Method for dissolving a solid in particulate form

Abstract

The present invention relates to a method for dissolving a solid which is a nuclear fuel, the method comprising the following steps: (a) bringing the solid in particulate form into contact with a first treatment solution which is an aqueous nitric acid solution, whereby part of the solid dissolves in the first treatment solution and a first suspension is obtained; (b) subjecting the first suspension obtained in step a) to solid / liquid separation, whereby a solid fraction comprising solid that has not dissolved in the first treatment solution and a liquid fraction are obtained; (c) bringing the solid fraction obtained in step b) into contact with a second treatment solution which is an aqueous nitric acid solution, whereby a second suspension is obtained; and (d) if the quantity of residual solids in the second suspension is greater than a predetermined value, subjecting the second suspension to at least one additional dissolution treatment until a quantity of residual solids in the suspension obtained following said at least one additional dissolution treatment is less than or equal to the predetermined value.

Description

[0001] METHOD FOR DISSOLVING A SOLID IN PARTICULAR FORM

[0002] TECHNICAL FIELD

[0003] The present invention relates to the general field of solid dissolution and in particular to the field of dissolution of heterogeneous solids composed of more or less reactive fractions, implemented in the context of the recycling of materials and waste and, in the field of nuclear material recycling, more particularly the recycling of spent or unspent nuclear fuels.

[0004] Indeed, the present invention proposes a process for dissolving a solid in particulate form such as spent or unspent nuclear fuel and employing a set of reactors in particular of the column type in series and / or in parallel into which the treatment solution is introduced.

[0005] PREVIOUS STATE OF THE ART

[0006] Many recycling processes involve an initial step in which solid compounds are subjected to chemical attack, causing them to dissolve. Subsequent steps following this dissolution may include extraction, such as liquid-liquid extraction, using extractants suited to the compound(s) to be recovered.

[0007] In general, when implementing dissolutions of heterogeneous solids composed of more or less reactive fractions, the slow-dissolving phases, even if they may in some cases represent only a small fraction of the material to be treated, impose very long operating times on the entire process, which are not very compatible with an industrial rate.

[0008] Conversely, if the dissolution operation must be carried out within a timeframe imposed to meet industrial production rates, but not adapted to the reaction times of the slowest phases, this can lead to incomplete dissolution of the solid and material recovery yields incompatible with the downstream end of the process. For nuclear materials, these low yields can potentially lead to the accumulation of fissile material, requiring, for criticality reasons, a very unfavorable design and control of the equipment's vacancy throughout the entire process.

[0009] As a reminder, the dissolution of nuclear fuels is largely influenced by their composition. Dissolution in nitric acid will differ depending on whether the nuclear fuels are uranium-based, particularly those made from uranium oxides, known as "UOX fuels," which have a plutonium content of less than 1% after irradiation, or uranium and plutonium-based fuels from the outset, particularly fuels made from mixed uranium and plutonium oxides, known as "MOX fuels." Indeed, some of these MOX fuels, whether irradiated or not, can contain varying amounts of chemical heterogeneities resulting from the manufacturing process itself. These heterogeneities appear as islands and are characterized by a plutonium content higher than that present in the rest of the fuel.

[0010] Furthermore, the manufacture of new plutonium-based nuclear fuel can generate manufacturing waste. Such manufacturing waste can consist of plutonium oxide powders, possibly containing americium, mixed uranium-plutonium oxide (U,Pu)O2 powders, and / or MOX-type mixed fuel pellets deemed non-compliant with specifications. These powders and / or pellets may also be contained within cladding, also known as "pencils." It is known that these unirradiated materials exhibit a more resistant behavior to dissolution by nitric acid than the same materials present in irradiated fuel.

[0011] To address these problems, the industrial approach to date has been to develop faster chemical processes. However, the industrial implementation of these processes introduces additional constraints regarding the cost of equipment and the downstream management of the chemicals introduced into the process.

[0012] Several studies have been conducted to improve the dissolution rate of plutonium dioxide (PuCh) in nitric acid, for example, by adding hydrofluoric acid (HF) or species that promote the oxidation of Pu, such as Ag(II), Co(II), or Ce(IV). However, these solutions are not easily feasible from an industrial perspective because, on the one hand, the presence of HF can corrode reprocessing equipment and interfere with downstream steps. On the other hand, the presence of these oxidizing species of Pu must be sufficiently high to have an effect in solution, which presents management challenges downstream of the plant. In particular, this necessitates their quantitative separation from the main production stream, such as U and Pu, and also requires a review of the management of final waste, for example, due to its necessary incorporation into solid waste such as glass.

