheaths to form the fuel rods which are themselves assembled into bundles to form the fuel assemblies, while the pellets deemed to be non-compliant are used to manufacture the chamotte.

All the operations of the MOX fuel manufacturing process are described in detail in the monograph of the CEA Nuclear Energy Directorate entitled "The treatment-recycling of spent nuclear fuel - The separation of actinides - Application to waste management" and published in 2008 (Éditions Le Moniteur, ISBN 978-2-281-11376-1), hereinafter reference [1],

In the final powder mixture, it is usual to introduce sintering additives and, in particular, a porogenic agent whose function is to create a certain porosity in the pellets during their sintering.

Currently, the reference pore-forming agent is an organic compound, namely azodicarbonamide with the formula: NH2-C(O)-N=N-C(O)-NH2, also known by the acronyms AZB or ADCA.

In the context of the prospects for implementing the process for future production of MOX fuels from nuclear materials, for example multi-recycled, likely to cause radiolysis of azodicarbonamide, in particular induced by the presence of a higher proportion of plutonium 238 and americium 241 isotopes, the decomposition of azodicarbonamide then leading to swelling of the raw pellets which would be prohibitive for their sintering. This phenomenon would be observed during storage of the raw pellets awaiting sintering.

As a result, the use of azodicarbonamide as a pore-forming agent in the manufacture of MOX fuel would generate a significant quantity of raw pellets which would be discarded.

It would therefore be desirable to have a porogenic agent which, while making it possible to create, in MOX fuel pellets during their sintering, a porosity at least as satisfactory as that produced by azodicarbonamide, does not present the disadvantages of the latter for the prospects of future manufacturing. The inventors therefore set themselves the goal of providing such a pore-forming agent and, to do this, of managing to manufacture porous microspheres that are resistant to radiolysis and thermolysis and that are compatible with nuclear fuel manufacturing processes and, in particular, with the MIMAS process, in particular in terms of composition, size and resistance to the crushing and shearing effects induced during the mixing operations that are carried out in the context of implementing these processes. In this context, they chose to manufacture microspheres with a size of between 20 LIIYI and 1,000 pm, comprising a matrix made of an actinide oxide and closed pores, of calibrated size, dispersed in this matrix.

From a completely different perspective than that of proposing a sintering additive and, more particularly, a pore-forming agent for the manufacture of MOX fuel, it has been proposed in the state of the art to manufacture solid or porous microspheres of uranium dioxide, UO2, or triuranium octoxide, U3O8, by internal gelation.

Schematically, internal gelation consists of preparing a solution called "sol", comprising a uranyl salt and polymerization precursors, dispersing the sol in the form of droplets in a heated liquid, immiscible with the sol, to obtain spherification by internal gelation of the droplets, in particular by the formation of a three-dimensional polymer network within which the uranyl ions are complexed. After various aging, washing and drying operations leading to the formation of uranyl hydroxides. The gelled microspheres obtained are subjected to a consolidation heat treatment during which the three-dimensional polymer network is eliminated and the uranyl hydroxides are transformed into UO2 if the heat treatment is carried out in a reducing atmosphere or into U3O8 if the heat treatment is carried out in an oxidizing atmosphere.

To obtain porous microspheres, two solutions have been proposed.

The first solution, which is described in US patent 4,218,430, hereinafter reference [2], consists in impregnating the microspheres in the gelled state with an organic compound, such as polyethylene glycol, glycerol or mannitol, which is subsequently eliminated by decomposition into volatile compounds during the heat treatment for consolidating the microspheres. Microspheres with open porosity are thus obtained, that is to say, whose pores communicate with each other via channels allowing the circulation of fluids.

The second solution, which is described by G. Colak et al. in Journal of Nuclear Materials 2022, 562, 153587 and in Journal of Nuclear Materials 2023, 577(1), 154319, respectively hereinafter references [3] and [4], consists in incorporating into the sol, before it is dispersed in the heated liquid medium, starch or graphite particles which are, there too, subsequently eliminated by decomposition into volatile compounds during the heat treatment of consolidation of the microspheres. It is shown in these references that the use of starch particles (more effective in creating porosity than that of graphite particles) leads to microspheres whose porosity decreases with the increase in the consolidation temperature and which is essentially a porosity accessible to the intrusion of an aqueous solution and, therefore, an open porosity.

