A process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2, and minor actinide oxides.

Abstract

The invention relates to a process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2 and minor actinide oxides, the minor actinides being selected from americium, neptunium and curium, comprising the steps of: a) cryogenic granulation of an aqueous solution comprising cations selected from uranium-based cations, plutonium-based cations and minor actinide-based cations; b) freeze-drying of the granules obtained in a); etc.; calcination of the granules from b). Applications: manufacture of nuclear fuels or blankets loaded with minor actinide(s).

Description

Title of the invention: Process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2 and minor actinide oxides. Technical field

[0001] The invention relates to a process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2 and minor actinide oxides.

[0002] For the remainder of this exposition, it is specified that by minor actinide, we mean the actinide elements other than uranium, plutonium and thorium, which are formed in reactors by successive captures of neutrons by the nuclei of standard fuel, the minor actinides being americium, curium and neptunium.

[0003] More specifically, the invention relates to a process for preparing a powder that is flowable, that can be pressed without prior mixing and that may, more specifically, have the following specific physicochemical characteristics: - a good aptitude for spontaneous flow; - a homogeneous particle size distribution centered in the range of 5 pm to 500 pm; - good homogeneity of the elements within the powder particles, when the latter contains several different actinide elements; - a limited level of structural carbon within the powder particles; - a minimum level of fine particles in the powder, in order to avoid their dissemination within equipment and glove boxes; - good compaction ability; and - excellent reactivity to natural sintering.

[0004] Due to the aforementioned physico-chemical characteristics, the powder obtained by the process of the invention can be suitable for the preparation of the following materials: - Uranium oxide fuels (UO2); - mixed oxide fuels of uranium and plutonium (U,Pu)O2, known as MOX fuels, currently used in light water reactors, or MOX fuels with high plutonium content that can be used in fast neutron reactors; - blankets loaded with minor actinide(s), such as minor actinide(s)-based transmutation targets intended for conducting nuclear transmutation experiments in fast neutron reactors, particularly with a view to better understand the transmutation mechanism of these minor actinide elements, these targets can consist of a MOX type material containing 1% to 5% by mass of minor actinide(s) (this material can be symbolized by the formula (U,Pu, Am,Np,Cm)O2) or a material comprising a uranium oxide matrix comprising 10% to 20% by mass of minor actinide(s) (this material can be symbolized by the formula (U,Am,Np,Cm)O2). Prior art

[0005] The manufacture of mixed uranium-plutonium oxide (U,Pu)O2 fuels, known as MOX fuels, has been the subject of various developments related to the desire to recycle plutonium recovered during the reprocessing of spent nuclear fuel. Plutonium recycling through the manufacture and irradiation of MOX fuels is now considered a means of limiting plutonium proliferation.

[0006] Several MOX fuel manufacturing processes have been developed over the past two decades, some involving complete grinding of UO2 and PuO2 powders to ensure intimate mixing, others limited to grinding only a fraction of these powders.

[0007] Currently, the preparation of the mixed (U,Pu)O2 oxide is carried out by dry mechanical mixing of UO2 and PuO2 oxide powders. The resulting mixture, after pressing, sintering, and grinding, allows the production of MOX fuel pellets meeting current specifications. The most proven industrial process comprises two main steps in the preparation of the powders: co-grinding of uranium oxide and plutonium oxide powders to produce a first mixture, called the master mixture, which is characterized by a plutonium content of 25% to 30%, followed by dry dilution of this master mixture with uranium oxide until the desired final plutonium content is obtained.

[0008] The PuO2 powder used in the manufacture of MOX fuel comes from the reprocessing of spent uranium fuel from light water reactors. This reprocessing is carried out via the PUREX liquid-liquid extraction process. This process yields concentrated solutions of depleted uranyl nitrate and plutonium nitrate. The concentrated plutonium nitrate solution is then converted into plutonium oxide (PuO2) powder by oxalic precipitation of the plutonium, filtration of the resulting plutonium oxalate solution, and subsequent dewatering, drying, and calcination of the plutonium oxalate precipitate.