[0013] From the 1950s onward, a significant number of dissolvers were developed for dissolving spent UOX fuel contained in the cut sections of fuel assemblies. Depending on their operating mode, these dissolvers can be classified into two categories: batch and continuous. The advantages of continuous operation over batch operation include a high production rate, ease of process operation, and improved safety due to a reduction in the number of handling steps required during operation. These advantages led to the development in France of a continuous dissolver better suited to processing fuel from its large nuclear fleet. Of all the dissolvers proposed, only the rotary wheel dissolver has been implemented on an industrial scale.This dissolution reactor allows for continuous operation of both the fuel and nitric acid, ensuring the dissolution of spent fuel as well as the potential recovery of the cut assembly sections (the shells). This last feature makes the dissolver bulky, with moving mechanical parts that complicate its maintenance, and unsuitable for handling more refractory fuels, since the long dissolution times of these residues would then impact the entire process head.

[0014] Thus, shell management is one of the main drawbacks of the rotary dissolver. Therefore, complete separation of the spent fuel from its cladding is being considered to avoid processing the shells in the dissolver. This separation can be achieved, for example, through a heat treatment that transforms the pellets into powder and removes them from the shells. This step contributes to reducing the size of the dissolution reactor, which is then fed only with powdered fuel, excluding structural components.

[0015] Powder dissolvers in the nuclear industry have been the subject of considerable design work. However, their applications have been limited to batch operation and, in some cases, in the presence of HF to accelerate the dissolution of plutonium-containing fuels.

[0016] Continuous-mode powder dissolvers have also been developed for the food industry. One of their main objectives, in addition to the complete dissolution of the solid, is its degassing and the potential recovery of these gases. Although gas management is an important step in nuclear fuel dissolution, applying the aforementioned devices to fuel dissolution would not be the most efficient. Indeed, their design is geared towards highly soluble powders, allowing for a short residence time within the system. This proves unfavorable in the case of the slow reaction between plutonium dioxide and nitric acid, such as the current rotary dissolver at the La Hague plant, which is optimized only for UOX fuel.

[0017] Furthermore, the nitric acid solution obtained from current nuclear fuel dissolution processes contains, in the liquid phase, uranium, plutonium, minor actinides, soluble fission products, and insoluble solids, including the insoluble fission products commonly referred to as "dissolution fines." These dissolution fines are small-grained solids, typically less than 1 µm, resulting from the failure of current dissolution processes to dissolve certain fission products such as platinum group metals and / or the partial dissolution of others such as molybdenum, zirconium, or technetium.

[0018] The inventors set themselves the goal of proposing a process that does not present the disadvantages of the processes currently used for the treatment of nuclear fuel, irradiated or not, and, consequently, of proposing a process allowing an improvement in the dissolution of this fuel, in particular an improvement in the dissolution of MOX fuels comprising high local plutonium contents.

[0019] The inventors also set themselves the goal of providing a process for dissolving nuclear fuel, irradiated or not, which optimizes the dissolution of valuable compounds contained in the dissolution fines from current dissolution processes for the purpose of recycling said compounds.

[0020] The inventors also set themselves the goal of providing a dissolution process that can operate both discontinuously and continuously and that allows for improved dissolution of any type of nuclear fuel, irradiated or not, with reasonable volumes of nitric acid solution and under optimal safety conditions.

[0021] The inventors also set themselves the goal of providing a dissolution process without the addition of any chemicals other than nitric acid.

[0022] In particular, this dissolution process must be as simple, compact and flexible as possible in order to optimize maintenance and limit production downtime, while ensuring safety and criticality risk management.

[0023] DESCRIPTION OF THE INVENTION

[0024] The stated goals and others are achieved by the invention which proposes a dissolution process adapted to solids, in particular of heterogeneous composition, in powder form.

[0025] The process according to the invention aims to dissolve solid nuclear fuel, particularly in a continuous manner, in a nitric acid solution alone to allow for quantitative recovery of the fissile material in a manner that is safe with respect to criticality. The process is designed based on the content of the least reactive phase in the solid, such as plutonium dioxide in the case of spent fuel.

[0026] The process consists of various reactors, primarily column reactors, fed with the solid to be dissolved and the treatment or etching solution. Each reactor's role is to retain and dissolve the residual, soluble portion of the solid, and to remove the remaining solution, thus creating differentiated overall flows depending on whether the residual solids are retained or the solution is removed. Depending on the operating conditions (temperature, feed rate, etc.), each reactor can be dedicated to dissolving a specific, more or less reactive, phase of the solid, the dissolution rate of which under the applied operating conditions is known.

[0027] The reactors can be assembled in series or parallel as needed. Implementing such reactors offers numerous advantages, including:

[0028] - Simplified maintenance due to the absence of moving parts within each reactor / dissolver. The presented approach, composed of several reactors in parallel, also allows for less demanding maintenance of the process, since a reactor can be shut down without a significant impact on treatment capacity;

[0029] - compactness with a reduction in reactor volumes and therefore easier management of criticality through geometry;

[0030] - flexibility with regard to the recycling of a wider spectrum of fuels such as UOX, MOX, MOX RNR fuels for "Fast Neutron Reactors" irradiated or not;

[0031] - flexibility in the operation of the process with the easy paralleling of several reactors in order to adapt the process to the incoming materials and the operational constraints of the plant;

[0032] - continuous processing of powdered solids at a high rate; and

[0033] - an absence of complex large-scale chemistry. Increasing the residence time of matter in a reactor eliminates the need for more complex and faster chemistries.