Thus, none of the solutions proposed in references [1] to [3] make it possible to obtain UO? and/or UsOs microspheres with calibrated closed porosity.

STATEMENT OF THE INVENTION

The invention aims precisely to fill the shortcomings of the state of the art by proposing, firstly, a material which comprises porous microspheres, in which:

- each porous microsphere comprises a matrix comprising an actinide oxide and one or more closed pores dispersed in the matrix, the microspheres have a size of between 20 LIIYI and 1,000 p.m, and

- the size of the pore(s) is calibrated; and which is obtained by a process comprising the steps of: a) preparing an aqueous suspension, or sol, by mixing an aqueous solution Al comprising a first and a second polymerization precursor, capable of forming together a polymer gel, and an aqueous suspension A2 comprising an actinide salt and beads of a sacrificial material; b) converting the sol into gelled microspheres by dispersing the sol in the form of droplets in a bath of an organic liquid immiscible with water, by means of whereby microspheres are obtained which are formed of a polymeric gel containing beads of sacrificial material; c) separating the gelled microspheres from the bath of organic liquid; d) washing and then drying the gelled microspheres; e) selective chemical dissolution of the beads of sacrificial material present in the gelled microspheres; and f) calcining the gelled microspheres, and in which, in step b), the size of the droplets is controlled by an automated dispensing system.

In the foregoing and the following, the term "size", applied to microspheres, is understood as corresponding to their diameter in the case where the microspheres are perfect spheres or to the diameter of circles which would have the same surface area as them (or equivalent diameter) in the case where the microspheres are not perfect spheres. As known per se, the size of the microspheres can be determined by light diffraction particle size analysis, for example by means of a laser particle size analyzer such as that marketed under the reference Mastersizer™ 3000 by Malvern Panalytical, or by morphogranulometry, for example by means of the morphogranulometer marketed under the reference Morphologi™ G3, also by Malvern Panalytical.

Moreover :

- “closed pore(s)” means a pore(s) which does not communicate with the surface of the microspheres in which it is/are located and, when there are two or more of them in the same microsphere, which do not communicate with each other, and

- the size of the pore(s) is considered to be calibrated by the diameter of the spherical imprint left in the microspheres by the ball or each of the balls of sacrificial material after selective dissolution of these balls (step e)) then calcination of the microspheres (step f)). The imprint of the sacrificial balls, which constitutes the closed porosity of the microspheres, retains its spherical shape during calcination but undergoes a partial reduction in its size depending on the temperature at which this calcination is carried out and its duration. According to the invention, the soil preparation step, or step a), advantageously comprises:

- the preparation of the aqueous solution Al by dissolving, with stirring, the first and second polymerization precursors in water, preferably demineralized, and cooling this solution between 0°C and 4°C so as to prevent these precursors from starting to form a polymer gel during the preparation of the sol (when mixing with solution A2);

- the preparation of the aqueous suspension A2 by dissolving, with stirring, the actinide salt in water, preferably demineralized, then the addition to the resulting solution of an aqueous suspension comprising the beads of sacrificial material and, optionally, a non-ionic surfactant (such as Triton™ X-100) also in preferably demineralized water, the presence of the non-ionic surfactant makes it possible to facilitate the dispersion of the beads in an aqueous medium; and

- mixing the aqueous solution Al with the aqueous suspension A2 and maintaining the resulting mixture at a temperature between 0°C and 4°C so as to prevent, here too, the first and second polymerization precursors from starting to form a polymer gel.

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

The actinide salt can be, a priori, any actinide salt soluble in water, that is to say having a solubility value greater than 1 mol/L at 20 °C.

However, it is preferably a nitrate, a sulfate, a citrate or an actinide chloride, the actinide being able to be uranium, thorium, plutonium, neptunium or americium depending on whether the matrix of the microspheres is intended to comprise a uranium oxide, a thorium oxide, a plutonium oxide, a neptunium oxide or an americium oxide.

