Process for the preparation of a powder comprising one or more oxides chosen from uranium UO2 oxides, plutonium puo2 oxides and oxides of minor actinides

Abstract

Cryogenic granulation, in the conventional way, is carried out on an aqueous suspension comprising one or more oxide powders, which is a preparation that can be a source of splashing and does not completely eliminate the risks.SOLUTION: The invention relates to a process for the manufacture of a powder comprising one or more oxides chosen from uranium oxide UO2, plutonium oxide PuO2 and oxides of minor actinides, the minor actinides being chosen from americium, neptunium and curium, comprising the following steps: a) cryogenic granulation of an aqueous solution comprising cations chosen from uranium-based cations, plutonium-based cations and minor actinide-based cations; b) freeze-drying of the granules obtained in step a); and c) calcining of the granules obtained from step b). An application of the invention is the manufacture of nuclear fuels or blankets filled with minor actinides.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a powder containing one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2, and oxides of minor actinides.

[0002] For the remainder of this specification, minor actinides means actinide elements other than uranium, plutonium, and thorium, which are formed in nuclear reactors by successive neutron capture by standard fuel nuclei, and it is specified that the minor actinides are americium, curium, and neptunium.

[0003] More specifically, the present invention relates to a method for preparing a powder that is moldable and can be compressed without pre-mixing, and more particularly that has the following specific physicochemical properties: - Good natural fluidity; - Uniform particle size distribution centered in the range of 5μm to 500μm; - good uniformity of elements in the powder particles, if the powder contains several different actinide elements; - limited structural carbon content within the powder particles; - Minimization of the fine particle content in the powder to avoid particles being dispersed into the equipment and glove box; - Excellent compressibility; and - Excellent reactivity to natural sintering.

[0004] Due to the above-mentioned physicochemical properties, the powder obtained by the method of the present invention may be suitable for the preparation of the following materials: - uranium oxide UO2 fuel; - Mixed uranium and plutonium oxide fuel (U, Pu)O2, denoted as MOX fuel, currently used in light water reactors or high plutonium content MOX fuel that can be used in fast neutron reactors - blankets filled with minor actinides, such as transmutation targets based on minor actinides, for the purpose of carrying out nuclear transmutation experiments in fast neutron reactors, in particular with the purpose of better understanding the transmutation mechanisms of minor actinide elements; these targets may consist of MOX-type material containing 1% to 5% by mass of minor actinides (this material may be symbolized by the formula (U, Pu, Am, Np, Cm)O2), or of material with a uranium oxide matrix containing 10% to 20% by mass of minor actinides (this material may be symbolized by the formula (U, Am, Np, Cm)O2). [Background technology]

[0005] The production of a mixture of uranium and plutonium oxides (U, Pu)O2, designated MOX fuel, has been the subject of various developments in connection with the drive to reuse plutonium recovered during spent nuclear fuel processing. Recycling of plutonium by producing MOX fuel and irradiating it is currently being considered with the goal of limiting plutonium proliferation.

[0006] Several methods of MOX fuel fabrication have been developed over the last two decades, some of which involve complete grinding of UO2 and PuO2 to ensure thorough mixing, while others are limited to grinding only fractions of these powders.

[0007] Currently, the production of mixed oxide (U, Pu)O2 is performed by dry mechanical blending of UO2 and PuO2 oxide powders. The resulting blend allows for post-compaction, sintering, and fractionation to produce MOX fuel meeting current specifications. The most tried and tested industrial method involves two major steps in powder preparation: cogrinding uranium oxide and plutonium oxide to produce a first blend, called the master blend, characterized by a plutonium content of 25% to 30%. The subsequent dry dilution of the master blend with uranium oxide until the desired final plutonium content is obtained.

[0008] The PuO2 powder used in the production of MOX fuel is sourced from the processing of spent uranium fuel from light water nuclear reactors. The processing is carried out by the PUREX process, by liquid-liquid extraction. After this process, a concentrated solution of depleted uranyl nitrate is obtained on the one hand, and a concentrated solution of plutonium nitrate on the other hand. The concentrated plutonium nitrate solution is then converted into plutonium oxide PuO2 powder by oxalic acid precipitation of plutonium, filtration of the resulting plutonium oxalate, followed by spinning, drying and calcination of the plutonium oxalate precipitate.