[0034] Indeed, with such reactors, production rates for the refractory solids dissolution stage are improved, and the equipment volume is reduced without requiring the introduction of additional chemicals. For nuclear applications, and more specifically for the recycling of high-plutonium fuel, this also allows for better management of criticality. The invention thus makes the dissolution stage more flexible and feasible in more compact equipment, while limiting the impact on downstream processes, particularly the liquid-liquid extraction process.

[0035] The originality of the process according to the invention thus lies in the dissolution of plutonium-rich nuclear fuels with a high recovery rate in a solution initially composed of nitric acid alone, all while minimizing the risk of criticality by the size and arrangement of the different reactors, the most fissile material being located in a reduced volume element.

[0036] Furthermore, the present invention applies not only to the dissolution of solid products from the manufacture of nuclear fuel elements or from the processing of irradiated nuclear fuels but also, more generally, to any solid product of heterogeneous composition for which the retention volumes of the process during dissolution must be controlled and limited, particularly in the context of the pharmaceutical industry.

[0037] More specifically, the present invention relates to a method for dissolving a solid comprising the following steps:

[0038] a) bringing said solid in particulate form into contact, in at least a first reactor, with a first treatment solution whereby a part of the solid dissolves in the first treatment solution and a first suspension is obtained;

[0039] b) subject the first suspension obtained in step a) to a solid / liquid separation whereby a solid fraction comprising solid undissolved in the first treatment solution and a liquid fraction are obtained;

[0040] c) In a second reactor, contact the solid fraction obtained in step b) with a second treatment solution, thereby obtaining a second suspension; and

[0041] d) in the case where the quantity of residual solids in the second suspension is greater than a predetermined value, subject the second suspension to at least one additional dissolution treatment until the quantity of residual solids in the suspension obtained following this at least one additional dissolution treatment is less than or equal to said predetermined value.

[0042] More specifically, the present invention relates to a process for dissolving a solid which is a nuclear fuel, comprising the following steps:

[0043] a) bringing into contact, in at least a first reactor (DI, D2, D40), said solid in particulate form with a first treatment solution which is an aqueous solution of nitric acid by means of which a part of the solid dissolves in the first treatment solution and a first suspension is obtained;

[0044] b) subject the first suspension obtained in step a) to a solid / liquid separation whereby a solid fraction comprising solid undissolved in the first treatment solution and a liquid fraction are obtained;

[0045] (c) In a second reactor (D3, D41), contact the solid fraction obtained in step (b) with a second treatment solution, which is an aqueous solution of nitric acid, thereby obtaining a second suspension; and (d) if the amount of residual solids in the second suspension is greater than 0.1% of the initial amount of solid introduced into the process, subject the second suspension to at least one additional dissolution treatment until the amount of residual solids in the suspension obtained after this at least one additional dissolution treatment is less than or equal to 0.1% of the initial amount of solid introduced into the process.

[0046] As previously mentioned, the solid subjected to dissolution in the process of the present invention may be a manufacturing scrap or solid product from the manufacture of nuclear fuel elements, a solid product from the processing of irradiated nuclear fuels, contaminated soil, solid waste from electrical or electronic equipment, industrial waste, solid compounds from the processing of used batteries, the processing of magnets or mining processing or even an ore.

[0047] In these cases, the compound(s) present in the solid to be dissolved is / are metallic element(s). Examples of such metallic elements include mercury, gold, platinum, lead, indium, gallium, aluminum, bismuth, tin, cadmium, copper, arsenic, nickel, zinc, titanium, cobalt, manganese, palladium, radium, ruthenium, thorium, uranium, plutonium, actinium, ytterbium, erbium, terbium, gadolinium, europium, neodymium, praseodymium, cerium, cesium, strontium, and lanthanum.

[0048] In the solid to be dissolved according to the process of the invention, this metallic element can be in a metallic form (degree of oxidation of zero), in an oxidized form or in the form of an alloy.

[0049] In the process according to the invention, the solid to be dissolved is in particulate form, i.e., in the form of particles. "Particles" are understood to mean small elements, i.e., having an average size less than or equal to 500 µm, in particular less than or equal to 250 µm, especially a size less than or equal to 100 µm, and more particularly a size less than or equal to 50 µm. By way of particular example, the solid in particulate form has an average size on the order of 10 µm, i.e., 10 µm ± 5 µm. The solid to be dissolved in particulate form may be the result of a preliminary step of grinding a compact material. In other words, the process according to the present invention may include a preliminary step of grinding a compact material, thereby obtaining a particulate solid.

[0050] The process according to the present invention uses at least two different reactors and in particular more than two different reactors. Thus, the process according to the present invention can use n different reactors with n representing an integer between 3 and 45 (inclusive) and in particular between 3 and 41 (inclusive).

