METHOD FOR PREPARING TARGETS FOR THE PRODUCTION OF RADIOISOTOPES OR NUCLEAR FUEL

Abstract

The present invention relates to a method for preparing targets for the production of radioisotopes or nuclear fuels, comprising a reactive sintering step. (no figures)

Description

Title of the invention: METHOD FOR PREPARING TARGETS FOR THE PRODUCTION OF RADIOISOTOPES OR NUCLEAR FUEL

[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 (99mTc) is a radioisotope for medical use obtained from the decay of molybdenum-99 (99Mo). To date, it is used in approximately 80 to 85% of diagnoses performed each year by nuclear medicine methods. Mo can be produced either from the fission of uranium-235 (U) or by neutron activation reactions of Mo isotopes.

[0003] The fission process for producing molybdenum 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 (I), xenon-133 (Xe), and yttrium-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 in 235U. Historically, the enrichments used could reach up to approximately 90% (in this 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 by the nature of the uranium-bearing material they contain. One of the most common types consists of plates whose cladding is made of aluminum alloy and whose core is composed 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 + UA12 + UA13 + UA14 can be found in varying proportions), but they can also be uranium-molybdenum (or other) alloys or even 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 concepts plus. "Exotic" methods, such as those based 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 mm2.

[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 a flat form or after bending to accommodate different assembly geometries.

[0007] The process commonly used to manufacture these targets in the form of plates is called the "picture-frame" method. It includes a preliminary step of pressing the mixture of the 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) can 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 for the production of "Mo", a powder of the compound UA12 is increasingly used as a starting material. This aluminide contains the most uranium (81.5 wt., compared to 74.6 wt. and 68.8 wt. for UA13 and UA14, respectively) and also has the highest density (8.1 g.cm³, compared to 6.8 and 6.1 g.cm³ for UA13 and UA14, respectively), which is favorable for increasing the U charge of the targets (or fuels), and therefore the production of radioisotopes (or neutrons, if it is a fuel). Increasing the U charge of the targets is particularly desirable when using LEU-type uranium. However, the volume fraction of UAl2 in the mixture with the Al powder is usually 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. Furthermore, the particle size of the powders used and the shape of the particles can influence the defects encountered at the end of manufacturing, and therefore require [further action]. an adaptation of the rolling range (temperature, pressure, number of passes, reduction ratio per pass).

[0009] The UA12 phase does not dissolve (or dissolves very poorly) in the alkaline solutions generally used for "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 manufacture. To achieve this, this phase must be transformed as much as possible into UA13 and / or UAI4 during the wafer manufacturing process, by reaction between UA12 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 twenty hours if the temperature is only 440°C. It is also preferable that this time be accumulated during the hot rolling operations, so that the volumetric variations induced by the transformation of the UA1X particles are eliminated by the rolling process.

[0010] Thus: - the use of the "picture-frame" process leads to a limitation of the volume fraction of uranium phase in the UA1X + Al mixture, otherwise prohibitive defects will be created in the targets (porosity...) and therefore a significant scrap rate will result from 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...) and the volume fraction of the uranium phase; - the increasingly frequent use of compound UA12 as the starting uranium phase (related to the transition from HEU to LEU) requires maintaining a temperature ranging from a few hours to about twenty hours, intended to transform UA12 into UA13 + UA14.

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

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

[0013] In particular, an objective of the invention is to propose a rapid manufacturing process, that is to say, for 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 provide a method for obtaining targets free or almost free of UAl2, which in particular, in the case of targets intended for the production of "Mo", makes it possible to significantly reduce the time maintaining the temperature necessary to obtain an optimized mixture composition 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 UAl3 and / or UA14,

[0017] said process comprising a step (i) of flash sintering (SPS) of a powder P consisting of or comprising a uranium source and aluminium or an aluminium alloy to form said sintered compact.

[0018] Flash sintering, or electric current-assisted sintering (SPS, FAST, or 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 therefore of the material (generally in powder form) placed within it. In the case of electrically conductive materials, part 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 material diffusion and sintering phenomena that are significantly accelerated compared to conventional sintering (in a resistive furnace).Furthermore, since sintering takes place under load in a matrix of defined dimensions, the dimensions of the sintered object are perfectly controlled.

