Method for preparing radioisotope or nuclear fuel production targets

Abstract

The present invention relates to a method for preparing targets for the production of radioisotopes or nuclear fuels comprising a reactive sintering step.

Description

[0001] The present invention relates to a method for preparing targets for the production of radioisotopes or nuclear fuels comprising a reactive sintering step.

[0002] Technetium-99m (99m<Tc) is a medical radioisotope obtained from the decay of molybdenum-99 (99<Mo). Currently, it is used in approximately 80 to 85% of diagnoses performed annually using nuclear medicine techniques. 99<Mo can be produced either from the fission of uranium-235 (235<U) or by neutron activation reactions of molybdenum isotopes.

[0003] The fission process for producing 99< Mo is by far the most widespread because its yield is significantly higher than that achieved by activation methods. It also has the advantage of allowing the simultaneous production of other isotopes used in nuclear medicine, such as iodine-131 (131< I), xenon-133 (133< Xe), and yttrium-90 (90< Y). It is implemented in so-called "experimental" or "research" nuclear reactors (as opposed to power-generating reactors) and is based on the irradiation of "targets" containing uranium enriched to 235< U. Historically, the enrichments used could reach up to approximately 90% (in which case it is referred to as highly enriched uranium, or HEU for "High Enriched Uranium").However, in order to comply with the nuclear non-proliferation treaty, efforts are currently being made internationally to use targets with a maximum enrichment of 20% (low-enriched uranium, or LEU for "Low Enriched Uranium").

[0004] There are different types of targets dedicated to the production of radioisotopes by fission. They are distinguished primarily by their geometry and the nature of the uranium-bearing material they contain. One of the most common types consists of plates with an aluminum alloy cladding and a core made up of a dispersion of uranium-based particles within an aluminum-based matrix. The uranium-bearing particles most often contain uranium and aluminum (in the form of uranium aluminide(s), which we will call UAlx and in which the phases U + UAl2 + UAl3 + UAl4 can be found in varying proportions), but they can also be uranium-molybdenum (or other) alloys or compounds such as uranium silicides (mainly U3Si2). Another type of target frequently encountered is in the form of tubes containing a sheet of a uranium-based material (such as metallic uranium).Other more "exotic" concepts based for example on the irradiation of liquid solutions containing uranium have also been studied, mainly on a laboratory scale.

[0005] The most common geometry of the targets is that of plates, typically about 1.3 - 1.5 mm thick, for a surface area of ​​about 30-50 x 100-200 mm 2<.

[0006] Similar plates, typically about 1.3–1.5 mm thick, with a surface area of ​​approximately 50–70 x 600–750 mm², can be used as nuclear fuel for research reactors. Such plates can be used in their flat form or after bending to accommodate different assembly geometries.

[0007] The process commonly used to manufacture these targets in plate form is called the "picture-frame" method. It includes a preliminary step of pressing the mixture of two powders (UAlx or other + Al) that will constitute the combustible core. This core is then placed in an aluminum alloy frame, and this assembly is sheathed by two plates, also made of aluminum alloy. The sheathing requires several stages of hot rolling (generally between 440 and 550°C, depending on the type of Al alloy used for the sheath) followed by cold rolling. An additional heat treatment (also between 440 and 550°C) may also be carried out before cold rolling. During the various temperature maintenance stages, reactions can occur between the uranium particles and the aluminum matrix, altering the composition of the combustible core.This is particularly the case when the core contains UAlx particles, because their reactivity towards the Al matrix is ​​high.

[0008] To manufacture targets with a 99< Mo content, a powder of the compound UAl 2 is increasingly used as a starting material. This aluminide has the highest uranium content (81.5 wt., compared to 74.6 wt. and 68.8 wt. for UAl 3 and UAl 4, respectively) and also the highest density (8.1 g.cm⁻³, compared to 6.8 and 6.1 g.cm⁻³ for UAl 3 and UAl 4, respectively). This is advantageous for increasing the uranium content of the targets (or fuels), and therefore the production of radioisotopes (or neutrons, if it is a fuel). Increasing the uranium content of the targets is particularly desirable when using LEU-type uranium. The volume fraction of UAl 2 in the mixture with Al powder is, however, most often limited to about 45% (50% vol being a value currently considered maximum by manufacturers), because fatal defects can appear in the targets during rolling when this fraction becomes too high.Indeed, there is no longer enough Al to accommodate the mechanical deformations. In addition, the particle size of the powders used and the shape of the particles can affect the defects encountered at the end of manufacturing, and therefore require an adaptation of the rolling range (temperature, pressure, number of passes, reduction ratio per pass).