[0051] By "first reactor" we mean a reactor in which, in addition to a first treatment solution, the solid is introduced in particulate form and in which part of the solid dissolves under the action of the first treatment solution.

[0052] The term "second reactor" refers to a reactor in which, in addition to a second treatment solution, the solid fraction obtained from the first suspension from at least one first reactor is introduced, and in which all or part of this solid fraction dissolves under the action of the second treatment solution. The reactors used in the present invention may also be referred to as "dissolution reactors" or simply "dissolvers." These terms and expressions are equivalent and interchangeable in the present invention.

[0053] In a particular embodiment, during step a), the solid in particulate form and the first treatment solution are introduced into at least two different first reactors.

[0054] In a more specific embodiment, during step a), the solid in particulate form and the first treatment solution are introduced into two separate reactors. This scenario corresponds, in the experimental section below, to scenario 1 in which the reactors are columns.

[0055] In another, more specific embodiment, during step a), the solid in particulate form and the first treatment solution are introduced into more than two different first reactors, in particular into m different first reactors, where m represents an integer between 3 and 45 (inclusive), and in particular between 3 and 40 (inclusive). Scenario 2 in the experimental section below describes a process using 40 column-type first reactors and one column-type second reactor.

[0056] These embodiments, in which the solid to be dissolved is introduced into several first reactors in parallel, make it possible to adapt the process to the incoming materials and, ultimately, to treat large quantities of material in small volumes.

[0057] Any reactor allowing liquid / solid or liquid / liquid contact can be used within the framework of the present invention.

[0058] Advantageously, the reactors implemented in the present invention are in the form of columns. Indeed, these columns have the function of creating a differentiated flow depending on whether it is a solid or a liquid solution and of sufficiently retaining the residual solids.

[0059] In one particular embodiment, the reactors used in the invention are fluidized bed columns. In other words, the first reactor(s) and the second reactor used in the present invention are fluidized bed columns.

[0060] The first suspension obtained in step a) of the process consists of the first treatment solution and the solid, partly in dissolved form in this treatment solution and partly in undissolved form.

[0061] Step b) of the process according to the present invention therefore aims to obtain, from the first suspension obtained in step a), a solid fraction corresponding to the part of the solid not dissolved in the first treatment solution and a liquid fraction corresponding to the first treatment solution in which a part of the solid is dissolved.

[0062] Any technique and means known to a person skilled in the art for achieving solid / liquid separation may be used in step a) of the process according to the present invention. By way of illustration and not limitation, decantation, centrifugation, and filtration may be cited as techniques that can be used for this solid / liquid separation. By way of illustration and not limitation, solid / liquid separators may be cited as sieves, strainers, decanters, hydrocyclones, or filters such as vacuum filters, tangential flow filters, pressure filters, compression filters, or gravity filters.

[0063] Step b) of the process according to the present invention can be carried out inside the first reactor(s).

[0064] Alternatively, step b) of the process according to the present invention is carried out outside the first reactor(s). In this alternative, the means used for solid / liquid separation, i.e., the solid-liquid separator, is positioned between the first reactor(s) and the second reactor and is in fluidic connection with these reactors. Scenarios 1 and 2 in the experimental section below correspond to this configuration.

[0065] In a particular embodiment, the introduction of the solid in particulate form, the first treatment solution, the first suspension, the solid fraction obtained from the first suspension and / or the second treatment solution is carried out continuously. In a further particular embodiment, the introduction of the solid in particulate form, the introduction of the first treatment solution, the introduction of the first suspension, the introduction of the solid fraction obtained from the first suspension and the introduction of the second treatment solution are carried out continuously at identical or different introduction rates or flow rates.

[0066] Advantageously, in the process according to the invention, the introduction of the solid in particulate form into the first reactor(s) is carried out at a speed of between 500 mg / s 1 and 60 g. s -1 , in particular between 750 mg / s1 and 50 g. s 1 and, in particular, between 1 g. s 1 and 10 g. s -1 .

[0067] Advantageously, in the process according to the invention, the introduction of the first treatment solution into the first reactor(s) is carried out at a flow rate between 1 L / h and 250 L / h, in particular between 5 L / h and 100 L / h and, in particular, between 15 L / h and 50 L / h.

[0068] When the process according to the present invention employs at least two different first reactors, the introduction of the solid in particulate form into each first reactor can be carried out at the same or different rates. Advantageously, the introduction of the solid in particulate form into each first reactor is carried out at the same rate.

[0069] Similarly, when the process according to the present invention uses at least two different first reactors, the introduction of the first treatment solution into each first reactor can be carried out at the same or different flow rates. Advantageously, the introduction of the first treatment solution into each first reactor is carried out at the same flow rate.

[0070] Advantageously, when step b) of the process according to the invention is carried out outside the first reactor(s), the introduction of the first suspension at the level of the means used to carry out the solid / liquid separation is carried out at a rate between 10 L / h and 1000 L / h, in particular between 25 L / h and 900 L / h and, in particular, between 50 L / h and 800 L / h.