[0009] Other liquid-liquid extraction processes have also been developed for the selective recovery of minor actinides (such as the selective extraction of americium by the EXAm process or the grouped extraction of minor actinides americium, curium and neptunium by the GANEX or SANEX processes).

[0010] For the manufacture of MOX fuel, the UO2 and PuO2 oxide powders used must meet specific characteristics. In particular, they must have good flowability, good compressibility, and the ability to be densified by sintering. The homogeneity of plutonium distribution is an important quality criterion for the final properties of the sintered material. Good homogeneity within each sintered pellet is, on the one hand, highly favorable for the behavior of the MOX fuel in the reactor, particularly with regard to increasing burnup rates, and, on the other hand, facilitates the complete dissolution of spent fuel during fuel reprocessing operations.

[0011] As for the transmutation targets, they have been the subject of extensive studies in order, in addition to their purposes mentioned above, to allow recycling of minor actinides from the processing of spent fuel from pressurized water reactors.

[0012] This type of recycling is carried out by two distinct routes known under the following names: heterogeneous recycling and homogeneous recycling.

[0013] In the case of heterogeneous recycling, minor actinides are separated from uranium and plutonium during the reprocessing of spent fuel and are then incorporated, at a high concentration (approximately 10% to 20% atomic percentage), into fuel elements comprising a non-fissile matrix (e.g., depleted UO2) distinct from the standard reactor fuel elements. The fuel elements containing the minor actinides may consist, for example, of blanket elements arranged around the periphery of a reactor core. This recycling method makes it possible, in particular, to avoid degrading the characteristics of the standard fuel through the incorporation of minor actinides by concentrating the problems generated by these actinides on a reduced material stream.

[0014] In the case of homogeneous recycling, minor actinides are mixed, at a low concentration (less than 5 atomic percent), and distributed almost uniformly throughout all the standard fuel elements of the reactor. To achieve this, during the reprocessing of spent fuel, uranium, plutonium, and minor actinides are treated together to form oxides, which are then used in the manufacture of said fuel.

[0015] Whether for the manufacture of nuclear fuels or transmutation targets, the processes recently proposed tend towards techniques limiting the dissemination of fine particles (and, therefore, the dusting of the glove boxes in which these fuels or targets are manufactured) and improving the homogeneity of the elements within the pellets.

[0016] This is the case of the WAR process (from Weak Acid Resin, so named because it is based on the use of a weakly acidic ion exchange resin), which aims to obtain homogeneous spherules of mixed (U,Am)O2 oxides without going through a granulation phase, which greatly limits the dissemination of fine particles, unlike conventional powder metallurgy processes, which implement granulation steps, such as grinding, sieving and mixing.

[0017] Another process involving an atomization-drying phase of an aqueous suspension comprising a UO2 powder obtained by dry process from UF6 was described in international application WO-A-00 / 30978, hereinafter referred to as [1]. This process, although not using grinding, sieving and mixing steps, still generates a non-negligible amount of fine particles during atomization.

[0018] Finally, it was proposed in international application WO-A-2019 / 038497, cited below. Following reference [2], a process that also prevents the formation and dissemination of fine particles during the fabrication of nuclear fuels or transmutation targets involves subjecting an aqueous suspension containing UO2 powder and, optionally, PuO2 powder and / or a powder of a minor actinide oxide to cryogenic granulation. The resulting granules are then freeze-dried and can be directly compacted into pellets. While this process undeniably offers numerous advantages, including producing oxide particles with remarkable physicochemical characteristics while limiting the risk of fine particle dissemination, it does not completely eliminate this risk. This is because the cryogenic granulation is performed on an aqueous suspension containing one or more oxide powders, the preparation of which may itself have been a source of dissemination.

[0019] The inventors have therefore set themselves the objective of providing a new process for preparing a powder comprising one or more actinide oxides which, while leading to the obtaining of oxide particles with physico-chemical properties just as interesting as those of the particles obtained by the process of reference [2], further reduces the risk of dissemination of fine particles.