[0009] The UAl₂ phase does not dissolve (or dissolves very poorly) in the alkaline solutions generally used for 99< Mo extraction. Therefore, it is recommended to have as little of it as possible (on the order of a few percent at most) in the targets after their fabrication. To achieve this, this phase must be transformed as much as possible into UAl₃ and / or UAl₄ during the wafer fabrication process, through a reaction between UAl₂ and Al: the total holding time of the target at temperature should be on the order of 6 to 10 hours at a temperature of 540-550°C and can reach up to about 20 hours if the temperature is only 440°C. It is also preferable that this holding time be cumulative during the hot rolling operations, so that the volumetric variations induced by the transformation of the UAl₂ particles are eliminated by the rolling process.

[0010] So : The use of the "picture-frame" process leads to a limitation of the volume fraction of uranium phase in the UAlx + Al mixture, otherwise it risks creating prohibitive defects in the targets (porosity, etc.) and therefore a significant scrap rate following the rolling operations; more generally, this manufacturing process may require a sometimes heavy development phase of the rolling range, depending on the characteristics of the starting powders (shape, particle size, etc.) and the volume fraction of uranium phase; the increasingly frequent use of the UAl 2 compound as the starting uranium phase (related to the transition from HEU to LEU) requires holding at temperature for a period ranging from a few hours to about twenty hours, intended to transform UAl 2 into UAl 3 + UAl 4.

[0011] These various limitations have repercussions on the production times and costs of the targets as well as on the production yield of the 99< Mo.

[0012] One objective of the invention is therefore to provide a method which does not present the aforementioned disadvantages.

[0013] In particular, one objective of the invention is to propose a rapid manufacturing process, that is to say, one in which the time required to obtain the targets is significantly reduced compared to the prior art, and this under moderate temperature and pressure conditions.

[0014] Another objective of the invention is to propose a process enabling the obtaining of targets free or almost free of UAl 2, which makes it possible in particular, in the case of targets intended for the production of 99< Mo, to significantly reduce the holding time at temperature necessary to obtain an optimized composition of the mixture for the extraction of this isotope, by chemical dissolution after irradiation.

[0015] Yet another objective of the invention is to propose a process which can quickly and easily adapt to the characteristics of the initial uranium powder (composition, particle size, etc.) and thus optimize the uranium charge of the powder mixture, and therefore, where applicable, the production yield of radioisotopes. SUBJECT OF THE INVENTION

[0016] Also, the invention relates to a method for preparing a target for the production of radioisotopes or nuclear fuels comprising a sintered compact made of or comprising UAl 3 and / or UAl 4, said method comprising a step (i) of flash sintering (SPS) of a powder P made of or comprising a uranium source and aluminium or an aluminium alloy to form said sintered compact.

[0017] Flash sintering, also known as electric current-assisted sintering (SPS), field-assisted sintering technique (FAST), or pulsed electric current sintering (PECS), is a sintering method that typically combines the effects of applying uniaxial pressure and a high-intensity, low-voltage pulsed current. This current passes through the matrix (made of a conductive material, often graphite or tungsten carbide) and induces rapid heating of the matrix by Joule heating, and consequently of the material (generally in powder form) placed within it. In the case of electrically conductive materials, some of the current passes through the powder bed itself. The effects induced at local scales are still poorly understood. In any case, they result in significantly accelerated diffusion and sintering phenomena compared to conventional sintering (in a resistive furnace).Furthermore, since sintering takes place under load in a matrix of defined dimension, the dimensions of the sintered object are perfectly controlled.