[0019] When the powder used is a mixture of two (or more) different materials that tend to react with each other under the effect of temperature, reactive sintering phenomena can occur. Their kinetics are again significantly higher than those observed under conventional reactive sintering conditions.

[0020] In the context of the present invention, "flash sintering" refers in particular to reactive flash sintering. Indeed, during flash sintering, the uranium source and the aluminum or aluminum alloy react together to form UAl3 and / or UAl14.

[0021] 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 UAl3 and / or UA14, and an envelope consisting of or comprising Al,

[0022] 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 UA13 and / or UA14; 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.

[0023] The aluminum is, for example, of A5 quality.

[0024] By "aluminium alloy", we mean in particular alloys 1100, 6061, AG3NET, AlFeNi.

[0025] According to a particular embodiment, the uranium source is chosen from, or comprises at least one element or compound chosen from: U; and - UA1X, for example UA12 or any U / UA12 mixture;

[0026] the uranium source being in particular UA12.

[0027] By "UA1X", we mean in particular UA12, UA13 and / or UA14, possibly with U in addition.

[0028] According to a particular embodiment, the process according to the present invention is a process for preparing a target for the production of radioisotopes.

[0029] According to a particular embodiment, the process according to the present invention is a process for preparing nuclear fuels.

[0030] 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.

[0031] 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%.

[0032] According to a particular embodiment, the particle size of the uranium and / or aluminum or aluminum alloy source is between 1 and 250 pm, in particular between 1 and 150 pm, especially between 1 and 100 pm.

[0033] By "particle size" is meant in particular the largest dimension of the particles.

[0034] This size can for example be measured by scanning electron microscopy (SEM), which is well known to those skilled in the art.

[0035] According to a particular embodiment, the particle size of the uranium source is between 1 and 150 pm, in particular between 1 and 100 pm, and / or the particle size of aluminium or aluminium alloy is between 1 and 50 pm, in particular between 1 and 45 pm.

[0036] According to a particular embodiment, which sintered compact contains less than 10%m of UAl2, in particular less than 9, 8, 7, 6, 5, 4, 3, 2 or 1%m of UAl2.

[0037] According to a particular embodiment, the sintered compact consists of UAl3 and / or UAl4, or comprises UAl3 and / or UAl4, in particular in a minimum total proportion of approximately 90%.

[0038] According to a particular embodiment, the sintered compact comprises in addition to aluminium, in particular at a rate of 40% by volume or less, for example at a rate of 30, 20 or 10% by volume or less.

[0039] 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.

[0040] 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.

[0041] According to a particular embodiment, the flash sintering of step (i) is carried out at a temperature Ta between 400 and 650°C, in particular between 500 and 600°C.

[0042] According to a particular embodiment, the temperature Ta is maintained for a period of 0 to 60 minutes, in particular from 1 to 10, 15 or 20 minutes.

[0043] According to a particular embodiment, the temperature Ta is reached by a temperature rise of between 10 and 500°C.min *, for example of about 100°C.min1.

[0044] According to a particular embodiment, step (i) is carried out under a pressure Pa between 10 and 400 MPa, in particular between 70 and 80 MPa.

[0045] According to a particular embodiment, step (i) is carried out at a temperature Ta between 400 and 650°C, in particular between 500 and 600°C, and under a pressure Pa between 10 and 400 MPa, in particular between 70 and 80 MPa, the pressure Pa being applied before the temperature Ta.

[0046] According to a particular embodiment, steps (i) and (ii) are simultaneous.

[0047] According to a particular embodiment, step (ii) is carried out by flash sintering, the steps (i) and (ii) being simultaneous.

[0048] 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 Tb between 10 and 100°C, in particular between 15 and 30°C, for example at a rate of about 100°C.min1.

[0049] According to a particular embodiment, step (i) is carried out at a temperature Ta between 400 and 650°C, in particular between 500 and 600°C, and under a pressure Pa between 10 and 400 MPa, in particular between 70 and 80 MPa, the pressure Pa being applied before the temperature Ta, and wherein step (i) is followed, prior to step (ii), by a cooling step (i'), in particular to a temperature Tb between 10 and 100°C, in particular between 15 and 30°C, the pressure Pa being released after reaching the temperature Tb.

[0050] The temperature Ta can possibly be reached and / or left by applying beforehand or afterward, respectively, one or more temperature steps, which is lower than Ta.