Among these salts, preference is given to nitrates and, in particular, to uranyl nitrate. The beads of sacrificial material typically have a diameter of between 20 nm and 100 pm, it being understood that, for the preparation of a batch of microspheres, all the beads of sacrificial material advantageously have the same diameter. The beads are preferably made of an organic polymer having the following characteristics: being hydrophobic, being stable, i.e. not exhibiting a phase transition between 0 °C and 90 °C, and being able to be dissolved in step e) by an organic solvent which does not dissolve the polymer gel forming the gelled microspheres.

Thus, it may in particular be polystyrene beads which can be dissolved by dimethylformamide, poly(methyl meth)acrylate beads which can be dissolved by chloroform or even polyethylene beads which can be dissolved by xylene (hot) or by dimethylformamide.

Among these, preference is given to polystyrene beads and, even more so, to polystyrene beads whose surface has been functionalized by carboxyl groups such as those available, in sizes ranging from 20 nm to 100 pm, from the company AlphaNanotech.

It goes without saying that the size and the number of balls of sacrificial material present in the sol are judiciously chosen according to the size of the microspheres that one wishes to manufacture and the closed porosity rate that one wishes to confer on these microspheres, knowing that, as shown in the experimental tests reported below, a phenomenon of "shrinkage" of the microspheres must be taken into account, that is to say a reduction in their size, which occurs between the start of their gelling and the end of their calcination with, as a corollary, a reduction in the size of the pores during the calcination of the microspheres. The importance of this shrinkage, which is a function of the operating conditions implemented in the manufacture of the microspheres, can be determined experimentally prior to this manufacture.

In this regard, it is specified that the closed porosity rate of the microspheres is an overall porosity rate, i.e. defined on the scale of a batch of prepared microspheres, and corresponds to the ratio (total volume of pores/total volume of microspheres) multiplied by 100. The closed porosity rate of the microspheres can therefore be adjusted on the scale of a batch of microspheres by playing on the size and number of balls of sacrificial material introduced into the soil.

According to the invention, the molar concentrations of HMTA and urea in aqueous solution Al, the molar concentration of actinide salt in aqueous suspension A2 as well as the volumes of aqueous solution Al and aqueous suspension A2 which are mixed together to form the sol are preferably chosen so that:

- on the one hand, the molar ratio between HMTA and urea present in the soil is between 0.1/2 and 1 and, better still, is equal to 2/3;

- on the other hand, the molar ratio between HMTA and the actinide ions present in the soil is between 1/2 and 4/3 and, better still, is equal to 1.

In step b), the dispersion of the sol in the bath of water-immiscible organic liquid may be carried out by means of any automated device making it possible to distribute in a liquid medium an aqueous suspension in the form of droplets of homogeneous size and without the droplets coming, at the time of their distribution, into contact with each other.

Thus, this device can in particular be an automatic syringe equipped with a needle measuring from 0.2 mm to 0.8 mm in internal diameter or, when it comes to manufacturing small microspheres, a nano-injector with a piezoelectric actuator such as that available under the reference Pipejet™ from the company BioFluidix.

In any event, the water-immiscible organic liquid bath is preferably a silicone oil bath, which is advantageously heated to a temperature of between 80°C and 95°C and, ideally, equal to 90°C ± 2°C so as to obtain gelation times (by polymerization of the first and second precursors) allowing gelation of the shell of the microspheres before their arrival at the bottom of the reactor in which step b) is carried out, the aim being to obtain non-deformed microspheres which do not fuse with each other. In any event, the temperature of the organic liquid bath must not be higher than the boiling point of water to avoid the microspheres bursting in the organic liquid bath.

According to the invention, step b) advantageously comprises, in addition to the dispersion of the sol in droplets in the bath of organic liquid immiscible with water, a ripening of the microspheres resulting from the gelling of these droplets by maintaining the microspheres in this bath for 30 minutes to 2 hours.

The separation of the gelled microspheres from the bath of water-immiscible organic liquid, or step c), can be carried out by any technique making it possible to recover gelled particles from a liquid phase without altering them, such as, for example, transferring this bath into a container through a sieve making it possible to retain the gelled microspheres.

Step d) of washing and drying the gelled microspheres recovered in step c) can be carried out directly after this step. However, it is also possible to provide, between steps c) and d), a step of aging the gelled microspheres in the open air, which may consist of simply leaving the gelled microspheres recovered in step c) to rest in contact with the air for several hours, for example from 6 hours to 24 hours and, ideally, for 12 hours.

