Use of oxoacid reductant in the molten salts for a nuclear fission (RSF) reactor

Abstract

The present invention relates to the use of at least one oxoacid reducing element in a molten salt for a molten salt nuclear fission reactor (MSNR). The present invention also relates to a nuclear fission process within a molten salt nuclear fission reactor (MSNR), said process being characterized in that it is carried out within the MSNR in which an oxoacid reducing element is placed.

Description

[0001] The present invention relates to the use of at least one oxoacid reducing element in a molten salt for a molten salt nuclear fission reactor (MSNR). The present invention also relates to a nuclear fission process within a molten salt nuclear fission reactor (MSNR), said process being characterized in that it is carried out within the MSNR in which an oxoacid reducing element is placed. The oxoacid reducing element is, for example, selected from aluminum, silicon, and beryllium, and can be used alone, in a mixture, or as an alloy.

[0002] Molten salt reactors (MFRs) consist of a reactor core in which a molten salt fuel made from heavy fissile or fissile nuclei undergoes self-sustaining nuclear fission reactions, and a primary heat exchanger in which the hot fuel salt transfers heat to a coolant molten salt. In an MFR, the molten fuel salt flows from the core to a heat exchanger where it transfers its heat to the coolant molten salt before returning to the reactor vessel. The molten fuel salt enters the core at a temperature of approximately 500-550°C and exits at a higher temperature of approximately 700-800°C.The implementation of an RSF therefore involves two types of salts: on the one hand, molten combustible salt which includes a fuel based on heavy fissile or fissile nuclei and, on the other hand, a heat transfer molten salt, which does not include any fuels.

[0003] In a fuel-sulfur reactor (FSR), the molten salts comprise at least one salt, for example, a chloride or a fluoride. The salts are in a solid state (crystals) at room temperature and become liquid above their melting point. Unlike heat transfer molten salts, fuel molten salts contain nuclear material, notably in the form of actinide chlorides or fluorides. The actinide chlorides and fluorides must be synthesized before being introduced into the FSR. For example, the synthesis of actinide chlorides, particularly plutonium chloride (PuCl₃) and uranium chloride (UCl₃), is carried out from the corresponding oxides: plutonium oxide (PuO₂) and uranium oxide (U₃O₈) in the case of PuCl₃ and UCl₃, respectively.

[0004] The use of molten salt reactors (MSRs) offers numerous advantages, including improved intrinsic safety and reduced waste. MSRs use liquid fuel that can be drained into a safety tank in the event of a failure, thus significantly reducing the risk of core meltdown. Furthermore, the molten salts expand as the temperature rises, leading to a decrease in the fission reaction (negative feedback coefficients), which contributes to the inherent safety of this design. In addition, MSRs utilize fuel more efficiently and can burn existing nuclear waste, thereby reducing the amount of long-term radioactive waste. However, the development of this technology relies on the availability of structural materials resistant to contact with molten salt, as molten fuel salt is highly corrosive.

[0005] In the 1950s-1970s, the Oak Ridge National Laboratory (ORNL) Molten Salt Reactor Experiment (MSRE) research program led to the development of the Hastelloy N alloy, a nickel-based alloy with alloying elements including molybdenum (15-17 wt. Mo), chromium (6-8 wt. Cr), iron (4-6 wt. Fe), carbon (0.04-0.08 wt. C), and alloying elements such as manganese, silicon, aluminum, and titanium. A number of other alloys have been developed, including GH3535 alloys, Nb-added alloys, Al-added alloys, and Ni-W-Cr series alloys. However, no material specifically resistant to molten chlorides / fluorides has yet been identified or optimized.

[0006] The main studies conducted in these environments have revealed the complexity of corrosion mechanisms, which involve the interaction between species dissolved in the salt and the immersed materials, as well as the interaction between the gaseous atmosphere and the molten salt. Reference can be made to the thesis of Alexandre Chmakoff, entitled "Understanding corrosion mechanisms in the environment of future reactors with molten salt fuel and coolant," Materials, Université Paris-Saclay, 2023.