[0051] Similarly, the pressure Pa can possibly be reached and / or released by applying beforehand or afterward, respectively, one or more pressure steps, which is less than Pa.

[0052] 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.

[0053] 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.

[0054] Step (ii) can be carried out by any sheathing method well known to a person skilled in the art.

[0055] According to a particular embodiment, step (ii) is carried out by flash sintering, rolling or hot isostatic compression.

[0056] 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.

[0057] By closed receptacle, we mean in particular a receptacle comprising an opening closed by a lid, or a frame of which each of the opposite openings is closed by a lid.

[0058] According to a particular embodiment, the sintered compact has a thickness between 200 and 1000 pm, the thickness being for example about 500 pm.

[0059] Alternatively, the sintered compact is in the form of a cylinder with a thickness of up to one centimeter.

[0060] 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 pm, the thickness being for example about 500 pm, and a diameter between 10 mm and 500 mm, the diameter being for example about 35 mm.

[0061] According to a particular embodiment, the sheathing around the sintered compact has a thickness between 0.2 and 2 mm.

[0062] According to a particular embodiment, the radioisotope production target is intended to be irradiated in a nuclear reactor, in order to produce 99Mo (precursor of 99mTc, used in nuclear medicine) or other radioisotopes, such as iodine 131.

[0063] According to a particular embodiment, the nuclear fuel plate is intended for research reactors. DETAILED DESCRIPTION OF THE INVENTION DEFINITIONS

[0064] 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 integer taken individually 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.

[0065] 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.

[0066] For the purposes of this description, percentages refer to percentages by mass relative to the total mass of the formulation, unless otherwise specified. FIGURES

[0067] Figure 1 is a synoptic diagram of an example of a method relating to the invention, with the main steps numbered. Steps 6 and 7 are optional in this example.

[0068] Figure 2 illustrates a quantification of the phases present in the UA1X mixture, in UA12 +A1 compacts sintered at 552°C, by analysis of X diffraction data using the Rietveld method.

[0069] Figure 3 is a SEM observation of polished sections of UA12 +A1 compacts sintered at 552°C (a) 0 min, (b) 15 min. (a) The light areas correspond to UA12, the black areas to Al and the intermediate grey areas to UA13 and UA14; (b) the light areas correspond to UA13, the black areas to Al and the intermediate grey areas to UA14.

[0070] Figure 4 shows a quantification of the phases present in the UA1X mixture, in UA12 + Al compacts sintered at 577°C, by analysis of X-ray diffraction data using the Rietveld method.

[0071] Figure 5 presents an observation by SEM, in backscattered electron mode, of atomized U-5A1 particles, coated (in an Al matrix) and polished.

[0072] Fig. 6 is a SEM observation of a polished cross-section of a U-5A1 + 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).

[0073] According to a particular embodiment, the process according to the invention is carried out from a UA12 and Al mixture, for example at a sintering temperature of 552°C.

[0074] By way of illustration, the following elements are given:

[0075] Powders used and mixture of these powders (steps 1 and 2, [Fig. 1]): the powder The uranium-bearing material used is a powder of the compound UA12, 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 micrometers to approximately 150 micrometers. This powder was manually mixed with an aluminum powder with a particle size of less than 44 micrometers (commercially available from STREM Chemicals).

[0076] A 50 / 50 volume ratio was chosen 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 UA12 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 / cm³ (for a mixture considered 100% dense). The quantities used were adjusted to obtain discs with a diameter of 10 or 18 mm and a thickness of approximately 500 µm after sintering.

[0077] Performing the sintering of the compact (steps 3 and 4, [Fig. 1]): The flash sintering furnace (or SPS, for "Spark Plasma Sintering") that was used is model HPD- 10 from the manufacturer FCT System GmbH. Graphite or tungsten carbide dies and pistons were used. 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.

[0078] A pressure of 76 MPa was applied to the powder bed (i.e., 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 of 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 / mm², and the pressure was released after cooling. Table 1 summarizes the sintering temperatures and times of the UA12 + Al discs whose characteristics are described by way of illustration for this particular embodiment and for the particular embodiment described below.

[0079] Table 1: Sintering temperatures and times of the UA12 + Al discs described for this particular embodiment and for the particular embodiment below. Temperature (°C) Sintering time at selected temperature (min) 552 0 5 10 15 577 0 5

[0080] 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.