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

[0010] For the production of MOX fuel, the UO2 and PuO2 oxide powders used must meet precise specifications. In particular, they must have good flowability, good compactibility, and be suitable for densification by sintering. The uniformity of plutonium distribution in each sintered pellet is an important quality measure for the final properties of the sintered material. Good uniformity in each sintered pellet is, on the one hand, extremely beneficial for the behavior of MOX fuel in the reactor, especially with the aim of increasing the burnup rate, and, on the other hand, promotes complete decomposition of spent fuel during the processing of these fuels.

[0011] Transmutation targets have been the subject of extensive research, not only for the purposes mentioned above, but also to enable the recycling of minor actinides from the processing of spent fuel from pressurized water reactors.

[0012] This type of recycling occurs in two separate routes: heterogeneous recycling and homogeneous recycling.

[0013] In heterogeneous recycling, minor actinides are separated from uranium and plutonium during the processing of spent fuel and then incorporated into fuel elements containing a high concentration (approximately 10%-20 atomic %) of a distinct non-fissile matrix (e.g., depleted UO2) from standard reactor fuel elements. Fuel elements containing minor actinides can consist of blanket elements located, for example, at the outer periphery of the reactor core. This recycling route makes it possible to avoid the impairment of standard fuel properties due to minor actinide contamination, particularly by focusing the problems caused by minor actinides on a reduced material flow.

[0014] In homogeneous recycling, minor actinides are mixed in low concentrations (less than 5 atomic percent) and distributed seemingly uniformly among all of the reactor's standard fuel elements. To achieve this, during spent fuel processing, uranium, plutonium, and minor actinides are processed together to form oxides, which are subsequently used in the fabrication of said fuel.

[0015] Recently proposed methods, whether for the production of nuclear fuel or transmutation targets, tend to be directed towards techniques that reduce particulate scattering (and hence dust accumulation in the glove boxes where these fuels or targets are produced) and improve the uniformity of elements within the pellets.

[0016] This is the case with the WAR (Weak Acid Resin, so named because it is based on the use of a weakly acidic ion exchange resin) process, which aims to obtain uniform globules of mixed oxide (U, Am)O2 without the need for a granulation step, which substantially suppresses the scattering of fine particles, unlike conventional powder metallurgy processes that perform granulation steps such as grinding, sieving, and mixing.

[0017] Another method involving a step of spray drying of an aqueous suspension containing UO2 powder obtained by drying from UF6 is described in International Application WO 00 / 30978, hereafter referred to as WO 00 / 30978. This method does not involve grinding, sieving and mixing steps, but still produces a non-negligible fine particle content during spray drying.

[0018] Finally, International Application WO 2019 / 038497, hereafter referred to as Patent Document 2, describes a method that makes it possible to avoid the formation and dispersion of fine particles during the production of nuclear fuel or transmutation targets, which method comprises subjecting an aqueous suspension containing UO powder and, optionally, PuO powder and / or powders of oxides of minor actinides, to cryogenic granulation, followed by freeze-drying the resulting granules and then directly compressing them into pellets. This method undoubtedly has many advantages, including the fact that it results in obtaining oxide particles with outstanding physicochemical properties while limiting the risk of dispersion of fine particles, but it does not completely eliminate the risk, since cryogenic granulation is carried out on an aqueous suspension containing one or more oxide powders, a preparation that can be a source of dispersion. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] WO00 / 30978 [Patent Document 2] WO2019 / 038497 Summary of the Invention [Problem to be solved by the invention]

[0020] The inventors therefore have an object to provide a novel method for producing powders comprising one or more actinide oxides, which results in obtaining oxide particles having advantageous physicochemical properties similar to those of the particles obtained by the method of Patent Document 2, while further reducing the risk of fine particle dispersion.

[0021] The inventors are further directed to a method that further enables: - Avoiding the limitations inherent in the preparation of actinide oxide powders, in particular the PuO2, oxalic acid precipitation, and filtration operations that can create filter clogging problems and thereby feeding problems for the furnaces used to calcinate the plutonium oxalate precipitate; - minimizing green compaction problems for the powders obtained according to the method, for example by dry compaction, by optimizing the rheological properties and structural stability of the powders obtained; - Minimizing the inherent problems of green compactibility when producing pellets from the powder obtained according to this method and, when the powder contains elements other than uranium, minimizing the scrap rate due to the uniform distribution of the various elements. [Means for solving the problem]