[0071] Advantageously, in the process according to the invention, the introduction of the solid fraction consisting of undissolved solid obtained following step b) into the second reactor is carried out at a rate between 1 mg.s 1 and 1 g. s ~ 1 , in particular between 5 mg / s 1 and 500 mg.s 1 and, in particular, between 10 mg / s 1 and 100 mg.s -1 Advantageously, in the process according to the invention, the introduction of the second treatment solution into the second reactor is carried out at a flow rate of between 10 mL / h and 10 L / h, in particular between 50 mL / h and 5 L / h and, in particular, between 100 mL / h and 1 L / h.

[0072] Any means for introducing a solid into a container is usable within the scope of the present invention for introducing the solid in particulate form into a first reactor. Typically, the solid in particulate form can be introduced into a first reactor by means of a screw conveyor or a dry chute. Alternatively, the material in particulate form can also be introduced into a first reactor under a flow of treatment solution or by air or liquid fluidization methods upstream of the introduction.

[0073] Everything described above for the introduction of the solid in particulate form applies mutatis mutandis to the introduction of the solid fraction into the second reactor.

[0074] Any means for introducing a liquid into a container can be used within the scope of the present invention to introduce a treatment solution into a reactor. Typically, a treatment solution is introduced into a reactor by means of a liquid feed pump and a hydrostatic pressure liquid feed means.

[0075] The term "treatment solution," also referred to as "attacking solution," means a solution suitable for reducing or oxidizing and dissolving compounds present in the solid in particulate form when in contact with it. A person skilled in the art will be able to choose, based on their knowledge and without inventive effort, the most appropriate treatment solution and dissolution times for the specific solid being dissolved.

[0076] Thus, the treatment solution that can be implemented within the framework of the present invention may be an acidic or even strongly acidic aqueous solution, or an alkaline or even strongly alkaline aqueous solution. "Strongly acidic" means an aqueous treatment solution with a pH of 5 or less, in particular less than 3 and, especially, less than 1. An acidic dissolving solution comprises one or more mineral acids. By way of example, an acidic aqueous dissolving solution may be selected from aqueous solutions of hydrochloric acid (HCl), nitric acid (HNO3), hydrofluoric acid (HF), sulfuric acid (H2SO4), phosphoric acid (H3PO4), and mixtures thereof. The acids or mixtures thereof used in the acidic or even strongly acidic treatment solution may be in the form of concentrated acids.

[0077] "Strongly alkaline" refers to an aqueous solution with a pH greater than 9, particularly greater than 11 and especially greater than 13. The term "strongly alkaline" is equivalent to and interchangeable with "strongly basic." An alkaline aqueous treatment solution contains one or more mineral bases. Examples of alkaline aqueous treatment solutions include aqueous solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (CafOH), lithium hydroxide (LiOH), cesium hydroxide (CsOH), and mixtures thereof.

[0078] The treatment solution can be aqueous, oxidizing, or reducing, depending on the compounds to be dissolved. It may contain at least one oxidizing agent or at least one reducing agent. Note that the mineral acids and mineral bases present in the treatment solution can act as either oxidizing or reducing agents. Alternatively, it may be necessary to add other oxidizing or reducing agents to the treatment solution.

[0079] A person skilled in the art is familiar with various methods of preparing such treatment solutions by diluting or mixing existing commercial compounds or compositions. They are also familiar with various methods of adjusting the pH to the desired value, if necessary, including the use of buffering additives for the solution(s).

[0080] The first and second treatment solutions used in the process according to the present invention may have identical or different compositions. In one particular embodiment, and especially when the solid to be dissolved is nuclear fuel, the first and second treatment solutions are aqueous solutions of nitric acid. More specifically, the treatment solutions used when the solid to be dissolved is nuclear fuel do not contain any other chemical compounds besides nitric acid. In other words, the treatment solutions used when the solid to be dissolved is nuclear fuel do not contain any compounds conventionally used to promote the dissolution and / or oxidation of plutonium, such as, for example, HF, Ag(II), Co(II), or Ce(IV).

[0081] In this particular embodiment, and especially when the solid to be dissolved is a nuclear fuel, the molar concentration of these treatment solutions, i.e., these aqueous solutions of nitric acid, is between 1 mol / L and 10 mol / L, and in particular between 3 mol / L and 8 mol / L. In other words, the molar concentration of nitric acid in the first treatment solution and the second treatment solution is between 1 mol / L and 10 mol / L, and in particular between 3 mol / L and 8 mol / L.

[0082] In this particular embodiment, and especially when the solid to be dissolved is a nuclear fuel, the molar concentration of the first treatment solution may be equal to, less than or greater than the molar concentration of the second treatment solution.

[0083] In a more specific embodiment, particularly when the solid to be dissolved is a nuclear fuel, the molar concentration of the first treatment solution is typically higher than the molar concentration of the second treatment solution. More specifically, the molar concentration of nitric acid in the first treatment solution is typically higher than the molar concentration of nitric acid in the second treatment solution.