[0020] They have also set themselves the objective that this process should also allow: - to overcome the constraints inherent in the preparation of actinide oxide powders and, in particular, in that of PuO2, the oxalic precipitation and filtration operations being able to generate problems of filter clogging and, consequently, of supplying the furnace used for calcining the plutonium oxalate precipitate; - to minimize, for the powders obtained at the end of the process, the problems of shaping in their raw state, for example, by dry pressing, thanks to the optimization and robustness of the rheological properties of the powders obtained; and - minimize, during the manufacture of pellets from the powders obtained at the end of the process, the scrap rate by minimizing the problems inherent in raw shaping and by having, when the powders contain elements other than uranium, a homogeneous distribution of the different elements. Description of the invention

[0021] The invention relates to a process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2 and minor actinide oxides, the minor actinides being selected from americium, neptunium and curium, comprising the steps of: a) cryogenic granulation of an aqueous solution comprising cations selected from uranium-based cations, plutonium-based cations and minor actinide-based cations; b) freeze-drying of the granules obtained in a); and c) calcination of the granules obtained from b); by which means the powder is obtained.

[0022] Thus, according to the invention, it is an aqueous solution containing "precursor" cations of the oxide or mixture of oxides intended to be present in the powder that is subjected to cryogenic granulation and not, an aqueous suspension comprising an oxide powder or a mixture of oxide powders as in reference [2].

[0023] In addition to fulfilling the objectives already mentioned above, the process of the invention also has the following advantages: - the use of water as a solvent is particularly interesting because it allows us to limit the use of organic products and thus limit impurities in the final powder obtained; - the simple, rapid, reproducible implementation leading, during step a), to a solution that can be brought by simple pumping to the injection nozzle of a cryogenic granulation device without any difficulty; - the combined use of a solution, cryogenic granulation and lyophilization allowing the obtaining of a powder comprising particles with controlled porosity, solid and well spherical with good homogeneity of distribution of elements and good flowability; - the possibility of obtaining powders without the oxalic acid precipitation and filtration steps usually used to recover uranium and, where applicable, plutonium from nitric acid solutions; and - the possibility of implementing this process in an industrial capacity production unit taking into account the criticality and therefore the geometry of the devices.

[0024] As previously indicated, step a) consists of a cryogenic granulation step of the aforementioned aqueous solution, this step being able to consist of spraying or atomizing - the two words being considered here as synonyms - this solution in the form of droplets, for example, by passing this solution through a nozzle, and putting the droplets thus formed into contact with a liquid at very low temperature (for example, liquid nitrogen) to fix the droplets in their form.

[0025] Such a step a) can be carried out in a commercial granulation device or in a device specially prepared in the laboratory for carrying out this step. This device may consist of a peristaltic pump that conveys the aqueous solution to a nozzle to allow the solution to be granulated. The microdroplets formed and projected by the nozzle fall into a Dewar flask filled with liquid nitrogen under agitation (by means, for example, of a magnetic stir bar) and are directly solidified into a spherical shape.

[0026] In the aqueous solution subjected to step a), the cations, whether based on uranium, plutonium, americium, neptunium and / or curium, can be associated with anions to form saline compounds and / or can be associated with organic ligands to form complexes, and, more specifically, coordination complexes.

[0027] The aqueous solution subjected to step a) is advantageously an aqueous nitric solution (or, in other words, an aqueous solution of nitric acid, for example with a concentration ranging from 0.5 mol / L to 15 mol / L, preferably between 1 mol / L and 8 mol / L). In such a context, if uranium-based cations are present, these cations are uranyl cations UO22+ coexisting with nitrate ions to form uranyl nitrate UO2(NO3)2; if plutonium-based cations are present, then these cations are Pu4+ cations associated with nitrate ions to form plutonium nitrate Pu(NO3)4, while if cations based on one or more minor actinides are present, then these cations are Mx+ cations associated with nitrate ions to form one or more nitrates M(NO3)X (M denoting Am, Np or Cm and x ranging from 3 to 6, the value of x being fixed so as to ensure the electroneutrality of M(NO3)X).