Step d) of washing and drying the gelled microspheres obtained either at the end of step c) or at the aging step is intended in particular to eliminate from these microspheres the residues left by the organic liquid from which they were separated in step c).

This step may comprise one or more operations of washing the microspheres, for example by soaking these microspheres in baths such as petroleum ether baths followed by ammonium hydroxide baths if the organic liquid is silicone oil, with at least the last washing operation being followed by an operation of drying the gelled microspheres, for example in an oven heated to 50°C-60°C.

According to the invention, the selective chemical dissolution of the beads of sacrificial material, or step e), preferably comprises soaking the gelled microspheres obtained at the end of step d) in a bath of an organic solvent which will infiltrate by permeation into these microspheres and will be capable of dissolving the beads of sacrificial material without dissolving the polymer gel forming the gelled microspheres. As previously indicated, the organic solvent may, for example, be dimethylformamide if the sacrificial material beads are polystyrene or polyethylene beads, (hot) xylene if the sacrificial material beads are polyethylene beads or chloroform if the sacrificial material beads are poly(methyl meth)acrylate beads.

In any event, the gelled microspheres are kept in the bath of organic solvent, optionally heated, for a sufficient time to ensure that all of the beads of sacrificial material are dissolved in their entirety. This time may be several hours, for example 24 hours.

The calcination of the gelled microspheres, or step f), which has the function of decomposing the polymeric gel forming the microspheres, of converting the actinides, which are present in this gel in the form of hydroxides, into an actinide oxide and of consolidating the microspheres, preferably comprises a heat treatment at a temperature between 600°C and 1,500°C, and this, under a reducing, neutral or oxidizing atmosphere depending on the type of microspheres that it is desired to obtain.

Thus, for example, to obtain microspheres whose matrix comprises uranium dioxide, UO2, the calcination is carried out under a reducing atmosphere such as an atmosphere composed of a mixture of argon and dihydrogen, for example in an Ar/H? volume ratio of 95/5, while, to obtain microspheres whose matrix comprises triuranium octoxide, U3O8, the calcination is carried out under an oxidizing atmosphere such as air, followed by cooling under a neutral atmosphere, for example argon, from a certain temperature, for example below 600 °C.

In accordance with the invention, the calcination may also comprise two successive heat treatments, 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 (with, for each treatment, a prior temperature increase), and this, under a reducing, neutral or oxidizing atmosphere.

The invention also relates to the use of a material as previously defined in the manufacture of nuclear fuels. In particular, the invention relates to the use of this material as a sintering additive in the manufacture of MOX nuclear fuel pellets and, more specifically, as a pore-forming agent.

In which case, the material preferably comprises microspheres whose matrix comprises a uranium oxide, with particular preference being given to microspheres whose matrix comprises uranium dioxide and, more specifically, to microspheres whose average size (i.e. the mode of the particle size distribution) is between 20 LIIYI and 100 LIIYI and, even better, between 20 LIIYI and 50 pm.

In this type of use and as known per se, the material is advantageously introduced into the final mixture of uranium dioxide and plutonium dioxide powders before this mixture is pressed into the form of raw pellets.

Alternatively, the material may also be used for the manufacture of particles of a TRISO fuel for a high-temperature nuclear reactor, or HTR, in which case the microspheres preferably have a size of between 400 pm and 1,000 pm. The principles of the manufacture of this type of fuel are described in particular in the monograph of the CEA Nuclear Energy Directorate entitled "Nuclear Fuels" and published in 2008 (Éditions Le Moniteur, ISBN 978-2-281-11325-9), hereinafter reference [5].

Other features and advantages of the invention will emerge from the additional description which follows.

It goes without saying that this additional description is given only as an illustration of the subject of the invention and must in no case be interpreted as a limitation of this subject.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 corresponds to an image taken with a scanning electron microscope (SEM), in secondary electron mode at a magnification of xl50, of a mechanically broken 1000 pm microsphere, prepared from double ammonium nitrate and cerium(IV), the image having been taken before dissolution of the polystyrene beads present in this microsphere. Figure 2 corresponds to an image similar to Figure 1 but taken at a magnification of x750, showing the spherical caps of the polystyrene microbeads (20 pm) protruding from the polymer gel.