[0007] The synthesis of actinide chloride salts (e.g., plutonium, uranium, and thorium) is generally initiated from the oxide forms, which are usually more stable (e.g., PuO₂, UO₂, U₃O₈, and ThO₂). The metal oxide dissociates into potentially soluble metal ions and oxide ions according to the following reaction (eq. 1): M x O y → xM + 2x + yO 2 − The oxide ion (O2-) content in salt, also known as oxoacidity, plays a significant role in material corrosion. Purification of both fuel salt and heat transfer salt to remove oxide ions is necessary, adding a step to the process. Furthermore, this purification prevents the precipitation of actinide oxides.

[0008] The use of toxic chlorinated gases such as Cl 2 (g) or HCl(g) as well as the synthesis of chlorine-enriched gaseous species 37, is described in the literature in order to purify molten chloride salts. References include the articles by (1) Michelle, et al., "The Synthesis of Plutonium Trichloride by Chlorination of Plutonium Dioxide with Phosgene" (1996), (2) Snyder, C., et al., "The Chlorination of Plutonium Dioxide," United States: N.p., 1988 Web., (3) Toni Y. Karlsson, et al., "Synthesis and Thermophysical Property Determination of NaCl-PuCl3 Salts," Journal of Molecular Liquids, Volume 387, 2023, and (4) Anderson, A., Mishra, B., "Investigation of the Carbochlorination Process for Conversion of Cerium and Neodymium Oxides into Their Chlorides," J. Sustain. Metall. 1, 189-198 (2015). Reference may also be made to patent application WO 83 / 01249 A1 (ALUMINIUM PECHINEY).

[0009] The inventors have identified a way to control the concentration of oxide ions in molten salts, and thus limit the corrosion of structural metals used in RSF equipment.

[0010] Within the scope of the present invention: The expression "between ... and ..." (for example, a range of values) should be understood as including the limits (for example, the limit values ​​of that range of values); any description relating to an embodiment is applicable and interchangeable with all other embodiments of the invention; and when an element or component is included in and / or selected in a list of elements or components, it should be understood that that individual element or component may be selected and combined with other individual elements, or may be selected to constitute a subgroup of two or more explicitly listed elements or components; also, any element or component cited in a list of elements or components may be omitted from that list.

[0011] The object of the present invention relates to the use of at least one oxoacid reducing element in a molten salt for a molten salt nuclear fission reactor (MSNR). The oxoacid reducing element is used to control the oxoacidity of the molten salt. Controlling the oxoacidity of the molten salts allows for the purification of the molten salts and prevents the precipitation of actinide oxides. Thus, by controlling the oxoacidity of the molten salts, the oxoacid reducing element allows the actinides to remain in a solubilized chloride form within the molten salts, thereby enabling their use in the nuclear fission process.

[0012] The nuclear fission process described in the present invention involves reactions that generate O2- oxide ions in the salt. Furthermore, the gaseous headspace is maintained by a gas sweep that is never completely free of moisture. As previously mentioned, the O2- oxide ion content in the salt, known as oxoacidity, significantly contributes to material corrosion. Therefore, purification of the molten salts is typically necessary to maintain reaction yield and efficiency, limit premature equipment wear, and prevent the precipitation of actinide oxides. However, this purification process involves the release of toxic chlorinated gases.The inventors identified that the use of an oxoacid reducing agent during the nuclear fission process makes it possible to control the oxoacidity of the molten salts, prevent the precipitation of actinide oxides, and, in particular, limit the corrosion of the structural metals used in RSF equipment. All these technical effects linked to the use of at least one oxoacid reducing agent contribute to improved reaction efficiency and the safety of the nuclear reactor.