[0018] When the powder used is a mixture of two (or more) different materials that tend to react with each other under the influence of temperature, reactive sintering phenomena can occur. Their kinetics are again significantly higher than those observed under conventional reactive sintering conditions.

[0019] In the context of this invention, "flash sintering" refers specifically to reactive flash sintering. In flash sintering, the uranium source and the aluminum or aluminum alloy react together to form UAl3 and / or UAl4.

[0020] According to one embodiment, the process of the present invention is a process for preparing a target for the production of radioisotopes or nuclear fuels consisting of or comprising a core consisting of or comprising UAl 3 and / or UAl 4, and a shell consisting of or comprising Al, said process comprising the following steps: (i) a flash sintering step (SPS) of a powder P consisting of or comprising a uranium source and aluminium or aluminium alloy to form a sintered compact consisting of or comprising UAl 3 and / or UAl 4, (ii) a step of coating the sintered compact obtained at the end of step (i) with aluminium or aluminium alloy to form the target.

[0021] The aluminum, for example, is of A5 quality.

[0022] The term "aluminium alloy" includes alloys such as 1100, 6061, AG3NET, and AlFeNi.

[0023] According to a particular embodiment, the uranium source is chosen from, or comprises at least one element or compound chosen from: U; and UAlx, for example UAl 2 or any mixture U / UAl 2; the uranium source being in particular UAl 2 .

[0024] By "UAlx", we mean in particular UAl 2, UAl 3 and / or UAl 4, possibly with U in addition.

[0025] According to a particular embodiment, the process according to the present invention is a process for preparing a target for the production of radioisotopes.

[0026] According to a particular embodiment, the process according to the present invention is a process for preparing nuclear fuels.

[0027] According to a particular embodiment, the uranium or aluminium or aluminium alloy source comprises less than 10% by mass of silicon and typically less than 1% by mass of silicon.

[0028] According to a particular embodiment, the volume of the uranium source powder relative to the total volume of the powder P is between about 30 and about 70%, in particular between about 40 and 50%.

[0029] According to a particular embodiment, the particle size of the uranium and / or aluminium or aluminium alloy source is between 1 and 250 µm, in particular between 1 and 150 µm, especially between 1 and 100 µm.

[0030] By "particle size", we mean in particular the largest dimension of the particles.

[0031] This size can, for example, be measured by scanning electron microscopy (SEM), which is well known to those skilled in the art.

[0032] According to a particular embodiment, the particle size of the uranium source is between 1 and 150 µm, in particular between 1 and 100 µm, and / or the particle size of aluminium or aluminium alloy is between 1 and 50 µm, in particular between 1 and 45 µm.

[0033] According to a particular embodiment, which sintered compact contains less than 10%m of UAl 2, in particular less than 9, 8, 7, 6, 5, 4, 3, 2 or 1%m of UAl 2.

[0034] According to a particular embodiment, the sintered compact consists of UAl 3 and / or UAl 4, or comprises UAl 3 and / or UAl 4, in particular in a minimum total proportion of approximately 90%m.

[0035] According to a particular embodiment, the sintered compact includes in addition to aluminium, in particular at a level of 40% by volume or less, for example at a level of 30, 20 or 10% by volume or less.

[0036] According to a particular embodiment, the process of the invention includes, prior to step (i), a step (o) of mixing a uranium source powder and an aluminum or aluminum alloy powder to obtain the powder P, in particular by means of a three-dimensional mixer, for example of the Turbula ® type, by attrition or with a mechanical pestle mixer.

[0037] According to a particular embodiment, the sintered compact has the shape of a cylinder, in particular a straight cylinder, more particularly a right circular cylinder, or a parallelepiped, in particular a rectangular parallelepiped.

[0038] According to a particular embodiment, the flash sintering of step (i) is carried out at a temperature T a between 400 and 650°C, in particular between 500 and 600 °C.

[0039] According to a particular embodiment, the temperature T a is maintained for a period of 0 to 60 minutes, in particular from 1 to 10, 15 or 20 minutes.

[0040] According to a particular embodiment, the temperature Ta is reached by a temperature rise of between 10 and 500°C.min -1< , for example of about 100°C.min -1< .