[0084] Thus, as described in scenario 1 of the experimental section below, the molar concentration of nitric acid in the first treatment solution can be on the order of 3.5 mol / L (i.e., 3.5 mol / L ± 0.3 mol / L) and that of the second treatment solution on the order of 2 mol / L (i.e., 2 mol / L ± 0.2 mol / L). Alternatively, and as described in scenario 2 of the experimental section below, the molar concentration of nitric acid in the first treatment solution can be on the order of 8 mol / L (i.e., 8 mol / L ± 1 mol / L) and that of the second treatment solution on the order of 4 mol / L (i.e., 4 mol / L ± 0.5 mol / L).

[0085] In the process according to the invention, the contact between the solid in particulate form and the first treatment solution during step a) is made at a temperature between 50°C and 98°C and, for example, at a temperature of 65°C or a temperature of 95°C.

[0086] In the process according to the invention, the contact between the solid fraction and the second treatment solution during step c) is made at a temperature between 50°C and 98°C and, for example, at a temperature of 65°C or a temperature of 95°C.

[0087] In the process according to the invention, the temperature at the time of contact between the solid in particulate form and the first treatment solution, and the temperature at the time of contact between the solid fraction and the second treatment solution, may be identical or different. Advantageously, these temperatures are identical.

[0088] Typically, the first reactor(s) and the second reactor are equipped with suitable heating means to directly heat the mixture formed by the solid in particulate form and the first treatment solution and the mixture formed by the solid fraction and the second treatment solution.

[0089] Step d) is optional. Indeed, if, in the second suspension obtained at the end of step c), the quantity of residual solids, i.e. the quantity of solid not having been dissolved in the second treatment solution, is less than or equal to a predetermined value, this second suspension can be used as is and in particular be subjected to a finishing step to recover at least one compound of interest obtained following the dissolution of the solid in the treatment solutions.

[0090] This finishing step may consist of

[0091] (i) promote the dissolution and / or oxidation of the compound(s) of interest. For example, when the compound of interest is plutonium, the finishing step consists of adding a compound such as, for example, HF, Ag(II), Co(II), or Ce(IV); or

[0092] ii) extract this or these compounds of interest via, for example, a liquid / liquid extraction, using extractants suitable for the compound(s) present in the second suspension and to be recovered.

[0093] Such a finishing step can also be implemented on the liquid fraction obtained following step b) of the process according to the present invention.

[0094] However, if the amount of residual solids in the second suspension obtained after step c) of the process exceeds a predetermined value, these residual solids must be further dissolved. To do this, the second suspension is subjected to one or more additional dissolution treatments until the amount of residual solids in the suspension obtained after these additional dissolution treatments is less than or equal to this predetermined value.

[0095] Any dissolution technique known to a person skilled in the art is usable in the context of step d) of the process according to the invention.

[0096] Advantageously, the second suspension obtained at the end of step c) i.e. after the reaction of the solid fraction with the second treatment solution can be introduced into a new reactor and put into contact with a new treatment solution, which corresponds to a repetition of steps b) and c) of the process as previously defined mutatis mutandis.

[0097] A person skilled in the art will be able to determine, without inventive effort, the predetermined value of residual solids suitable and relevant for step d) of the process according to the present invention, depending on the nature of the solid to be dissolved, the compound(s) of interest to be recovered following the dissolution of this solid, and / or the operating conditions of the envisaged finishing steps. In the process according to the invention, this predetermined value typically corresponds to a quantity of residual solids in the second suspension equal to 0.1% of the initial quantity of solid introduced into the process. Since reference is made to solids, it is evident that the above percentage is a percentage expressed by mass. As previously mentioned, the dissolution process according to the invention is particularly suitable for dissolving nuclear fuels.In other words, in the dissolution process according to the invention, the solid in particulate form is a nuclear fuel.

[0098] Typically, the nuclear fuel to be dissolved by the dissolution process according to the invention comprises at least one plutonium oxide and / or at least one mixed oxide of plutonium and at least one other actinide. This other actinide is in particular selected from the group consisting of uranium, thorium, neptunium, americium, and curium.

[0099] In particular, the nuclear fuel to be dissolved by the dissolution process according to the invention is a mixed oxide of plutonium and uranium, i.e., it is a MOX fuel.

[0100] More specifically, the nuclear fuel as previously defined and to be dissolved by the dissolution process according to the invention is irradiated or non-irradiated nuclear fuel.

[0101] More specifically, the nuclear fuel as previously defined and to be dissolved by the dissolution process according to the invention is irradiated nuclear fuel.

[0102] Finally, the nuclear fuel, as previously defined and to be dissolved by the dissolution process according to the invention, may be confined within a cladding. In this case, the dissolution process according to the invention may include a preliminary cladding removal step carried out prior to the possible grinding step and to step a) of the process.

[0103] Other features and advantages of the present invention will become apparent to those skilled in the art upon reading the examples given below, which are provided by way of illustration and not limitation, with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 shows a diagram of the process according to the present invention in accordance with dissolution scenario 1. [Ac] represents the concentration of actinides in solution.