Figure 3 corresponds to a SEM image, in secondary electron mode at a magnification of xl 400, of a fractured microsphere as shown in Figures 1 and 2 but after chemical dissolution of the polystyrene beads and consolidation of this microsphere by calcination at 1200°C.

Figure 4 corresponds to a SEM image, in secondary electron mode at a magnification of x800, of a polished section of a microsphere prepared from uranyl nitrate, the section having been carried out after chemical dissolution of the polystyrene beads and consolidation of this microsphere by calcination at 1400°C.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The tests reported below were carried out in order to verify:

- on the one hand, the possibility of preparing, by internal gelation, microspheres based on an actinide oxide using a piezoelectric nano-injector for the dispersion of the sol, and

- on the other hand, the possibility of preparing, by internal gelation, microspheres based on an actinide oxide which have a calibrated closed porosity.

I - Preparation of microspheres by dispersion of the sol using a nano-injector

This test was carried out using a double salt of ammonium and cerium(IV) instead of an actinide salt, cerium being conventionally used as a simulant for uranium(VI) and plutonium(IV), in particular for reasons of radiation protection of experimenters, in the work of developing and validating experimental protocols relating to these actinides.

In this test, microspheres were prepared following the following operating protocol. Soil preparation:

A series of soils of identical composition was prepared by proceeding for each soil to:

- dissolving in a first beaker, with stirring, hexamethylenetetramine (HMTA) and urea in 500 LIL of demineralized water to obtain an aqueous solution comprising 3 mol/L of HMTA and 3 mol/L of urea and cooling the solution thus obtained in an ice bath for 30 minutes;

- dissolving in a second beaker, with stirring, double ammonium nitrate and cerium(IV), ((NH4)2Ce(NO3)e), and ammonium hydroxide, NH4OH, in 500 LIL of demineralized water to obtain an aqueous solution comprising 2 mol/L of (NH4)2Ce(NO3)e and 1 mol/L of NH4OH; the addition of NH4OH increases the pH and promotes polymerization; then

- mixing, with stirring, the two aqueous solutions thus obtained in proportions such that the molar ratios HMTA/Ce ⁴⁺ and OH/Ce ⁴⁺ are respectively equal to 3/2 and 1/2, and cooling the resulting sol in an ice bath for 15 minutes.

Conversion of soils into gelled microspheres:

The sols obtained above were dispersed in the form of spherical droplets in a silicone oil bath heated to 90 °C using the BioFluidix Pipejet™ piezoelectric nano-injector, whose frequency was set at 4 Hz, using delivery pipes with internal diameters ranging from 125 LIIYI to 500 LIIYI and capable of delivering droplets with volumes ranging from 6 nL to 63 nL.

The gelled microspheres resulting from the gelation of the sols in contact with heated silicone oil were allowed to mature for 2 hours in this oil, after which they were recovered and aged for 12 hours and then subjected to 3 washes of one hour each with 20 mL of petroleum ether, then to 3 washes of 15 minutes each with 10 mL of an aqueous solution of 0.5 M NH4OH, the last of these washes being followed by drying in an oven at 60 °C. Consolidation of gelled microspheres by calcination:

For their consolidation, the gelled microspheres were subjected to a heat treatment at 1,400°C in air with:

- a temperature rise from 40°C to 1,400°C at a rate of 5°C/min,

- a 2-hour stage at 1,400°C, and

- a drop in temperature from 1,400 C to 40 °C.

Images of the microspheres were taken by scanning electron microscopy (SEM) to determine their size as a function of the volume of the droplets injected into the silicone oil bath, at three different stages of their preparation, namely: after their maturation in this oil, after their last washing with NH4OH and after their calcination.

The results of these analyses, which were carried out on 50 microspheres for each volume injected, are presented in the table below.

This table shows that, whatever the size of the sol droplets when dispersed in the silicone oil bath, the size of the microspheres resulting from the gelation of these droplets decreases drastically during their maturation in this bath, their washing and their calcination. This phenomenon, more simply called "shrinkage", must be taken into consideration when manufacturing microspheres of a desired diameter. It further shows that by dispersing, in a heated silicone oil bath, a sol in the form of droplets with a volume of less than 6 nL - which is possible with the piezoelectric nano-injector used in the present test - it is possible to obtain microspheres whose average size (i.e. the mode of the particle size distribution) after calcination is between 20 LIIYI and 50 pm, i.e. in the range of average sizes which is particularly preferred for microspheres which are intended to be used as a pore-forming agent in the manufacture of a MOX nuclear fuel.