[0013] In the context of the present invention, the term "oxoacid reducing element" indicates an element that: when in metallic form it is capable of donating electrons (oxidation), transforming into a metallic ion and which, in the presence of oxide ions, is converted into a metallic oxide; when in ionic form or as a salt it is converted into a metallic oxide in the presence of oxide ions.

[0014] Examples of reducing elements used in the context of the present invention are aluminium (Al), silicon (Si) and beryllium (Be), these elements being in metallic form, in ionic form or in the form of salts (e.g. chlorides, fluorides, iodides or bromides).

[0015] In the case of aluminum metal, the aluminum loses electrons (either chemically or by electrolysis) to form aluminum ions (Al³⁺) which react with oxide ions present in the molten salt to form aluminum oxide (Al₂O₃). Al → Al³⁺ + 3e⁻ 3 O 2 − + 2 Al 3 + ↔ Al 2 O 3 s

[0016] Within the framework of the present invention, the oxoacid reducing element is likely to be found in various forms as described below.

[0017] When the oxoacid reducing element is aluminium and is in salt form, it can thus be found in the form M x1 Al y1 X z1 (1) according to which: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x1 ≤ 10, preferably x1= 0 or 1, 1 ≤ y1 ≤ 2, preferably y1= 1, and 1 ≤ z1 ≤ 13, preferably 2 ≤ z1 ≤ 10 or 3 ≤ z1 ≤ 8.

[0018] Thus, when the reducing element oxoacid is aluminium, it can, for example, be found in the form of Al, in the ionic form Al 3+< , in the form of a salt, for example in the form of chloride AlCl 3 , Al 2 Cl 6 , KAlCl 4 , NaAlCl 4 , or LiAlCl 4 .

[0019] When in salt form, particularly in chloride form, the reducing element aluminium oxoacid can be in liquid, solid or gaseous form.

[0020] In one embodiment, the reducing agent aluminum oxoacid is used in gaseous chloride form, for example NaAlCl4(g), Na2Al2Cl8(g), AlCl3(g) or Al2Cl6(g). Thus, according to this embodiment, the reducing agent aluminum oxoacid is injected in gaseous form into the molten salt, for example by bubbling.

[0021] When the oxoacid reducing element is silicon and is in salt form, it can thus be found in the form M x2 Si y2 X z2 (II) according to which: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x2 ≤ 10, preferably x2= 0 or 1, 1 ≤ y2 ≤ 2, preferably y2= 1, and 4 ≤ z2 ≤ 14, preferably 4 ≤ z2 ≤ 10 or 4 ≤ z2 ≤ 8.

[0022] Thus, when the reducing oxoacid element is silicon, it can, for example, be found in the form of Si, in the ionic form Si 4+< , or in the form of chloride SiCl 4 , KSiCl 5 , NaSiCl 5 , or LiSiCl 5 .

[0023] When in salt form, particularly in chloride form, the reducing element silicon oxoacid can be in liquid, solid or gaseous form.

[0024] In one embodiment, the reducing agent silicon oxoacid is used in gaseous chloride form, for example SiCl4(g). Thus, in this embodiment, the reducing agent aluminium oxoacid is injected in gaseous form into the molten salt, for example by bubbling.

[0025] When the oxoacid reducing element is beryllium and is in salt form, it can thus be found in the form M x3 Be y3 X z3 (III) according to which: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x3 ≤ 10, preferably x3 = 0 or 1, 1 ≤ y3 ≤ 2, preferably y3 = 2, and 2 ≤ z3 ≤ 12, preferably 2 ≤ z3 ≤ 6.

[0026] Thus, when the oxoacid reducing element is beryllium, it can, for example, be found in the form of Be, in the ionic form Be 2+< , or in the form of chloride BeCl 2 , KBeCl 3 , NaBeCl 3 , or LiBeCl 3 .

[0027] When in salt form, particularly in chloride form, the reducing element beryllium oxoacid can be in liquid, solid or gaseous form.