[0028] This aqueous nitric solution may, in particular, originate from liquid-liquid extraction processes such as the PUREX or GANEX / EXAm process, the the concentration of this solution can be adjusted beforehand by evaporation prior to the implementation of the process of the invention.

[0029] It goes without saying that the process is not limited to the cryogenic granulation of an aqueous nitric solution comprising cations associated with nitrate ions and that other acidic aqueous solutions such as, for example, an aqueous solution of sulfuric acid in which the cations are associated with sulfate ions are likely to be suitable.

[0030] The aqueous solution subjected to step a) comprises, in particular, a total concentration of actinide element(s) (uranium and / or plutonium and / or minor actinide(s)) ranging from 5 g / L to 300 g / L.

[0031] When the aqueous solution subjected to step a) is an aqueous solution of uranium-based cations, it may contain a trace amount of plutonium-based cations, depending on the process by which this solution was obtained. Conversely, when the aqueous solution subjected to step a) is an aqueous solution of plutonium-based cations, it may contain a trace amount of uranium-based cations, depending on the process by which this solution was obtained.

[0032] In accordance with the invention, it is also possible that the aqueous solution subjected to step a) may comprise uranium-based cations and plutonium-based cations (but without minor actinide-based cations) with a mole (or atomic) proportion of plutonium (as determined by the ratio Pu / (U+Pu)) ranging from 1% to 99% depending on the intended use of the powder to be prepared (use for scientific research purposes, use in the experimental or industrial manufacture of new nuclear fuels, etc.).

[0033] By way of example, for the manufacture of MOX fuels intended for light water reactors or RELs (pressurized water reactors and boiling water reactors), then the aqueous solution subjected to step a) preferably has a mole (or atomic) proportion of plutonium ranging from 3% to 12%, while for the manufacture of MOX fuels intended for fast neutron reactors or FNRs, then said aqueous solution preferably has a mole (or atomic) proportion of plutonium ranging from 15% to 40%.

[0034] When the solution comprises uranium-based cations and cations based on one or more minor actinides (but without plutonium-based cations), then the molar (or atomic) proportion of minor actinide(s) will preferably range from 1% to 50% (this being determined by the ratio M / (U+M)), M representing the minor actinide(s).

[0035] Furthermore, the aqueous solution subjected to step a) may comprise at least one additive selected from water-soluble organic polymers, organic compounds nitrogenous compounds and mixtures thereof, this or these additives being advantageously present in such quantity that the dynamic viscosity (for a shear rate of 1500 s *) of the aqueous solution does not exceed 1000 mPa.s and, preferably, does not exceed 100 mPa.s.

[0036] The advantage of using such additives lies in their ability to increase the viscosity of the solution, in order to control the shape of the granules obtained subsequently during the cryogenic granulation step.

[0037] Examples of water-soluble organic polymers include polyvinyl alcohol (PVA), polyethylene glycol (PEG), poly(vinyl butyral) (known by the abbreviation PVB), acrylic latex or a mixture thereof.

[0038] As examples of nitrogenous organic compounds, mention may be made of amide compounds or amine compounds.

[0039] The dynamic viscosity is conventionally measured using a rheometer at a shear rate of 1500 s⁻¹ with a cone-cylinder configuration system at ambient temperature and pressure (i.e., without the application of any external heating or pressurization other than ambient temperature and pressure, the ambient temperature being 20 °C and the ambient pressure being atmospheric pressure). Preferably, the dynamic viscosity does not exceed 100 mPa·s, which corresponds to a very fluid solution that can easily flow through the feed pipes and the atomization nozzle of the cryogenic granulation device.

[0040] In addition, the solution may include one or more complex stabilizing agents, when uranium-based cations, plutonium-based cations and / or cations based on one or more minor actinides are associated with organic ligands, to form complexes.