II - Preparation of microspheres with calibrated closed porosity:

In order to verify the possibility of creating a calibrated closed porosity in microspheres prepared by internal gelation, two tests were carried out:

- the first from a double salt of ammonium and cerium(IV) for the same reasons as those specified in point I above, and

- the second from a uranium(VI) salt.

11.1 - Test using the double salt of ammonium and cerium(IV):

In this test, microspheres were prepared following the following operating protocol.

Soil preparation:

In a first beaker, hexamethylenetetramine and urea were dissolved, with stirring, in 500 LIL of demineralized water to obtain an aqueous solution comprising 3.18 mol/L of HMTA and 3.18 mol/L of urea and the solution thus obtained was cooled in an ice bath for 30 minutes.

In a second beaker, ammonium cerium(IV) nitrate and ammonium hydroxide were dissolved, with stirring, in 500 LIL of demineralized water to obtain an aqueous solution comprising 1.67 mol/L of (NH4)2Ce(NO3)e and 0.8 mol/L of NH4OH to which were added 500 LIL of an aqueous suspension comprising 20 mg/mL of carboxylated polystyrene beads of 20 LIIYI diameter each (AlphaNanotech).

The contents of the two beakers were mixed together and the resulting sol was cooled in an ice bath for 15 minutes. Conversion of soil into gelled microspheres:

The sol obtained above was dispersed in the form of droplets in a silicone oil bath heated to 90 °C using a syringe equipped with a needle with an internal diameter of 0.8 mm.

As in point I above, the microspheres were allowed to mature for 2 hours in this oil, after which they were recovered and aged for 12 hours and then subjected to 3 washes of one hour each with 20 mL of petroleum ether, then to 3 washes of 15 minutes each with 10 mL of an aqueous solution of 0.5 M NH4OH, the last of these washes being followed by drying in an oven at 60 °C.

Chemical dissolution of polystyrene beads:

For the dissolution of polystyrene beads, the microspheres were immersed in 10 mL of /V,/V-dimethylformamide (DMF) and left in this solvent for 24 hours at the end of which they were recovered and dried in an oven at 60 °C.

Consolidation of microspheres by calcination:

For their consolidation, the microspheres were subjected to heat treatment at 1,200°C, in air with:

- a temperature rise from 40°C to 1,200°C at a rate of 1°C/minute,

- a 2-hour stage at 1,200°C, and

- a drop in temperature from 1,200 C to 40 °C.

SEM images of sections of the microspheres thus obtained were taken before dissolution of the polystyrene beads by DMF (figures 1 and 2) as well as after dissolution of the polystyrene beads by DMF and consolidation of these microspheres by calcination (figure 3).

In figures 1 and 2, the polystyrene beads are clearly visible, whereas in figure 3, we no longer see any beads but one of the pores, materialized by the arrow fl, left by the chemical dissolution of the polystyrene beads.

In addition to confirming that it is possible to chemically dissolve the beads of a sacrificial material, which are present in gelled microspheres, by simply soaking the gelled microspheres in an appropriate solvent and thereby creating a calibrated closed porosity within these microspheres, Figure 3 shows that a Consolidation of the gelled microspheres by calcination, for example at 1200°C, allows this porosity to be preserved.

11.2 - Test using a uranium(VI) salt:

In this test, microspheres were prepared following the following operating protocol.

Soil preparation:

In a first beaker, hexamethylenetetramine and urea were dissolved, with stirring, in 500 LIL of demineralized water to obtain an aqueous solution comprising 2 mol/L of HMTA and 3 mol/L of urea and the solution thus obtained was cooled in an ice bath for 30 minutes.

In a second beaker, uranyl nitrate, UC^NOsh, was dissolved, with stirring, in 390 μL of demineralized water to obtain an aqueous solution comprising 2 mol/L of 1102(1x103)2 to which were added 100 μL of an aqueous suspension comprising 50 mg/mL of carboxylated polystyrene beads of 30 μL diameter each (AlphaNanotech) as well as 1 μL of the surfactant Triton™ X-100.