[0028] In one embodiment, the reducing agent beryllium oxoacid is used in gaseous chloride form, for example BeCl 2(g). Thus, according to this embodiment, the reducing agent aluminium oxoacid is injected in gaseous form into the molten salt, for example by bubbling.

[0029] In one embodiment, the oxoacid reducing element is metallic, for example, in the form of a metallic part positioned in the RSF reactor in contact with the salt. When the oxoacid reducing element is metallic, it may, for example, and without limitation, be in the form of a rod, bar, strand, powder, filament, granule, sheet, and / or plate.

[0030] In another embodiment, the oxoacid reducing agent is in ionic form or as a salt, preferably as a chloride, and is introduced into the reactor before or during the nuclear fission reaction. The oxoacid reducing agent is, for example, introduced in solid, liquid (or molten), or gaseous form.

[0031] When the oxoacid reducing agent is introduced into the reactor during the nuclear fission reaction, it can be introduced discontinuously, either all at once or in several stages, for example, in solid or liquid (molten) form. The oxoacid reducing agent can also be introduced continuously into the RSF reactor, for example, in gaseous form, such as by bubbling through molten salt.

[0032] In one embodiment, the oxoacid reducing agent is in ionic or salt form, preferably as a chloride, and is mixed with the fuel salt (molten or unmolten) before the latter is introduced into the RSF reactor. For example, aluminum chloride (AlCl3) is added in solid or gaseous form to the molten fuel salt just before it is introduced into the RSF reactor for the nuclear fission process. In another example, the oxoacid reducing agent is mixed with the fuel salt (molten or unmolten) to purify it. In this example, the oxoacid reducing agent can be removed before the fuel salt is introduced into the RSF reactor, or alternatively, left in place.Purification may include introducing the oxoacid element in saline form to the combustible salt, waiting for the purification reaction, waiting for the natural elimination of excess aluminum chloride, silicon or beryllium that has not reacted, and filtering the salt to separate the precipitated oxides.

[0033] In the context of the present invention, the combustible salt is broadly defined. It may, in particular, include actinide chlorides; for example, it may be a salt comprising ThCl₄, AmCl₃, PuCl₃, UCl₄, and / or UCl₃, including a combination of these three, four, or five chlorides. The combustible salt may also include NaCl and / or AlCl₃ chlorides, used in the synthesis of actinide chlorides. In one embodiment, the combustible salt comprises a mixture of NaCl-AlCl₃-UCl₃, NaCl-AlCl₃-PuCl₃, or NaCl-AlCl₃-UCl₃-PuCl₃ chlorides.

[0034] In one embodiment, the oxoacid reducing agent is in ionic form or as a salt, preferably as a chloride, and is mixed with the coolant (whether molten or not). For example, aluminum chloride (AlCl3) is added in solid or gaseous form to the molten coolant salt just before it is introduced into the RSF reactor for the nuclear fission process. In another example, the oxoacid reducing agent is mixed with the coolant (whether molten or not) to purify it. In this example, the oxoacid reducing agent can be removed before the fuel salt is introduced into the RSF reactor, or alternatively, left in place.Purification may include introducing the oxoacid element in saline form to the heat transfer salt, waiting for the purification reaction, waiting for the natural elimination of excess aluminum chloride, silicon or beryllium that has not reacted, and filtering the salt to separate the precipitated oxides.

[0035] Also, when the oxoacid reducing element is used in solid form, it may be found mixed with other salts. For example, when the solid oxoacid reducing element is AlCl3, it may be used in powder form as AlCl3-NaCl, AlCl3-NaCl-PuCl3, AlCl3-NaCl-PuCl3-UCl3, AlCl3-NaCl-UCl3, AlCl3-NaCl-ThCl4, AlCl3-NaCl-ThCl4-UCl3.

[0036] As previously indicated, these embodiments can be combined with each other. For example, within the framework of the present invention, an oxoacid reducing element in metallic form, positioned in the RSF reactor, and an identical or distinct oxoacid reducing element in ionic form or in salt form, for example chloride, can be used, which is placed in the RSF reactor before or during the nuclear fission reaction.