[0041] Prior to step a), the process of the invention may include a step of preparing the solution comprising uranium-based cations, plutonium-based cations and / or cations based on one or more minor actinides by bringing the different ingredients of this solution into contact and in the desired proportions.

[0042] By way of example, the aqueous solution can be prepared by contacting and then mixing different nitric solutions comprising the different desired elements followed possibly by concentration by evaporation of the water in order to obtain the desired concentrations.

[0043] According to the process of the invention, after the cryogenic granulation step, the granules obtained are subjected to a freeze-drying step, for example, by placing them in a freeze dryer to allow the sublimation of the frozen water and to preserve the shape of the granules (and in particular their sphericity) and their individualities.

[0044] At the end of the freeze-drying, the residual moisture of the granules is very low, which makes it possible to avoid drying these granules before their calcination.

[0045] When the aqueous solution subjected to step a) is an aqueous solution of nitric acid, the granules from the lyophilization step are granules comprising uranyl nitrate UO2(NO3) and / or plutonium nitrate Pu(NO3)4 and / or one or more nitrates M(NO3)X (M denoting Am, Np or Cm and x ranging from 3 to 6, the value of x being fixed so as to ensure the electroneutrality of M(NO3)X).

[0046] After the freeze-drying step, the process of the invention includes a step of calcining the granules.

[0047] This calcination can be oxidizing or reducing, or oxidizing then reducing, depending on the actinide elements retained and the desired valency adjustment.

[0048] If the granules are free of uranium-based cations, that is to say they comprise only plutonium-based cations or cations based on one or more minor actinides or a mixture of plutonium-based cations and cations based on one or more minor actinides, then calcination can be carried out in a single step, that is to say either under an oxidizing atmosphere or under a reducing atmosphere, preference being given however to an oxidizing atmosphere.

[0049] If the granules comprise uranium-based cations (alone or with other cations), then calcination can be carried out in a single step under a reducing atmosphere, but it is preferred that it be carried out in two successive steps, a first step under an oxidizing atmosphere allowing the elimination of any organic matter and the formation of U3O8, followed by a second step under a reducing atmosphere allowing the conversion of U3O8 into UO2.

[0050] Calcination under an oxidizing atmosphere can consist of a heating operation, for example under air or under an oxygen-enriched atmosphere such as an atmosphere comprising 80% by volume of oxygen at a temperature ranging from 100 °C to 1200 °C, preferably less than 800 °C for a duration of up to 12 hours, preferably less than 4 hours. This calcination is isomorphic, in that its implementation does not affect the shape of the granules subjected to this step.

[0051] Calcination under a reducing atmosphere can consist of a heating operation, for example under hydrogenated argon, at a temperature ranging from 300 °C to 1200 °C, preferably less than 900 °C, for a period of up to 12 hours, preferably less than 4 hours.

[0052] At the end of the process of the invention, a powder is obtained which may specifically have the following characteristics: - a homogeneous particle size distribution centered in the range of 5 pm to 500 pm; - sufficient cohesion of the granules to withstand the handling required for the preparation of lozenges; - excellent flow properties, including good spontaneous flow properties; - good compaction capacity; - an excellent aptitude for natural sintering; - good homogeneity of element distribution within the powder, when the powder contains several actinide elements (uranium and / or plutonium and / or minor actinide(s)); and - a minimum of fine particles within the powder, thus limiting the risks of dissemination and contamination.

[0053] The almost perfect sphericity of these granules allows very good flowability in the pressing molds for obtaining pellets which will then be sintered.

[0054] As for the homogeneity of the distribution of elements, it is particularly important with regard to the element plutonium if it is present. Once the powder is compacted and sintered to make MOX fuel, the homogeneity of the plutonium distribution is highly favorable for the behavior of the fuel in the reactor, particularly with a view to increasing burnup rates, and also facilitates the complete dissolution of the spent fuel during future reprocessing operations.

[0055] According to the invention, the powder is preferably a UO2 powder, a PuO2 powder, a powder comprising a mixture of UO2 and PuO2 such as a powder having a Pu / (U+Pu) molar ratio of 12% (REL type) or 30% (RNR type), any preference being given to a powder comprising a mixture of UO2 and PuO2.