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

Conversion of soil into gelled microspheres:

The sol obtained above was dispersed in the form of droplets of 50 nL each in a silicone oil bath heated to 90 °C using the piezoelectric nano-injector used in point I above, the frequency of which was set at 5 Hz.

The microspheres were allowed to mature for 2 hours in this oil, after which they were successively recovered on a sieve, allowed to age on this sieve for 12 hours in the open air, rinsed with demineralized water in the sieve, subjected to 3 washes of one hour each with 20 mL of petroleum ether, dried under a hood, then subjected to 3 washes of 15 minutes each with 10 mL of an aqueous solution of 0.5 M NH4OH, the last of these washes being followed by drying in an oven at 50 °C. Chemical dissolution of polystyrene beads:

For the dissolution of the polystyrene beads, the microspheres were immersed in 10 mL of DM F and left in this solvent for 1 day at the end of which they were recovered and dried in an oven at 50 °C.

Consolidation of microspheres by calcination:

For their consolidation, the microspheres were subjected to a heat treatment at 1400 °C, under an Ar/H? mixture (95/5, v/v), with:

- an initial temperature rise from 40°C to 350°C in 1 hour and 20 minutes,

- a first stage of 30 minutes at 350°C,

- a second temperature increase from 350°C to 1400°C in 5 hours,

- a second 2-hour stage at 1,400°C, and

- a drop in temperature from 1,400 C to 40 °C.

The microspheres thus obtained were subjected to infrared spectroscopy analyses as well as to SEM analyses.

IR spectroscopy analyses confirmed that the urea-formaldehyde resin forming the three-dimensional network at the gelation stage was completely decomposed during calcination and that the consolidated microspheres no longer comprise, as matrix, only uranium dioxide.

An SEM image of a polished section of one of the microspheres is shown in Figure 4. As visible in this figure, this microsphere is perfectly spherical and has a diameter of the order of 100 LIIYI with a closed pore size of the order of 18 μm. The closed pore, visible in Figure 4, is also perfectly spherical.

Ill - Determination of the volume of aqueous suspension of sacrificial material beads to be introduced into the soil to obtain the desired closed porosity rate:

For the preparation of a batch of N microspheres of chosen average size, it is possible to determine the volume of aqueous suspension of sacrificial material beads to be introduced into the soil in order to obtain, at the batch scale, a desired closed porosity rate. Thus, for example, for the preparation of a batch of 20,000 UOz-based microspheres, measuring 100 LUYI on average, using an aqueous suspension comprising 50 mg/mL of polystyrene beads 30 LUYI in diameter and having a density (hereinafter denoted dps) equal to 1.05 g/cm ³ , and targeting a closed porosity rate of 5%, then:

1) the volume of a microsphere, noted V^phère, is equal to:

5,23 x 10^-7cm³ ; Vp.sphere

5.23 x 10 ^-7 cm ³ ;

2) the total volume of microspheres, noted Vtotai, is equal to:

Vtotal = 20,000 x (5.23 x 10“ ⁷ ) = 0.0104 cm ³ ;

3) at the batch scale, the closed porosity volume of the microspheres, noted V _por them, is equal to:

0,0005 cm³ ; A poor man

0.0005 cm ³ ;

4) the total mass of polystyrene beads, denoted mwies PS, to be introduced into the soil is equal to: mbiiies PS ⁼ porous ^x d _PS = 0.0005 x 1.05 = 0.000525 g = 0.525 mg.

Knowing that the aqueous suspension of polystyrene beads comprises 50 mg of beads per mL, the volume of this suspension to be introduced into the soil will therefore be equal to: T r vsuspension aqueous to be introduced into the soil

REFERENCES CITED

[1] “Treatment-recycling of spent nuclear fuel - Separation of actinides - Application to waste management”, 2008, Éditions Le Moniteur, ISBN 978-2-281-11376-1

[2] US Patent 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, Le Moniteur Editions, ISBN 978-2-281-11325-9