[0022] The present invention provides a method for producing a powder containing one or more oxides selected from uranium oxide UO2, plutonium oxide PuO2, and oxides of minor actinides, comprising: The minor actinides are selected from americium, neptunium, and curium by the following process: a) cryogenically granulating an aqueous solution containing cations selected from uranium-based cations, plutonium-based cations, and minor actinide-based cations; b) freeze-drying the granules obtained in step a); and c) calcining the granules obtained from step b); The present invention relates to a method for producing a powder comprising the steps of: DETAILED DESCRIPTION OF THE INVENTION

[0023] Thus, according to the present invention, an aqueous solution containing "precursor" cations of the oxide or mixture of oxides intended to be present in the powder is subjected to cryogenic granulation, rather than an aqueous suspension containing an oxide powder or mixture of oxide powders as in US Pat. No. 5,649,999.

[0024] Apart from meeting the above objectives, the method of the present invention also has the following advantages: The use of water as solvent is particularly advantageous, as it makes it possible to limit the use of organic products and therefore the impurities in the final powder obtained; - a simple, fast and reproducible implementation resulting in a solution that can be transported without any difficulty during step a) by simply pumping it into the injection nozzle of the cryogenic granulator; - the combined use of cryogenic granulation and freeze-drying solutions, which make it possible to obtain powders containing solid, spherical, controlled-porosity particles with good elemental uniformity and good malleability; - The option to obtain powders while avoiding the oxalic acid precipitation and filtration steps usually performed to recover uranium, and, if applicable, plutonium, from nitric solutions; and - The option of carrying out the method at industrial capacity units, taking into account the criticality and therefore the location of the equipment.

[0025] As already mentioned, step a) consists of a step of cryogenic granulation of the above-mentioned aqueous solution, which may consist, for example, of spraying or atomizing this solution in droplet form, by passing it through a nozzle and bringing the droplets thus formed into contact with a liquid at very low temperature (for example liquid nitrogen), in order to solidify them in that form - these two terms being considered synonymous in the present specification.

[0026] Such step a) can be carried out in a commercial granulation device or in a device specially prepared in a laboratory for carrying out this step. This device can consist of a peristaltic pump that conveys the aqueous solution to a nozzle and makes it possible to granulate the solution. The fine droplets formed or sprayed by the nozzle are dropped into a Dewar filled with liquid nitrogen under stirring (for example, by a magnetic bar) and are directly solidified into spheres.

[0027] In the aqueous solution subjected to step a), the cations, whether they are based on uranium, plutonium, americium, neptunium and / or curium, may associate with anions to form salt compounds and / or with organic ligands to form complexes, more particularly coordination complexes.

[0028] The aqueous solution subjected to step a) is advantageously an aqueous nitric solution (or in other words an aqueous nitric acid solution, for example with a concentration ranging from 0.5 mol / L to 15 mol / L, preferably from 1 mol / L to 8 mol / L). Under such circumstances, if uranium-based cations are present, these cations will react with uranyl UO2, which coexists with nitrate ions. 2+ cations, forming uranyl nitrate UO2(NO3)2; if plutonium-based cations are present, then these cations associate with the nitrate ion to form Pu 4+ cation, forming plutonium nitrate Pu(NO3)4, while if one or more minor actinide-based cations are present, then these cations associate with the nitrate ion to form the cation M x+and one or more nitrates M(NO3) x (wherein M represents Am, Np, or Cm, and x ranges from 3 to 6, and the value of x is M(NO3) x (determined to ensure electroneutrality of the

[0029] The aqueous solution of nitrogen is obtained in particular from a liquid-liquid extraction process, such as the PUREX process or the GANEX / EXAm process, and the concentration of this solution can be pre-adjusted by evaporation before carrying out the process of the invention.

[0030] It goes without saying that the method is not limited to the cryogenic granulation of aqueous solutions of nitrogen containing cations associated with nitrate ions, but other acidic aqueous solutions may also be suitable, such as, for example, aqueous sulfuric acid solutions in which the cations are associated with sulfate ions.

[0031] The aqueous solution subjected to step a) contains in particular actinide elements (uranium and / or plutonium and / or minor actinides) in a total concentration ranging from 5 g / L to 300 g / L.

[0032] If the aqueous solution subjected to step a) is an aqueous solution of uranium-based cations, it may contain traces of plutonium-based cations, depending on the method of the invention from which it was obtained. Conversely, if the aqueous solution subjected to step a) is an aqueous solution of plutonium-based cations, it may contain traces of uranium-based cations, depending on the method from which it was obtained.