[0105] Figure 2 shows a diagram of the process according to the present invention in accordance with dissolution scenario 2. [Ac] represents the concentration of actinides in solution.

[0106] DETAILED DESCRIPTION OF SPECIFIC IMPLEMENTATION METHODS

[0107] Two scenarios were calculated to illustrate the application to the reprocessing of spent nuclear fuel, composed, among other things, of uranium oxides (quick to dissolve), plutonium oxides (very slow to dissolve) and mixed uranium and plutonium oxides with various plutonium contents.

[0108] In these simulations, the elements enabling the desired solid / liquid flow separation are fluidized bed columns. Their principle is to suspend and disperse solid particles simply by entraining a fluid fed at the bottom of the column. Knowing the characteristics of these solids and controlling the flow rates of the upward fluid through the particle bed, it is possible to achieve different fluidization regimes: for example, total confinement of the particles within the column and / or separation of the particles according to their size (with the smaller particles being carried with the fluid towards the column outlet). These columns can be arranged in parallel and / or in series.

[0109] These simulations and the associated results required the acquisition of the following basic data:

[0110] - characteristics of the materials to be dissolved: powder particle sizes, density, and uranium and plutonium content, and

[0111] - dissolution rates of these various materials under varied and controlled operating conditions (temperature, acidity, concentration of reactants).

[0112] These data were acquired for (U,Pu)Û2 compounds with varying Pu / U+Pu contents, but for each compound, the Pu content was homogeneous throughout the solid. Based on these data, a physicochemical model was developed to account for the dissolution mechanisms identified during the experiments. This physicochemical model, combined with a hydrodynamic model of the columns, enabled the two simulations detailed below.

[0113] The assumptions about input flows are based on current industrial production:

[0114] - Scenario 1: mass of spent MOX PWR fuel (10% Pu / U+Pu) produced in France per year and stored pending a process enabling the treatment of this fuel at an industrial rate of 120 tonnes / year,

[0115] - Scenario 2: processing of a MOX fuel with a high plutonium content (30% in Pu / U+Pu) at a rate equivalent to that of the current processing of spent UOX fuels at the La Hague plant of 1000 tonnes / year.

[0116] The objective is to achieve a percentage of Pu solid or dissolved solid greater than 99.9% at the process outlet, which corresponds to an example of a predetermined value as previously defined.

[0117] Scenario 1

[0118] This first scenario is dedicated to the dissolution of MOX PWR fuel, the production of which currently averages 120 tonnes per year (or 6 g.s). 1(for 8 months of continuous annual processing). The fuel, composed of a 10% mixed oxide of Pu / (U+Pu), is in the form of a powder with an average size of 10 µm and dissolves at a rate of:

[0119] - 148 pm.s 1 at 3.5 mol.L 1 of nitric acid at 65°C,

[0120] - 28 pm.s 1 at 2 mol.L 1 of nitric acid at 65°C.

[0121] The process consists of three column-type reactors (Figure 1):

[0122] - two parallel columns, DI and D2, in which the fuel is subjected to primary dissolution by 3.5 mol.L nitric acid 1 at 65°C introduced into these columns and

[0123] - another in series, D3, dedicated to the exhaustion of undissolved solids in the first two columns, fed with 2 mol.L nitric acid 1at 65°C. Columns DI and D2 correspond to first reactors and column D3 to a second reactor as previously defined.

[0124] The introduction of powder can be done by many techniques, such as by a screw conveyor, a dry chute or under flow of dissolution solution or by air or liquid fluidization methods upstream of the introduction.

[0125] The fuel is introduced into columns DI and D2, using, for example, a screw conveyor and at a speed of 3 g.s -1 .

[0126] The suspension obtained after primary dissolution is collected from columns DI and D2. It contains 251 g / L of dissolved actinides (Pu + U). It is carried over at a rate of 12 mg / s -1, up to a solid / liquid separator S for example of the filter type allowing the separation of the solid elements contained in this suspension (solid fraction) from the liquid fraction of the latter.

[0127] The solid fraction, free of dissolved actinides, is introduced into column D3 at a rate of 12 mg.s -1 .

[0128] Suitable means for conveying the suspension obtained after the primary dissolution of column DI (or column D2) to the solid / liquid separator can be any means of conveying a liquid fluid from one compartment to a separate compartment. These means include liquid feed pumps and hydrostatic pressure liquid feed methods.

[0129] Each parallel column (DI and D2) has a diameter of 200 mm and a height of 1.2 m (the height corresponds to the fluidized zone). Column or dissolver D3 has a diameter of 40 mm and a height of 360 mm. Each column is fed with fresh acid solutions at a rate of 43 L / h for columns DI and D2 and 0.4 L / h for column or dissolver D3.

[0130] The suspension obtained after the exhaustion of undissolved solids in the first two columns can be recovered at the outlet of column D3.