[0041] According to a particular embodiment, step (i) is carried out under a pressure P a between 10 and 400 MPa, in particular between 70 and 80 MPa.

[0042] According to a particular embodiment, step (i) is carried out at a temperature T a between 400 and 650°C, in particular between 500 and 600°C, and under a pressure P a between 10 and 400 MPa, in particular between 70 and 80 MPa, the pressure P a being applied before the temperature T a.

[0043] According to a particular embodiment, steps (i) and (ii) are simultaneous.

[0044] According to a particular embodiment, step (ii) is carried out by flash sintering, steps (i) and (ii) being simultaneous.

[0045] According to a particular embodiment, the process according to the present invention is a process as defined above in which: Steps (i) and (ii) are simultaneous, or step (i) is followed, prior to step (ii), by a cooling step (i'), in particular to a temperature T u between 10 and 100°C, in particular between 15 and 30°C, for example at a rate of about 100°C.min -1.

[0046] According to a particular embodiment, step (i) is carried out at a temperature T a between 400 and 650°C, in particular between 500 and 600°C, and under a pressure P a between 10 and 400 MPa, in particular between 70 and 80 MPa, the pressure P a being applied before the temperature T a, and wherein step (i) is followed, prior to step (ii), by a cooling step (i'), in particular to a temperature T b between 10 and 100°C, in particular between 15 and 30°C, the pressure P a being released after reaching the temperature T b.

[0047] The temperature T a can possibly be reached and / or left by applying beforehand or afterward, respectively, one or more temperature steps, which is lower than T a.

[0048] Similarly, the pressure P a can possibly be reached and / or released by applying beforehand or afterward, respectively, one or more pressure steps, which are less than P a.

[0049] According to a particular embodiment, the powder P is placed in a flash sintering chamber (or mold), which is lined with at least one sheet of graphite.

[0050] According to a particular embodiment, step (i) is followed, prior to step (ii), by a demolding step of the sintered compact obtained by flash sintering.

[0051] Step (ii) can be carried out by any method of sheathing well known to a person skilled in the art.

[0052] According to a particular embodiment, step (ii) is carried out by flash sintering, rolling or hot isostatic compression.

[0053] According to a particular embodiment, the sintered compact has the shape of a cylinder, in particular a straight cylinder, more particularly a right circular cylinder, or a parallelepiped, in particular a rectangular parallelepiped, and in which step (ii) is carried out by flash sintering, through the formation of a closed receptacle of shape and dimensions adapted to said sintered compact, forming the sheath of the sintered compact.

[0054] A closed receptacle is understood to include, in particular, a receptacle comprising an opening sealed by a lid, or a frame in which each of the opposite openings is sealed by a lid.

[0055] According to a particular embodiment, the sintered compact has a thickness between 200 and 1000 µm, the thickness being for example about 500 µm.

[0056] Alternatively, the sintered compact is in the form of a cylinder with a thickness of up to one centimeter.

[0057] According to a particular embodiment, the sintered compact has the shape of a cylinder, in particular a straight cylinder, more particularly a straight circular cylinder, or a parallelepiped, in particular a rectangular parallelepiped, said cylinder having a thickness between 200 and 1000 µm, the thickness being for example about 500 µm, and a diameter between 10 mm and 500 mm, the diameter being for example about 35 mm.

[0058] According to a particular embodiment, the sheathing around the sintered compact has a thickness between 0.2 and 2 mm.

[0059] According to a particular embodiment, the radioisotope production target is intended to be irradiated in a nuclear reactor, with a view to producing 99< Mo (precursor of 99m< Tc, used in nuclear medicine) or other radioisotopes, such as iodine 131.

[0060] According to a particular embodiment, the nuclear fuel plate is intended for research reactors. DETAILED DESCRIPTION OF THE INVENTION DEFINITIONS

[0061] As understood here, ranges of values ​​in the form of "xy" or "from x to y" or "(between) x and y" include the bounds x and y, the integers between these bounds, and all other real numbers between these bounds. For example, "1-5", or "from 1 to 5", or "between 1 and 5" refers to the integers 1, 2, 3, 4, and 5, as well as all other real numbers between 1 and 5. Preferred embodiments include each individual integer within the range of values, as well as any subcombination of these integers and any set of real numbers between these integers. As an example, preferred values ​​for "1-5" may include the integers 1, 2, 3, 4, 5, 1-2, 1-3, 1-4, 1-5, 2-3, 2-4, 2-5, etc.