[0037] The oxoacid reducing element is chosen for example from aluminium, silicon and beryllium, and can be used alone, in mixture or in alloy form.

[0038] Within the framework of the present invention, the oxoacid reducing element can be used in combination with a reducing element, the latter being different from the oxoacid reducing element. The use of a reducing element in combination with an oxoacid reducing element also makes it possible to reduce the electrochemical potential of the reaction and to stabilize the oxoacid reducing element. For example, aluminum is most stable at moderately negative potentials, around -2V, compared to Cl₂ / Cl⁻.

[0039] In a preferred embodiment, this reducing element differs from the oxoacid reducing element in that it is metallic uranium. When used in combination with the oxoacid reducing element, metallic uranium can, for example, be in the form of pellets, beads, powder, filament, wire, bars, or rods. It can also be used as an alloy with the oxoacid reducing element.

[0040] According to a preferred embodiment of the present invention, the oxoacid reducing element is aluminium and / or silicon and is used in combination with uranium metal.

[0041] According to a preferred embodiment of the present invention, the oxoacid reducing element is aluminium and / or silicon and is used in combination with uranium metal in alloy form.

[0042] For example, an alloy of the oxoacid reducing element (e.g., aluminum) and the distinct reducing element (e.g., uranium) is used. The alloy may contain several of each of these species. The mass percentage of the distinct reducing element (e.g., uranium) (or of the mixture of these elements, if applicable) can vary from 5 to 95% by weight, relative to the total weight of the alloy.

[0043] According to a preferred embodiment of the present invention, a binary alloy of aluminum and uranium is used. This alloy preferably comprises (or consists of): from 1 to 99%, for example from 5 to 95% molar uranium metal, and from 1 to 99%, for example from 5 to 95% molar aluminium metal, relative to the total weight of the alloy. Such a U-A1 alloy has the advantage of a higher melting point than aluminum metal alone, and therefore better resistance to the operating temperatures of the nuclear fission process. Examples include the following alloys: Al 4 U, Al 3 U and Al 2 U.

[0044] The present invention also relates to a nuclear fission process within a molten salt nuclear fission reactor (MSF), comprising the steps of: a) place a combustible molten salt based on fissile or fissile heavy nuclei in the core of the RSF reactor; b) carry out self-sustaining nuclear fission reactions; c) circulate the hot combustible salt through a primary heat exchanger so that it transfers heat to a heat transfer molten salt; said process being characterized in that it is carried out within the RSF reactor in which an oxoacid reducing element is placed (or introduced).

[0045] The nuclear fission process of the present invention can have various objectives. These include, but are not limited to, the production of heat, the production of electricity and the transmutation of nuclear waste (including americium and plutonium in certain cases).

[0046] As previously described in the context of using an oxoacid reducing agent to reduce the oxoacidity of molten salt, examples of reducing agents placed within the RSF reactor during nuclear fission are aluminum (Al), silicon (Si), and beryllium (Be). These elements can be in metallic, ionic, or salt form, as described previously. For example, aluminum could be in the form of Al, in its ionic form Al³⁺, or as AlCl₃ chloride.

[0047] The oxoacid reducing agent can, for example, be placed (or introduced) into the RSF reactor in the fuel molten salt and / or in the coolant molten salt. It can also be placed (or introduced) into the fuel molten salt just before it is introduced into the RSF reactor for nuclear fission.

[0048] In one embodiment, the oxoacid reducing element is metallic, for example, in the form of a metallic part positioned in the RSF reactor, in contact with the salt. When the oxoacid reducing element is metallic, it may, for example, be in the form of a rod, bar, strand, powder, filament, granule, sheet, and / or plate.

[0049] According to one embodiment, the oxoacid reducing element is in ionic form or in salt form, preferably in chloride form, and is introduced into the reactor before or during the nuclear fission reaction.