[0081] Demolding of the compact (step 5, [Fig. 1]): lining the molds with graphite foil allows for easy demolding of the compacts after sintering. Fragments of graphite sheet sometimes adhere to the surface of the samples and can be easily removed by light mechanical polishing.

[0082] Steps 6 and 7 ([Fig. 1]), which are 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#:

[0083] The good densification of the disks was verified by scanning electron microscopy (SEM) (few pores visible) on polished sections of disks thus obtained, as well as by density measurement using the Archimedes method.

[0084] It was also possible to quantify the microstructural changes also highlighted by SEM examinations and by means of X-ray diffraction (XRD) analyses followed by a refinement of the diffractograms obtained by the Rietveld method.

[0085] The results thus obtained on the four compacts sintered at 552°C are presented in [Fig. 2]. The indicator chosen to monitor the progress of the reactive sintering reactions is the relative mass fraction (in %m) of each aluminide in the mixture UA12 + UA13 + UA14 (denoted UA1X). The accuracy of this value is estimated to be within a few %m.

[0086] The results of this quantification are consistent with the microstructures observed by SEM ([Fig. 3]). Indeed, the predominant presence of UAl2 (76 wt% in the UA1X 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 only 4.5 wt%. Increasing the sintering time then leads to the formation of an increasingly large proportion of UAl4 at the expense of UAl3.

[0087] According to another particular embodiment, the process according to the invention is carried out from a UA12 and Al mixture, for example at a sintering temperature of 577°C.

[0088] By way of illustration, the following elements are given:

[0089] 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).

[0090] Figure 4 illustrates the quantification of phases in the UA1X mixture on the basis X-ray diffraction (XRD) analysis.

[0091] From the end of the temperature rise and with a holding time of 0 minutes, the fraction of UAl2 is extremely low (only 3%m) and becomes zero after 5 minutes.

[0092] According to a particular embodiment, the process according to the invention is carried out from a mixture of U-5A1 (U-Al alloy with 5%m of Al) and Al, for example at a sintering and sheathing temperature of 527°C.

[0093] By way of illustration, the following elements are given:

[0094] Powders used and mixture of these powders (steps 1 and 2, [Fig. 1]): in this mode For this particular fabrication, the uranium powder used is a U-5Al powder produced by centrifugal atomization by the Korea Atomic Energy Research Institute (KAERI), based in South Korea. This powder consists 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 / cm³ (± 0.1 g / cm³). According to X-ray diffraction analysis, it contains the aU and UA12 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 ([Fig. 5]).

[0095] Such a powder has the advantage of containing significantly more uranium than the UA12 compound. It was mixed with the same aluminum powder as that used in the two preceding specific embodiments, in a volume ratio of 40 / 60, instead of 50 / 50. The choice to use a higher volume fraction of Al (60% vol instead of 50% vol) is due to the fact that increased consumption of Al will be necessary during reactive sintering to transform the U into UA1X. Such volume proportions lead to a U charge in the mixture (assumed to be 100% dense) of approximately 5.1 g.cm³ (compared with the 3.3 g.cm³ corresponding to the UA12 + 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.

[0096] Reactive sintering (step 4, [Fig. 1]): The same pressure (76 MPa) and the same temperature ramps (100°C.min⁻¹) were used as in the two preceding specific embodiments. The sintering consisted of holding the particles at 527°C for 10 minutes. Microstructural examinations revealed high reactivity of the U-5Al₂ 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. UA₂ + 80 wt. UA₃ + 10 wt. UA₄.

[0097] Coating (steps 6 and 7, [Fig. 1]): Coating tests were carried out on 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 disc made of AG3 grade aluminum alloy (commonly used to manufacture fuel plates), with a total thickness of 0.9 mm, comprising a central part of 35 mm in diameter hollowed out to a height of 0.5 mm intended to serve as a receptacle for the disc (2), - (2) a disk 35 mm in diameter and 0.5 mm thick obtained by sintering by SPS of a U-5A1 + Al mixture (the sintered core), - (3) a 40 mm disc made of AG3 alloy, 0.5 mm thick, for use as lid.

[0098] In these tests, parts (1) and (3) were milled from a cylindrical bar of aluminum alloy. 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 above, with the holding time at 527°C set at 5 minutes. 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.