[0062] As used in this description, the term "approximately" refers to a range of values ​​within ±10% of a specific value. For example, the expression "approximately 20" includes values ​​within 20 ± 10%, that is, values ​​from 18 to 22.

[0063] For the purposes of this description, percentages refer to percentages by mass relative to the total mass of the formulation, unless otherwise stated. FIGURES

[0064] There [ Fig 1 ] is a synoptic diagram of an example of a process that is the subject of the invention, with the main steps numbered. Steps 6 and 7 are optional in this example. The [ Fig 2 ] illustrates a quantification of the phases present in the UAlx mixture, in UAl 2 +Al compacts sintered at 552°C, by analysis of X-ray diffraction data using the Rietveld method. The [ Fig 3] is a SEM observation of polished sections of UAl₂+Al compacts sintered at 552°C (a) 0 min, (b) 15 min. (a) The light areas correspond to UAl₂, the dark areas to Al, and the intermediate gray areas to UAl₃ and UAl₄; (b) the light areas correspond to UAl₃, the dark areas to Al, and the intermediate gray areas to UAl₄. The [ Fig 4 ] presents a quantification of the phases present in the UAlx mixture, in UAl 2 + Al compacts sintered at 577°C, by analysis of X-ray diffraction data using the Rietveld method. The [ Fig 5 ] presents a SEM observation, in backscattered electron mode, of atomized U-5Al particles, coated (in an Al matrix) and polished. The [ Fig 6 ] constitutes an observation by SEM of a polished cross section of a U-5Al + Al disk sintered and sheathed by SPS at 527°C, (a) general view, (b) examination at higher magnification of the area framed in red on image (a).

[0065] According to a particular embodiment, the process according to the invention is carried out from a mixture of UAl 2 and Al, for example at a sintering temperature of 552°C.

[0066] The following elements are given as examples: Powders used and mixture of these powders (steps 1 and 2, figure 1 )The uranium powder used is a powder of the compound UAl₂, obtained by grinding small ingots (mass: a few grams) of this compound, produced by arc smelting. The particle morphology is angular, due to the fragile nature of this compound. Their size ranges from a few µm to approximately 150 µm. This powder was manually mixed with an aluminum powder with a particle size of less than 44 µm (commercially available from STREM Chemicals). A 50 / 50 volume ratio was used for mixing the two powders. Given that their densities differ by a factor of three (8.1 g / cm³ and 2.7 g / cm³, respectively for UAl₂ and Al), the mass fraction of UAl₂ in the mixture is therefore 75% (± 2%). This results in a U charge of approximately 3.3 g.cm -3 (for a mixture considered to be 100% dense).The quantities used were adapted to obtain discs with a diameter of 10 or 18 mm and a thickness of approximately 500 µm, after sintering. Performing the sintering of the compact (steps 3 and 4, figure 1 )The flash sintering (SPS, for "Spark Plasma Sintering") furnace used was the HPD-10 model from FCT System GmbH. Graphite or tungsten carbide dies and pistons were employed. The dies were equipped with a thermocouple placed in a hole at sample height to control the temperature during the sintering cycle. For pistons with a diameter of 18 mm and larger, the temperature was checked by laser pyrometry with the probe aimed at the center of the disc. The inside of the molds was lined with graphite foil (Papyex®, marketed by MERSEN), 0.2 to 0.4 mm thick (depending on the mold), to facilitate demolding and prevent contamination of the mold by the material to be sintered. Two graphite discs were also placed between the powder bed and the pistons for the same reasons.A pressure of 76 MPa was applied to the powder bed (corresponding to a force of 6 kN for sintering 10 mm diameter pellets, or 19 kN for an 18 mm diameter). Once this pressure was reached, a temperature ramp at 100°C / min was applied until the desired sintering temperature was reached. The temperature was then held for 0 to 15 minutes. The temperature was then cooled at a rate of 100°C / min, and the pressure was released after cooling. Table 1 summarizes the sintering temperatures and times of the UAl₂ + Al discs whose characteristics are described for illustrative purposes for this particular embodiment and for the specific embodiment described below. Table 1 : temperatures and sintering times of the UAl 2 + Al disks described for this particular embodiment and for the particular embodiment below. Temperature (°C) Sintering time at the chosen temperature (min) 552 0 5 10 15 577 0 5 It is recalled that the hot rolling and annealing temperatures used in the prior art picture-frame process are generally between 440 and 550°C. Demolding the compact (step 5, figure 1 ) The lining of the molds with graphite foil allows for easy demolding of the compacts after sintering. Fragments of graphite foil sometimes adhere to the surface of the samples and can be easily removed by light mechanical polishing. Steps 6 and 7 ( figure 1 ), optional, were not carried out during this series of tests and will be illustrated in a third particular manufacturing embodiment. Characteristics of the compacts obtained :