[0056] The powder obtained according to the process of the invention can be used directly (i.e. without requiring the addition of other ingredients) to constitute a compacted material, for example, in the form of nuclear fuel pellets.

[0057] Thus, the invention also relates to a process for preparing pellets of nuclear fuel comprising successively the steps of: i) implementation of the powder preparation process as defined above; ii) compaction of the powder obtained in i) into pellets; and iii) sintering of the pellets obtained in ii).

[0058] The compaction step ii) may consist, on the one hand, of placing the powder in a mold of suitable shape to form one or more pellets and, on the other hand, of subjecting this powder to uniaxial pressing, for example, using a piston applying pressure to the powder placed in the mold, this pressure being going from 150 MPa to 1000 MPa for a duration ranging from 1 second to 10 minutes.

[0059] The sintering step iii) may consist of heating the aforementioned pellets, for example, to a temperature ranging from 1,000 °C to 1,800 °C, for a holding period ranging from 1 hour to 8 hours, preferably from 3 hours to 5 hours, under a neutral gas atmosphere, such as argon, possibly including dry or humidified hydrogen, the hydrogen being present in the mixture at a content of up to 5% by volume and the water being present in the mixture at a content of up to 20,000 ppm.

[0060] Thus, for example, the sintering of UO2 pellets can be carried out under an atmosphere consisting solely of argon as well as under a mixture of argon and dry or moistened hydrogen, while for the sintering of pellets comprising a mixture of UO2 and PuO2, a mixture of argon and dry or moistened hydrogen is typically used.

[0061] Alternatively, between step i) and step ii), a powder of a uranium oxide, such as a powder of U3O8, a powder of PuO2 and / or at least a powder of a minor actinide oxide, may be added to the powder from step i) in order to adjust the intended composition, if necessary.

[0062] In any event, the nuclear fuel is, preferably, a MOX fuel.

[0063] Other features and advantages of the invention will become apparent from the following supplementary description, which relates to an example of the preparation of a mixed powder and fuel pellets according to embodiments in accordance with the processes of the invention.

[0064] Of course, this additional description is given only as an illustration of the invention and does not in any way constitute a limitation thereof.

[0065] Detailed description of particular embodiments

[0066] Example 1: preparation of a mixed UO2 / PuO2 powder

[0067] This example illustrates the implementation of the process of the invention for the preparation of a mixed powder comprising uranium oxide UO2 and plutonium oxide PuO2 in a Pu / (U+Pu) ratio of 10% by mass, this preparation being entirely carried out in a glove box.

[0068] A 5 mol / L aqueous solution of nitric acid, comprising uranyl nitrate and plutonium nitrate in a Pu / (U+Pu) ratio of approximately 10 wt% (U] = 200 g / L; [Pu] = 21 g / L), 2 wt% polyethylene glycol 3400, and having a dynamic viscosity of less than 20 mPa·s, is placed in a beaker to be picked up by a peristaltic pump (flow rate of 33 mL / min, air pressure of 15 kPa) and sprayed through a nozzle which generates droplets of this solution.

[0069] The aforementioned dynamic viscosity is measured using an ANTON PAAR RHEOLAB QC rheometer, at a shear rate of 1500 s1.

[0070] The droplets thus generated fall into a Dewar flask filled with liquid nitrogen, under magnetic stirring at 300 rpm, whereby they are instantly frozen and form granules which retain the original shape of the droplets.

[0071] After cryogenic granulation, the granules are quickly placed in a freeze dryer to sublimate the frozen water and preserve their spherical shape, an operation that takes several hours. Once all the water has been removed from the granules, they are calcined under an oxidizing atmosphere (80% by volume O2) at 600 °C for 1 hour to transform the nitrates into oxides while preserving their morphology, and then under a reducing atmosphere (for example, under hydrogenated argon) to reduce the U3O8 phase to UO2 and produce a powder comprising both UO2 and PuO2.