[0033] According to the present invention, the aqueous solution subjected to step a) may also contain uranium-based cations and plutonium-based cations (but no minor actinide-based cations), in a molar (or atomic) proportion of plutonium (determined by the ratio Pu / (U+Pu)) that may range from 1% to 99%, depending on the intended use of the powder prepared (such as for scientific research purposes, experimental or industrial production of new nuclear fuels).

[0034] For example, for the production of MOX fuel for light water reactors or LWRs (pressurized water reactors and boiling water reactors), then the aqueous solution subjected to step a) preferably has a molar (or atomic) proportion of plutonium in the range of 3% to 12%, while for the production of MOX fuel intended for fast neutron nuclear reactors, or FNRs, then said aqueous solution preferably has a molar (or atomic) proportion of plutonium in the range of 15% to 40%.

[0035] If the solution contains uranium-based cations and one or more minor actinide-based cations (but no plutonium-based cations), then the molar (or atomic) ratio of minor actinides is preferably in the range of 1% to 50% (as determined by the ratio M / (U+M), where M is the minor actinide).

[0036] Furthermore, the aqueous solution subjected to step a) may contain at least one additive selected from water-soluble organic polymers, nitrogen-containing organic compounds and mixtures thereof, and this or these additives may be present in the aqueous solution within 1500s -1 Advantageously, the viscosity of the emulsion is present in an amount such that the dynamic viscosity (relative to a shear rate of 1000 mPa·s) does not exceed 1000 mPa·s, preferably does not exceed 100 mPa·s.

[0037] The advantage of using such additives lies in their ability to increase the viscosity of the solution during the cryogenic granulation process in order to control the morphology of the resulting granules.

[0038] As examples of water-soluble organic polymers, mention may be made of polyvinyl alcohol (PVA), polyethylene glycol (PEG), poly(vinyl butyral) (known by the abbreviation PVB), acrylic latex, or mixtures thereof.

[0039] Examples of nitrogen organic compounds include amide compounds and amine compounds.

[0040] Dynamic viscosity was measured at room temperature and ambient pressure (i.e., without the application of external heat and pressure other than the temperature and pressure of the surrounding atmosphere; room temperature may be a temperature of 20°C, and ambient pressure may be atmospheric pressure) using a rheometer in a cylinder-cone geometry system for 1500 s -1 Preferably, the dynamic viscosity does not exceed 100 mPa·s, which corresponds to a very fluid solution, able to flow easily through the feed pipes and spray nozzles of a cryogenic granulator.

[0041] Additionally, the solution may include one or more complex stabilizers where the uranium-based cations, plutonium-based cations, and / or one or more minor actinide-based cations are associated with organic ligands to form complexes.

[0042] Prior to step a), the method of the present invention may comprise a step of preparing a solution comprising uranium-based cations, plutonium-based cations, and / or one or more minor actinide-based cations by contacting the various ingredients of the solution in the desired proportions.

[0043] For example, aqueous solutions can be prepared by contacting and mixing various nitric solutions containing various desired elements, optionally followed by concentration by evaporation of the water to reach the desired concentration.

[0044] According to the method of the present invention, after cryogenic granulation, the granules obtained are subjected to a freeze-drying step, for example by placing them in a freeze-dryer, in order to sublimate the frozen water and preserve the shape of the granules (and in particular their spherical shape) as well as their characteristics.

[0045] At the end of freeze-drying, the residual moisture within the granules is very low, preventing the granules from drying out before baking.

[0046] When the aqueous solution used in step a) is an aqueous solution of nitric acid, the granules obtained in the freeze-drying step contain uranyl nitrate UO2(NO3) and / or plutonium nitrate Pu(NO3)4 and / or one or more nitrates M(NO3) x (M means Am, Np or Cm, x ranges from 3 to 6, and the value of x is M(NO3) x The granules contain a cation (determined to ensure electrical neutrality of the cation).

[0047] After the freeze-drying step, the method of the present invention includes a step of calcining the granules.

[0048] This calcination can be oxidative or reductive, or oxidation followed by reduction, depending on the actinide element to be retained and the desired valence adjustment.

[0049] If the granules are free of uranium-based cations, i.e., contain only plutonium-based cations, or one or more minor actinide-based cations, or a mixture of plutonium-based cations and one or more minor actinide-based cations, then calcination can be carried out in a single step, i.e., in either an oxidizing or reducing atmosphere, but preferentially in an oxidizing atmosphere.