[0067] The good densification of the disks was verified by scanning electron microscopy (SEM) (few visible pores) on polished sections of the resulting disks, as well as by density measurement using Archimedes' method. Furthermore, it was possible to quantify the microstructural changes also revealed by SEM examinations and by means of X-ray diffraction (XRD) analyses followed by refinement of the diffractograms obtained using the Rietveld method. The results thus obtained on the four compacts sintered at 552°C are presented in Figure 2 . The indicator chosen to monitor the progress of the reactive sintering reactions is the relative mass fraction (in wt%) of each aluminide in the mixture UAl₂ + UAl₃ + UAl₄ (denoted UAlₓ). The accuracy of this value is estimated to be within a few wt%. The results of this quantification are consistent with the microstructures observed by SEM. ( Figure 3 ).Indeed, the predominant presence of UAl₂ (76 wt% in the UAl₂x mixture) at 0 minutes and its absence at 15 minutes are confirmed. Moreover, after only 2 minutes of holding at 552°C, its content is already reduced to just 4.5 wt%. Increasing the sintering time then leads to the formation of an increasingly large proportion of UAl₄ at the expense of UAl₃.

[0068] According to another particular embodiment, the process according to the invention is carried out from a mixture of UAl 2 and Al, for example at a sintering temperature of 577°C.

[0069] For illustrative purposes, the following elements are given: The powders used and the manufacturing steps of the compacts are identical to those described previously. The only difference is the sintering temperature, namely 577°C instead of 552°C. Two sintering dwell times at 577°C are considered: 0 minutes and 5 minutes (see Table 1). The Figure 4illustrates the quantification of phases in the UAlx mixture based on XRD analyses. From the end of the temperature rise and with a holding time of 0 minutes, the fraction of UAl 2 is extremely low (only 3 wt%) and becomes zero after 5 minutes.

[0070] According to a particular embodiment, the process according to the invention is carried out from a mixture of U-5Al (U-Al alloy with 5%m of Al) and Al, for example at a sintering and sheathing temperature of 527°C.