Claims

1. Material comprising porous microspheres, wherein: - each porous microsphere comprises a matrix comprising an actinide oxide and one or more closed and spherical pores, dispersed in the matrix, the microspheres have an average size of between 20 LIIYI and 1000 μm, and the size of the pore(s) is calibrated; and which is obtained by a method comprising the steps of: a) preparing a sol by mixing an aqueous solution Al comprising a first and a second polymerization precursor, capable of forming together a polymeric gel, and an aqueous suspension A2 comprising an actinide salt, optionally a non-ionic surfactant and beads of a sacrificial material; b) converting the sol into gelled microspheres by dispersing the sol in the form of droplets in a bath of a water-immiscible organic liquid, whereby microspheres are obtained which are formed of a polymeric gel containing beads of sacrificial material; c) separating the gelled microspheres from the organic liquid bath; d) washing and then drying the gelled microspheres; e) selective chemical dissolution of the sacrificial material beads present in the gelled microspheres; and f) calcining the gelled microspheres; and wherein, in step b), the droplet size is controlled by an automated dispensing system. 2. Material according to claim 1, for which step a) comprises: - the preparation of the aqueous solution Al by dissolving, with stirring, the first and second polymerization precursors in water and maintaining this solution at a temperature between 0°C and 4°C; - the preparation of the aqueous suspension A2 by dissolving, with stirring, the actinide salt in water and then adding to the resulting solution an aqueous suspension comprising the beads of sacrificial material and, optionally, a non-ionic surfactant in water; and - mixing the aqueous solution Al with the aqueous suspension A2 and maintaining the resulting mixture at a temperature between 0°C and 4°C. 3. Material according to claim 1 or claim 2, wherein the first and second polymerization precursors are hexamethylenetetramine and urea. 4. Material according to any one of claims 1 to 3, wherein the actinide salt is a nitrate, a sulfate, a citrate or a chloride of uranium, thorium, plutonium, neptunium or americium, preferably a nitrate and, more still, a nitrate of uranium. 5. Material according to any one of claims 1 to 4, for which the beads of sacrificial material are made of a hydrophobic organic polymer, not exhibiting a phase transition between 0°C and 90°C and which an organic solvent can dissolve in step b) without dissolving the polymer gel forming the gelled microspheres. 6. Material according to claim 5, wherein the organic polymer is polystyrene, poly(methyl meth)acrylate or polyethylene, preferably polystyrene. 7. Material according to claim 3, for which: - the molar ratio between hexamethylenetetramine and urea present in the soil is between 1/2 and 1 and, preferably, equal to 2/3, and - the molar ratio between hexamethylenetetramine and the actinide ions present in the soil is between 1/2 and 4/3 and, preferably, equal to 1. 8. Material according to any one of claims 1 to 7, for which the sol is dispersed in the form of droplets in the bath of organic liquid by means of a piezoelectric nano-injector. 9. Material according to any one of claims 1 to 8, for which the bath of water-immiscible organic liquid is a bath of silicone oil whose temperature is between 80°C and 95°C, preferably equal to 90°C ± 2°C. 10. Material according to any one of claims 1 to 9, for which step b) comprises maturing the gelled microspheres in the organic liquid bath for from 30 minutes to 2 hours. 11. Material according to any one of claims 1 to 10, for which step e) comprises soaking the gelled microspheres obtained at the end of step d) in a bath of an organic solvent capable of dissolving the beads of sacrificial material without dissolving the polymer gel forming the gelled microspheres. 12. Material according to claim 11, wherein the organic solvent is dimethylformamide, xylene or chloroform. 13. Material according to any one of claims 1 to 12, for which step f) comprises a heat treatment at a temperature between 600°C and 1,500°C, under a reducing, neutral or oxidizing atmosphere. 14. Material according to any one of claims 1 to 12, for which 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 under a reducing, neutral or oxidizing atmosphere. 15. Material according to any one of claims 1 to 14, in which the actinide oxide of the matrix of the microspheres is uranium dioxide or triuranium octoxide. 16. Use of a material according to any one of claims 1 to 15, as a sintering additive in the manufacture of pellets of a MOX type nuclear fuel. 17. Use according to claim 16, in which the material is used as a pore-forming agent. 18. Use according to claim 16 or claim 17, in which the material comprises microspheres whose matrix comprises a uranium oxide, preferably uranium dioxide.