[0099] The microstructure obtained after the sheathing step, 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 ([Fig. 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 pm, the U-5A1 particles are completely transformed and the remaining quantity of Al is very small, - that the fully transformed areas correspond very predominantly to UA13 (as has been verified elsewhere by EDS), UAl4 being encountered mainly in the form of a border of about 10 pm thick at the core / sheath interface.

[0100] If there are still some U-5A1 particles partially transformed, these can easily be transformed according to the present invention by adding Al and / or maintaining the applied temperature for a longer time.

[0101] The particular embodiments described above have shown the ability of the process of the invention to ensure the reactive sintering of a UA1X+ Al mixture in order to obtain a sintered mixture containing a reduced quantity of the UA12 phase during a heat treatment cycle whose total duration (rise + plateau + descent included) does not exceed 30 minutes, for tested temperatures between 527 and 577°C. This temperature range can be extended, but avoiding approaching the melting point of Ai (approximately 660°C).

[0102] 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, for which developing lamination ranges is complex and time-consuming.

[0103] Three sets of parameters ensuring the flexibility of the process according to the invention are considered in particular below. They relate to: - the characteristics of the initial powder mixture; - the sintering conditions; - the sheathing stage.

[0104] 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 UA1X + Al mixture and its reactivity; - the method of mixing these powders; - the method of filling the mold of the SPS machine.

[0105] The second set of parameters that can be adjusted is the pressure sintering cycle, namely: - the creation of intermediate temperature and / or pressure levels; - the use of more advanced parameters such as direct or pulsed current (duration of bursts and pauses, etc.)

[0106] Reactive sintering is a thermally activated phenomenon, and therefore its rate increases with temperature. The applied pressure also influences the reaction kinetics. Adjusting these various parameters, through optimization, can favor the formation of one or the other uranium aluminide, depending on the desired composition of the final mixture. Thus, increasing the sintering temperature will favor the formation of the UA14 phase at the expense of the UA13 phase.

[0107] The third set of parameters to be adjusted relates to the sheathing step, which can be carried out by SPS but also by other methods (rolling, hot isostatic pressing, etc.). It should be noted that sheathing by SPS offers the same major advantages as sintering by SPS, namely its speed and flexibility. The important point to remember is that, regardless of the method used, this step is likely to induce a change in the characteristics of the core that has been previously sintered by SPS (change in thickness, composition, etc.). Regardless of the method If this is retained, it may be necessary to adjust all the manufacturing steps of the targets, up to and including the sheathing, to obtain a product that conforms both to the specifications of the operators of the reactors in which the targets are irradiated and to those of the plants that extract radioisotopes from the irradiated targets.

Claims

Demands

1. A method for preparing a target for the production of radioisotopes or nuclear fuels comprising a sintered compact made of or comprising UAl3 and / or UA14, 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.

2. A method according to claim 1 of preparing a target for the production of radioisotopes or nuclear fuels consisting of or comprising a core consisting of or comprising UAl3 and / or UA14, 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 aluminum or an aluminum alloy to form a sintered compact consisting of or comprising UAl3 and / or UA14; ii. a cladding step of the sintered compact obtained at the end of step (i) with aluminum or an aluminum 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 - UA1X, for example UA12 or any U / UA12 mixture; the uranium source being in particular UA12.

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 aluminium or aluminium alloy source is between 1 and 250 pm, in particular between 1 and 150 pm, in particular between 1 and 100 pm.

6. A method according to any one of the preceding claims, which comprises, prior to step (i), a step (o) of

7.

8.

9.

10. mixing of a uranium source powder and an aluminium or aluminium alloy powder to obtain the P powder, in particular by means of a three-dimensional mixer, for example of the Turbula® type, by attrition or with a mechanical hammer mixer. A method according to any one of the preceding claims, wherein: - the flash sintering of step (i) is carried out at a temperature Ta between 400 and 650°C, in particular between 500 and 600°C; and / or - step (i) is carried out under a pressure Pa between 10 and 400 MPa, in particular between 70 and 80 MPa. A method according to any one of the preceding claims, wherein steps (i) and (ii) are simultaneous. A method according to any one of the preceding claims, wherein step (ii) is carried out by flash sintering, rolling or hot isostatic compression. A method according to any one of the preceding claims, wherein the sheathing around the sintered compact has a thickness between 0.2 and 2 mm.

Images