[0050] If the granules contain uranium-based cations (alone or with other cations), then calcination can be carried out in a single step in a reducing atmosphere, but preferably in two successive steps: a first step in an oxidizing atmosphere to remove organics and produce U3O8, followed by a second step in a reducing atmosphere to convert U3O8 to UO2.

[0051] Calcination in an oxidative atmosphere may consist of a heating operation in air or an oxygen-rich atmosphere, such as an atmosphere containing 80% by volume of oxygen, at a temperature ranging from 100°C to 1200°C, preferably below 800°C, for a duration of up to 12 hours, preferably less than 4 hours. This calcination is isomorphous in that its implementation does not affect the shape of the granules subjected to this process.

[0052] Calcination in a reducing atmosphere can consist, for example, of a heating operation in argon hydride at a temperature ranging from 300° C. to 1200° C., preferably below 900° C., for a duration of up to 12 hours, preferably less than 4 hours.

[0053] According to the method of the present invention, a powder is obtained which may have the following characteristics in particular: - Uniform particle size distribution centered in the range of 5μm to 500μm; - Sufficient granule cohesion to withstand processing for pellet preparation; - Good flow properties, in particular good natural flowability; - Good compressibility; - Excellent natural sinterability; - if the powder contains several actinide elements (uranium, and / or plutonium, and / or minor actinides), a good distribution uniformity of the elements in the powder; and - Minimizing fine particles in the powder, thereby reducing the risk of scattering and contamination.

[0054] The almost perfectly spherical morphology of these granules allows very good malleability in pressure molds to obtain pellets which are subsequently sintered.

[0055] The uniformity of elemental distribution is particularly relevant for plutonium, if present. Once the powder is compressed and sintered to form MOX fuel, uniformity of plutonium distribution is extremely beneficial to the fuel's behavior in the reactor, particularly for the purpose of increasing the burn-up rate, and also promotes complete dissolution of the spent fuel during future processing operations.

[0056] According to the present invention, the powder is preferably a UO2 powder, a PuO2 powder, or a powder containing a mixture of UO2 and PuO2, such as a powder having a molar ratio Pu / (U+Pu) of 12% (LWR type) or 30% (FNR type), and preferably a powder containing a mixture of UO2 and PuO2.

[0057] The powder obtained by the method of the present invention can be used directly (ie, without the need for the addition of other raw materials) to form compacted materials, for example in the form of nuclear fuel pellets.

[0058] The present invention therefore also relates to a method for producing nuclear fuel pellets, comprising the following successive steps: i) carrying out the method for producing a powder as described above; ii) compressing the powder obtained in step i) into pellets; and iii) sintering the pellets obtained in step ii).

[0059] The compacting step ii) may consist, on the one hand, in placing the powder in a mold of a shape adapted to form one or more pellets, and, on the other hand, in subjecting the powder contained in the mold to uniaxial pressing, for example by applying pressure using a piston, which pressure may range from 150 MPa to 1000 MPa and for a duration ranging from 1 second to 10 minutes.

[0060] The sintering step iii) may consist of heating the above-mentioned pellets, for example to a temperature in the range of 1000°C to 1800°C, for a process duration of 1 hour to 8 hours, preferably 3 hours to 5 hours, under a neutral gas atmosphere, for example argon, optionally containing 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 20000 ppm.

[0061] Thus, for example, sintering of UO2 pellets can be carried out both in an atmosphere consisting of argon alone and in a mixture of argon and dry or humidified hydrogen, whereas for sintering pellets containing a mixture of UO2 and PuO2, a mixture of argon and dry or humidified hydrogen is typically used.

[0062] Alternatively, between steps i) and ii), a powder of uranium oxide, such as a powder of U3O8, a powder of PuO2, and / or a powder of at least one oxide of a minor actinide may be added to the powder obtained from step i) to adjust the target composition, if necessary.

[0063] In all cases, the nuclear fuel is preferably MOX fuel.

[0064] Other aspects and advantages of the present invention will become apparent from the following additional description of examples of preparation of mixed powders and fuel pellets according to method embodiments of the present invention.

[0065] Of course, this additional description is meant to be illustrative of the invention and not limiting. [Example]

[0066] Example 1: Preparation of UO2 / PuO2 mixed powder This example illustrates the implementation of the method of the invention for the preparation of a mixed powder containing uranium oxide UO2 and plutonium oxide PuO2 in a ratio Pu / (U+Pu) of 10% by mass, the preparation being carried out entirely inside a glove box.