[0131] This type of configuration makes it possible to achieve the objective by recovering more than 99.9% of the fed solid in solution.

[0132] Scenario 2: In this second scenario, the aim is to dissolve 1000 tonnes per year of a 30% mixed oxide in Pu / (U+Pu) form as a powder with a particle size centered on 10 µm, with the process operating for 8 months per year, corresponding to a production rate of 48 g / s -1This solid is characterized by the following dissolution rates:

[0133] - l,lxl0 -4 pm.s 1 at 8 mol.L 1 of nitric acid and 95°C,

[0134] - 7.3xl0 -5 pm.s 1 at 4 mol.L 1 of nitric acid and 95°C.

[0135] Due to a higher processing rate and slower dissolution kinetics compared to the mixed oxide in Scenario 1, the number of columns and the acidity of the attack solutions are increased. This scenario consists of 41 columns (Figure 2): 40 in parallel (from DI to D40, of which only columns DI, D2, and D40 are shown) ensuring primary dissolution (majority of the dissolved solid), and one column in series (D41) dedicated to the removal of residual undissolved solids from the 40 parallel columns.

[0136] The fuel is introduced into columns DI to D40 at a rate of 1.2 g.s -1The suspension obtained after primary dissolution is collected from the DI to D40 columns. It contains 243 g / L of dissolved actinides (Pu + U). It is carried along at a rate of 40 mg / s -1 , up to a solid / liquid separator S for example of the filter type allowing the separation of the solid elements contained in this suspension (solid fraction) from the liquid fraction of the latter.

[0137] The solid fraction, free of dissolved actinides, is introduced into column D41 at a rate of 40 mg / s -1 .

[0138] Suitable means for conveying the suspension obtained after the primary dissolution of column DI (or any of columns D2 to D40) to the solid / liquid separator can be any means of conveying a liquid fluid from one compartment to a separate compartment. These means include liquid feed pumps and hydrostatic pressure liquid feed methods.

[0139] Each parallel column (DI to D40) has a subcritical diameter of 160 mm and a height of 0.77 m (the height corresponds to the fluidized zone). Each column is fed with fresh 8 mol / L nitric acid. 1 at 95°C. The D41 column or dissolver has a diameter of 70 mm and a height of 100 mm. It is fed with fresh 4 mol / L acid. 1 at 95°C.

[0140] The suspension obtained after the exhaustion of undissolved solids in the first 40 columns can be recovered at the outlet of column D41.

[0141] This type of configuration makes it possible to achieve the objective by recovering more than 99.9% of the fed solid in solution.

Claims

DEMANDS 1. A process for dissolving a solid which is a nuclear fuel, comprising the following steps: a) bringing into contact, in at least a first reactor (DI, D2, D40), said solid in particulate form with a first treatment solution which is an aqueous solution of nitric acid by means of which a part of the solid dissolves in the first treatment solution and a first suspension is obtained; b) subject the first suspension obtained in step a) to a solid / liquid separation whereby a solid fraction comprising solid undissolved in the first treatment solution and a liquid fraction are obtained; (c) In a second reactor (D3, D41), contact the solid fraction obtained in step (b) with a second treatment solution, which is an aqueous solution of nitric acid, thereby obtaining a second suspension; and (d) if the amount of residual solids in the second suspension is greater than 0.1% of the initial amount of solid introduced into the process, subject the second suspension to at least one additional dissolution treatment until the amount of residual solids in the suspension obtained after this at least one additional dissolution treatment is less than or equal to 0.1% of the initial amount of solid introduced into the process.

2. Dissolution process according to claim 1, characterized in that, during step a), said solid in particulate form and said first treatment solution are introduced into at least two different first reactors.

3. Dissolution process according to claim 1 or 2, characterized in that said first and second reactors are fluidized bed columns.

4. Dissolution process according to any one of claims 1 to 3, characterized in that the introduction of the solid in particulate form, of the first treatment solution, of the first suspension, of the solid fraction obtained from the first suspension and / or of the second treatment solution is carried out continuously.

5. Dissolution process according to any one of claims 1 to 4, characterized in that the molar concentration of nitric acid of said first treatment solution and of said second treatment solution is between 1 mol / L and 10 mol / L and in particular between 3 mol / L and 8 mol / L.

6. Dissolution process according to any one of claims 1 to 5, characterized in that the molar concentration of nitric acid of said first treatment solution is greater than the molar concentration of nitric acid of said second treatment solution.

7. Dissolution process according to any one of claims 1 to 6, characterized in that said nuclear fuel comprises at least one plutonium oxide and / or at least one mixed oxide of plutonium and at least one other actinide.

8. Dissolution process according to claim 7, characterized in that said other actinide is selected from the group consisting of uranium, thorium, neptunium, americium and curium.

9. Dissolution process according to any one of claims 1 to 8, characterized in that said nuclear fuel is a MOX fuel.

10. Dissolution process according to any one of claims 1 to 9, characterized in that said nuclear fuel is irradiated nuclear fuel.

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