[0071] The following elements are given as examples: Powders used and mixture of these powders (steps 1 and 2, figure 1 ) :In this particular embodiment, the uranium powder used is a U-5Al powder manufactured by centrifugal atomization by the Korea Atomic Energy Research Institute (KAERI), based in South Korea. This powder is in the form of spherical particles with a median diameter of approximately 60 µm (measured by laser granulometry). Its density, determined by helium pycnometry, is 13.6 g.cm⁻³ (± 0.1 g.cm⁻³). According to X-ray diffraction analysis, it contains the αU and UAl₂ phases in the following respective mass proportions: 63 wt. and 37 wt. (values ​​determined to ± 5 wt.). Its microstructure is as solidified and is characterized by the presence of fine UAl₂ dendrites in a U matrix. ( Figure 5 ).This powder has the advantage of containing significantly more uranium than the UAl₂ compound. It was mixed with the same aluminum powder used in the two preceding specific embodiments, in a volume ratio of 40 / 60, instead of 50 / 50. The choice of using a higher Al volume fraction (60% vol instead of 50% vol) is due to the fact that increased Al consumption will be required during reactive sintering to convert U to UAl₂. Such volume proportions result in a U charge in the mixture (assumed to be 100% dense) of approximately 5.1 g.cm⁻³ (compared with the 3.3 g.cm⁻³ corresponding to the UAl₂ + Al mixture, in 50 / 50 volume proportions). The quantities used were adapted to obtain discs with a diameter of 10 to 35 mm and a thickness of approximately 500 µm, after sintering. Reactive sintering (step 4, figure 1 ) :The same pressure (76 MPa) and the same temperature ramps (100°C.min⁻¹) were used as in the two preceding specific embodiments. Sintering consisted of holding at 527°C for 10 minutes. Microstructural examinations revealed high reactivity of the U-5Al particles with the Al matrix, although there were sometimes significant variations in the extent of reactions at the local level, related to the availability of Al in the immediate environment of the uranium-bearing particles. According to XRD analyses, the aluminide fractions obtained at the end of this step were approximately as follows: 10 wt% UAl₂ + 80 wt% UAl₃ + 10 wt% UAl₄. Core strengthening (steps 6 and 7, figure 1 ) : Tests were carried out on coating 35 mm diameter discs obtained after the reactive sintering described above. The various constituent elements of the final object (4) are as follows: (1) a 40 mm diameter disc made of AG3 grade aluminum alloy (commonly used to manufacture fuel plates), with a total thickness of 0.9 mm, having a central part with a diameter of 35 mm hollowed out to a height of 0.5 mm intended to serve as a receptacle for the disc (2), (2) a disc with a diameter of 35 mm and a thickness of 0.5 mm obtained by SPS sintering of a U-5Al + Al mixture (the sintered core), (3) a 40 mm diameter disc made of AG3 alloy, with a thickness of 0.5 mm, for use as a lid. In these tests, parts (1) and (3) were milled from a cylindrical aluminum alloy bar. This explains their striated surface finish. They underwent careful cleaning and pickling before the SPS coating operation, using the method employed for the prior art "picture-frame" industrial process. Parts (1), (2), and (3) were placed in the SPS machine's die and subjected to the same pressure and thermal cycle as those used for the sintering step described previously, with a holding time of 5 minutes at 527°C. The sequence of sintering and coating steps therefore corresponds, in the case described here, to a total holding time at 527°C of 10 + 5 = 15 minutes. The microstructure obtained after the sheathing stage, observed by SEM, shows very good continuity between the core and the sheath, demonstrating the ability of the SPS method to ensure the welding of the core and the sheath. ( figure 6 ). It also shows: that the core thickness increased from approximately 500 µm before sheathing to approximately 700 µm after sheathing, that on either side of this central part of the core, over a thickness of approximately 250 µm, the U-5Al particles are completely transformed and the remaining quantity of Al is very small, that the completely transformed areas correspond predominantly to UAl 3 (as has been verified elsewhere by EDS), UAl 4 being encountered mainly as a rim approximately 10 µm thick at the core / sheath interface.

[0072] If there are still some U-5Al particles that are still partially transformed, these can easily be transformed according to the present invention by adding Al and / or by maintaining the applied temperature for a longer time.

[0073] The specific embodiments described above have demonstrated the ability of the process of the invention to perform reactive sintering of a UAl X + Al mixture to obtain a sintered mixture containing a reduced quantity of the UAl 2 phase during a heat treatment cycle whose total duration (including rise, plateau, and fall) does not exceed 30 minutes, for tested temperatures between 527 and 577°C. This temperature range can be extended, while avoiding approaching the melting point of Al (approximately 660°C).

[0074] As is well known to those skilled in the art, degrees of freedom allow for rapid optimization of this process. This is a significant advantage compared to the picture-frame process, where developing lamination ranges is complex and time-consuming.

[0075] Three sets of parameters ensuring the flexibility of the process according to the invention are considered below. They relate to: the characteristics of the initial powder mixture; the sintering conditions; the coating stage.