[0072] The powder is ready to be pressed into pellets before the sintering step.

[0073] The aforementioned cryogenic granulation is implemented in a device comprising the following elements: - a beaker which holds the aqueous solution, this beaker being connected to a peristaltic pump which allows to convey this solution to a spray nozzle, the flow rate of the pump being a maximum of 2 L / h with an air pressure of 15 kPa; and - a Dewar flask filled with liquid nitrogen, equipped with a magnetic stirrer (300 rpm), which is connected to the spray nozzle and allows the instant freezing of the droplets of solution formed by this nozzle.

[0074] Example 2: preparation of UO2 / PuO2 pellets

[0075] This example illustrates the preparation of pellets of a nuclear fuel from the powder obtained in Example 1 above.

[0076] To achieve this, the powder is subjected to uniaxial cold pressing at 500 MPa with external lubrication using stearic acid, resulting in pellets 4.5 mm in diameter and 4 mm in height. These pellets are then subjected to a sintering operation for 4 hours at 1700 °C, under an atmosphere of argon with 4% by volume of hydrogen and 1200 vpm of water, the temperature of 1700 °C being reached by a temperature increase of 2 °C / min under an atmosphere of argon with 4% by volume of dry hydrogen.

[0077] The pellets thus sintered have relative densities of about 94-96% with good homogeneity of the U and Pu elements within the pellets (thanks to good homogeneity of these elements within the powder).

[0078] Example 3: preparation of UO2 / PuO2 pellets

[0079] In this example, pellets of 4.5 mm in diameter and 4 mm in height are prepared from a powder comprising uranium oxide UO2 and plutonium oxide PuO2 in a Pu / (U+Pu) ratio of 10 atomic %, this powder having been obtained by a process similar to that described in example 1 above except that polyethylene glycol was not added to the aqueous solution subjected to cryogenic granulation.

[0080] To obtain the pellets, the powder is subjected to uniaxial cold pressing at 600 MPa, with external lubrication by stearic acid (no internal lubrication of the granules), then the pellets are sintered for 4 hours at 1700 °C, under an atmosphere of argon with 4% by volume of hydrogen and 1200 vpm of water, the temperature of 1700 °C being reached by a temperature rise of 2 °C / min under an atmosphere of argon with 4% by volume of dry hydrogen.

[0081] The pellets obtained have relative densities between 97% and 98% with good homogeneity of the U and Pu elements within the pellets (due to the good homogeneity of these elements within the granules).

[0082] Example 4: preparation of a powder and pellets of UO2

[0083] In this example, a UO2 powder is prepared by placing a 1 mol / L aqueous solution of nitric acid, comprising uranyl nitrate, in a beaker to be taken up by a peristaltic pump (flow rate of 40 mL / min, air pressure of 30 kPa) and sprayed through a nozzle which generates droplets of this solution.

[0084] The droplets thus generated fall into a Dewar flask filled with liquid nitrogen, under magnetic stirring at 300 rpm, thereby obtaining granules.

[0085] These granules are quickly placed in a freeze dryer for several hours. Once all the water has been removed from the granules, they (10-300 µm) are calcined under an oxidizing atmosphere (80% by volume oxygen) at 600 °C for 1 hour. A reduction step under an argon atmosphere with 4.3% hydrogen is then carried out at 750 °C for 1 hour to reduce U3O8 to UO2.

[0086] The UO2 powder thus obtained is then pressed into pellets 4.5 mm in diameter and 4 mm high by uniaxial cold pressing at 500 MPa, with external lubrication using stearic acid (no internal lubrication of the granules), then the pellets are sintered for 4 hours at 1700 °C, under an atmosphere of argon with 4% by volume of hydrogen and 1200 vpm of water, the temperature of 1700 °C being achieved by a temperature rise of 2 °C / min under an argon atmosphere with 4% dry hydrogen.

[0087] The pellets obtained have relative densities of approximately 93%.