[0067] A 5 mol / L aqueous nitric acid solution containing uranyl nitrate and plutonium nitrate in a ratio of approximately 10% by mass Pu / (U + Pu) ([U] = 200 g / L; [Pu] = 21 g / L), 2% by mass polyethylene glycol 3400, and having a dynamic viscosity of less than 20 mPa·s was placed in a beaker and pumped in by a peristaltic pump (flow rate of 33 mL / min, air pressure of 15 kPa) and sprayed through a nozzle that allowed the generation of small droplets of this solution.

[0068] The dynamic viscosity mentioned above was measured using an ANTON PAAR RHEOLAB QC rheometer at 1500 s -1 The measurement was carried out at a shear rate of .

[0069] Under magnetic stirring at 300 rpm, the droplets so produced were dropped into a Dewar vessel filled with liquid nitrogen, which caused the droplets to instantly freeze and form granules that retained the original shape of the droplets.

[0070] After cryogenic granulation, the granules were immediately placed in a freeze dryer for several hours to sublimate the frozen water and preserve their spherical shape. When all the water had been removed from the granules, they were then baked in an oxidizing atmosphere (80% O by volume) at 600°C for 1 hour to convert the nitrates to oxides while preserving their morphology, followed by calcination in a reducing atmosphere (e.g., in argon hydride) to reduce UO to UO. This resulted in a powder containing both UO and PuO.

[0071] The powder is prepared to be compressed into pellets prior to the sintering process.

[0072] The cryogenic granulation described above is carried out in an apparatus comprising the following components: - a beaker containing an aqueous solution, said beaker being connected to a peristaltic pump making it possible to convey the solution to the spray nozzle, the flow rate of the pump being a maximum of 2 L / h and the air pressure being 15 kPa; and - A Dewar vessel filled with liquid nitrogen, equipped with magnetic stirring (300 rpm), connected to a spray nozzle, allowing the instant freezing of the droplets of solution formed by the nozzle.

[0073] Example 2: Preparation of UO2 / PuO2 pellets This example illustrates the preparation of nuclear fuel pellets from the powder obtained in Example 1 above.

[0074] To do this, the powder was subjected to cold uniaxial pressing at 500 MPa with an external stearic acid lubricant to obtain pellets 4 mm in diameter and 4 mm in height, which were then subjected to a sintering operation for 4 hours at 1700°C in an argon atmosphere with 4% by volume hydrogen and 1200 vpm of water, the temperature of 1700°C being reached at a rate of 2°C / min in an argon atmosphere with 4% by volume dry hydrogen.

[0075] The pellets sintered in this way had a relative density of about 94-96%, maintaining a good uniformity of the elements U and Pu within the pellet (thanks to the good uniformity of these elements in the powder).

[0076] Example 3: Preparation of UO2 / PuO2 pellets In this example, pellets with a diameter of 4.5 mm and a height of 4 mm were prepared from a powder containing uranium oxide UO2 and plutonium oxide PuO2 in a ratio Pu / (U+Pu) of 10 atomic %, the powder being obtained in a manner similar to that described in Example 1 above, except that no polyethylene glycol was added to the aqueous solution subjected to the cryogenic granulation.

[0077] To obtain pellets, the powder was subjected to low-temperature uniaxial pressing at 600 MPa with external stearic acid lubrication (no internal lubrication of the granules), after which the pellets were sintered at 1700°C for 4 hours under an argon atmosphere with 4 vol% hydrogen and 1200 vpm water, the temperature of 1700°C being reached by heating at 2°C / min under an argon atmosphere with 4 vol% dry hydrogen.

[0078] The resulting pellets had a relative density of 97-98% with good uniformity of the elements U and Pu in the pellets (due to the good uniformity of these elements in the granules).

[0079] Example 4: Preparation of UO2 powder and pellets In this example, UO powder was prepared by placing an aqueous solution of 1 mol / L nitric acid containing uranyl nitrate in a beaker, drawing it in with a peristaltic pump (flow rate 40 mL / min, air pressure 30 kPa), and spraying it through a nozzle that allows the solution to produce small droplets.

[0080] The droplets thus produced were dropped into a Dewar vessel filled with liquid nitrogen under magnetic stirring at 300 rpm, thereby obtaining granules.