[0076] The following parameters concerning the initial powder mixture can be adjusted first: the shape and particle size of the particles of this powder and of the Al powder which will influence both the homogeneity of the UAlx + Al mixture and its reactivity; the method of mixing these powders; the method of filling the mold of the SPS machine.

[0077] The second set of parameters that can be adjusted is the pressure sintering cycle, namely: the achievement of intermediate temperature and / or pressure levels; the use of more advanced parameters such as direct or pulsed current (pulse and pause duration, etc.)

[0078] Reactive sintering is a thermally activated process, and therefore progresses more rapidly at higher temperatures. The applied pressure also influences the reaction kinetics. Adjusting these parameters, through optimization, can favor the formation of one or the other uranium aluminide, depending on the desired composition of the final mixture. For example, increasing the sintering temperature will promote the formation of the UAl₄ phase at the expense of the UAl₃ phase. The third set of parameters to adjust concerns the cladding stage, which can be performed by SPS (Selective Pressure Sachets) or other methods (rolling, hot isostatic pressing, etc.). It should be noted that SPS cladding offers the same major advantages as SPS sintering, namely its speed and flexibility.The key point to remember is that, regardless of the method used, this step is likely to induce changes in the characteristics of the core that has been previously sintered by SPS (changes in thickness, composition, etc.). Regardless of the method chosen, it may be necessary to adjust all the manufacturing steps of the targets, including the cladding, to obtain a product that conforms to both the specifications of the reactor operators in which the targets are irradiated and those of the facilities that extract radioisotopes from the irradiated targets.

Claims

1. A process for preparing a radioisotope production target comprising a sintered compact made of or comprising UAl3 and / or UAl4, said process comprising a step (i) of flash sintering (SPS) of a powder P made of or comprising a uranium source and aluminium or an aluminium alloy to form said sintered compact, which contains less than 10%m of UAl2.

2. A method according to claim 1 for preparing a radioisotope production target consisting of or comprising a core consisting of or comprising UAl3 and / or UAl4, and a shell consisting of or comprising Al, said method comprising the following steps: (i) a flash sintering step (SPS) of a powder P consisting of or comprising a uranium source and aluminium or aluminium alloy to form a sintered compact consisting of or comprising UAl3 and / or UAl4, which contains less than 10%m of UAl2; (ii)a step of coating the sintered compact obtained at the end of step (i) with aluminium or an aluminium alloy to form the target.

3. A method according to any one of the preceding claims, wherein the uranium source is selected from or comprises at least one element or compound selected from: - U; and - UAlx, for example UAl2 or any U / UAl2 mixture; the uranium source being in particular UAl2.

4. A method according to any one of the preceding claims, wherein the volume of the uranium source powder relative to the total volume of the powder P is between about 30 and about 70%, in particular between about 40 and 50%.

5. A method according to any one of the preceding claims, wherein the particle size of the uranium and / or aluminum or aluminum alloy source is between 1 and 250 µm, in particular between 1 and 150 µm, especially between 1 and 100 µm.

6. A method according to any one of the preceding claims, comprising, prior to step (i), a step (o) of mixing a uranium source powder and an aluminum or aluminum alloy powder to obtain powder P, in particular by means of a three-dimensional mixer, for example of the Turbula type ® , by attrition or with a mechanical pestle mixer.

7. A method according to any one of the preceding claims, wherein: - the flash sintering of step (i) is carried out at a temperature T a between 400 and 650°C, in particular between 500 and 600°C; and / or - step (i) is carried out under a pressure P a between 10 and 400 MPa, in particular between 70 and 80 MPa.

8. A method according to claim 1, said method comprising a step (i) of flash sintering (SPS) of a powder P consisting of or comprising a uranium source and aluminium or aluminium alloy to form a sintered compact consisting of or comprising UAl3 and / or UAl4, containing less than 10%m of UAl2, which is sheathed by aluminium or aluminium alloy to form the target.

9. Method according to claim 2 or 8, wherein the sheathing is carried out by flash sintering, rolling or hot isostatic compression.

10. Method according to claim 2, 8 or 9, wherein the sheathing around the sintered compact has a thickness between 0.2 and 2 mm.

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