[0088] Example 5: preparation of a PuO2 powder

[0089] This example illustrates the implementation of the process of the invention for the preparation of a powder comprising plutonium oxide PuO2, this preparation being entirely carried out in a glove box.

[0090] An aqueous solution of nitric acid at 1.5 mol / L, comprising plutonium nitrate ([Pu] = 35 g / L) is placed in a beaker to be taken up by a peristaltic pump (flow rate of 33 mL / min, air pressure of 15 kPa) and sprayed through a nozzle which makes it possible to generate droplets of this solution.

[0091] The droplets thus generated fall into a Dewar flask filled with liquid nitrogen, under magnetic stirring at 300 rpm, whereby they are instantly frozen and form granules which retain the original shape of the droplets.

[0092] After cryogenic granulation, the granules are quickly placed in a freeze dryer to sublimate the frozen water and maintain their spherical shape, an operation that takes several hours. Once all the water has been removed from the granules, they are calcined under an oxidizing atmosphere (80% by volume O2) at 600 °C for 30 minutes to convert the plutonium nitrate to PuO2.

[0093] The PuO2 powder thus obtained is ready to be mixed with UO2 powder to obtain UO2 / PuO2 pellets. References cited

[0094] [1] WO-A-00 / 30978 [2] WO-A-2019 / 038497

Claims

Demands

1. A process for preparing a powder comprising one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2 and minor actinide oxides, the minor actinides being selected from americium, neptunium and curium, comprising the steps of: a) cryogenic granulation of an aqueous solution comprising cations selected from uranium-based cations, plutonium-based cations and minor actinide-based cations; b) lyophilization of the granules obtained in a); and c) calcination of the granules obtained from b); thereby obtaining the powder.

2. A method according to claim 1, wherein the cations present in the aqueous solution subjected to step a) are associated with anions to form saline compounds and / or are associated with organic ligands to form complexes.

3. A method according to claim 1 or claim 2, wherein the aqueous solution subjected to step a) is an aqueous solution of nitric acid in which the cations are associated with nitrate ions.

4. A method according to claim 3, wherein the aqueous solution subjected to step a) comprises at least one nitrate selected from uranyl nitrate UO2(NO3)2, plutonium nitrate Pu(NO3)4 and nitrates M(NO3)X, M being one of the minor actinides and x being an integer from 3 to 6.

5. A process according to any one of claims 1 to 4, wherein the aqueous solution subjected to step a) further comprises one or more additives selected from water-soluble organic polymers, nitrogenous organic compounds and mixtures thereof.

6. A process according to claim 5, wherein the additive(s) are present in such an amount that the dynamic viscosity of the aqueous solution does not exceed 1,000 mPa.s for a shear rate of 1,500 s1.

7. A method according to claim 6, wherein the dynamic viscosity of the aqueous solution does not exceed 100 mPa.s.

8. A method according to any one of claims 5 to 7, wherein the water-soluble organic polymer(s) are selected from polyvinyl alcohol, polyethylene glycol, poly(vinyl butyral) and acrylic latex.

9. A method according to any one of claims 5 to 7, wherein the nitrogenous organic compound(s) are selected from amide compounds and amine compounds.

10. A process according to any one of claims 1 to 9, wherein the aqueous solution subjected to step a) comprises a total concentration of actinide element(s) ranging from 5 g / L to 300 g / L.

11. A process according to any one of claims 1 to 10, wherein the calcination of the granules is an oxidizing or reducing calcination or is a calcination which is successively oxidizing and then reducing.

12. A method according to any one of claims 1 to 11, wherein the powder is a UO2 powder, a PuO2 powder or a powder comprising a mixture of UO2 and PuO2, preferably a powder comprising a mixture of UO2 and PuO2.

13. A process for preparing pellets of nuclear fuel, comprising successively the steps of: i) carrying out a process according to any one of claims 1 to 12 for the preparation of a powder; ii) compacting the powder obtained in i) into pellets; and iii) sintering the pellets obtained in ii).

14. A method for preparing pellets of a nuclear fuel according to claim 13, wherein the nuclear fuel is a MOX fuel.