[0081] The granules were immediately placed in a freeze dryer for several hours. Once all the water had been removed from the granules, the (10-300 μm) granules were calcined in an oxidizing atmosphere (80% oxygen by volume) at 600 °C for 1 hour. A reduction step in an argon atmosphere with 4.3% oxygen was then carried out at 750 °C for 1 hour to reduce U3O8 to UO2.

[0082] The UO2 powder so obtained was compacted in the form of pellets 4.5 mm in diameter and 4 mm in height by uniaxial cold pressing at 500 MPa with external stearic acid lubricant (without internal lubrication of the granules), after which the pellets were sintered at 1700°C for 4 hours under an argon atmosphere with 4% by volume hydrogen and 1200 vpm of water, the temperature of 1700°C being reached by heating at 2°C / min under an argon atmosphere with 4% dry hydrogen.

[0083] The resulting pellets had a relative density of about 93%.

[0084] Example 5: Preparation of PuO2 powder This example illustrates the implementation of the method of the invention for the preparation of a powder containing plutonium oxide PuO2, the preparation being carried out entirely inside a glove box.

[0085] An aqueous solution of 1.5 mol / L nitric acid containing plutonium nitrate ([Pu] = 35 g / L) was placed in a beaker, taken in by a peristaltic pump (33 mL / min, air pressure 15 kPa) and sprayed through a nozzle allowing the generation of small droplets of the solution.

[0086] The droplets so produced were dropped into a Dewar vessel filled with liquid nitrogen under magnetic stirring at 300 rpm, whereby the droplets instantly froze and formed granules that retained the original shape of the droplets.

[0087] After cryogenic granulation, the granules were immediately placed in a freeze dryer for several hours to sublimate the frozen water and preserve the spherical shape of the granules. When all the water had been removed from the granules, they were calcined at 600°C for 30 minutes in an oxidizing atmosphere (80% by volume of O2) to convert the plutonium nitrate to PuO2.

[0088] The PuO2 powder so obtained is ready to be mixed with UO2 powder in order to obtain UO2 / PuO2 pellets.

Claims

1. Uranium oxide UO ２ , plutonium oxide PuO ２ and oxides of minor actinides, The minor actinides are selected from americium, neptunium, and curium by the following process: a) cryogenically granulating an aqueous solution containing cations selected from uranium-based cations, plutonium-based cations, and minor actinide-based cations; b) freeze-drying the granules obtained in step a); and c) calcining the granules obtained from step b); whereby a powder is obtained.

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

3. 3. The method according to claim 1 or 2, wherein the aqueous solution subjected to step a) is an aqueous nitric acid solution, and the cation is associated with nitrate ions.

4. The aqueous solution used in step a) is uranyl nitrate UO ２ (NO ３ ) ２ , plutonium nitrate Pu(NO ３ ) ４ and nitrate M(NO ３ ) ｘ wherein M is one of the minor actinides and x is an integer ranging from 3 to 6.

5. The method 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. The additive has a dynamic viscosity of 1500 s －１ 6. The method of claim 5, wherein the viscosity of the polymeric polymer is present in an amount not exceeding 1000 mPa s for a shear rate of 1000 mPa s.

7. 7. The method of claim 6, wherein the dynamic viscosity of the aqueous solution does not exceed 100 mPa·s.

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

9. The method according to any one of claims 5 to 7, wherein the nitrogenous organic compound is selected from amide compounds and amine compounds.

10. 10. The method according to any one of claims 1 to 9, wherein the aqueous solution subjected to step a) contains a total concentration of actinide elements in the range of 5 g / L to 300 g / L.

11. The method according to any one of claims 1 to 10, wherein the calcination of the granules is oxidative calcination or reduction calcination, or oxidative calcination is followed by reduction calcination in succession.

12. The powder is UO ２ Powder, PuO ２ Powder or UO ２ and PuO ２ a powder comprising a mixture of ２ and PuO ２ The method according to any one of claims 1 to 11, wherein the powder comprises a mixture of

13. The following consecutive steps: i) carrying out the method of any one of claims 1 to 12 to prepare a powder; ii) compressing the powder obtained in step i) into pellets; and iii) sintering the pellets obtained in step ii). A method for producing nuclear fuel pellets, comprising:

14. 14. The method for manufacturing nuclear fuel pellets according to claim 13, wherein the nuclear fuel is MOX fuel.