[0050] According to one embodiment, the oxoacid reducing element is in ionic form, preferably in chloride form, and is mixed with the molten fuel salt before it is introduced into the RSF reactor.

[0051] According to one embodiment, the oxoacid reducing element is in ionic form, preferably in chloride form, and is mixed with the molten heat transfer salt.

[0052] The oxoacid reducing element can, for example, be chosen from aluminium, silicon and beryllium, and can be used alone, in mixtures or as an alloy.

[0053] The oxoacid reducing element is preferably aluminium.

[0054] When the oxoacid reducing agent is in ionic form, it can be introduced into the RSF reactor or the molten salts in solid or liquid form. When the oxoacid reducing agent is introduced into the process in liquid form, it is preheated to a temperature at least equal to its melting point. For example, when the oxoacid reducing agent is aluminum chloride and is introduced into the process of the invention in liquid form, the process includes a preheating step to a temperature of at least 120°C or at least 150°C.

[0055] As described previously, the oxoacid reducing element can be used in combination with a reducing element different from the oxoacid reducing element.

[0056] In a preferred embodiment, this reducing element differs from the oxoacid reducing element in that it is metallic uranium. When used in combination with the oxoacid reducing element, metallic uranium can, for example, be in the form of pellets, beads, powder, filament, wire, bars, or rods. It can also be used as an alloy with the oxoacid reducing element.

[0057] In the process of the present invention, an alloy of the oxoacid reducing element (e.g., aluminum) and the distinct reducing element (e.g., uranium) is preferably used, which offers the advantages described above. The technical characteristics of this alloy are the same as those mentioned previously. ASPECTS OF THE PRESENT INVENTION

[0058] The present invention relates in particular to the following aspects: Aspect 1. Use of at least one oxoacid reducing agent in a molten salt for a molten salt nuclear fission reactor (MSNR) to control the oxoacidity of the molten salt. Aspect 2. Use of at least one oxoacid reducing agent in a molten salt for a molten salt nuclear fission reactor (MSNR) to prevent the precipitation of actinide chlorides to actinide oxide during the nuclear fission reaction. Aspect 3. Use of at least one oxoacid reducing agent in a molten salt for a molten salt nuclear fission reactor (MSNR) to purify the fuel salt or the coolant salt, for example, to purify the fuel salt before its introduction into the MSNR. Aspect 4. Use according to any one of aspects 1-3, wherein the oxoacid reducing agent is in the form of a metallic component positioned in the MSNR, in contact with the salt. Aspect 5.Use according to any one of aspects 1-3, wherein the oxoacid reducing agent is in ionic or salt form, preferably as a chloride, and is introduced into the reactor before or during the nuclear fission reaction (e.g., in solid, liquid, or gaseous form). Aspect 6. Use according to any one of aspects 1-3, wherein the oxoacid reducing agent is in ionic or salt form, preferably as a chloride, and is mixed with the fuel salt (melted or unmelted) before the latter is introduced into the RSF reactor (e.g., in solid, liquid, or gaseous form). Aspect 7. Use according to any one of aspects 1-3, wherein the oxoacid reducing agent is in ionic or salt form, preferably as a chloride, and is mixed with the heat transfer salt (melted or unmelted) (e.g., in solid, liquid, or gaseous form). Aspect 8.Use according to any one of the preceding aspects, according to which the oxoacid reducing element is chosen from aluminium, silicon and beryllium, and can be used alone, in mixture or in alloy form. Aspect 9. Use according to any one of the preceding aspects, where the oxoacid reducing element is aluminium and is in the form of a salt (1): M x1 Al y1 X z1 (1) where: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x1 ≤ 10, preferably x1 = 0 or 1, 1 ≤ y1 ≤ 2, preferably y1 = 1, and 1 ≤ z1 ≤ 13, preferably 2 ≤ z1 ≤ 10 or 3 ≤ z1 ≤ 8. Aspect 10.Use according to any one of the preceding aspects, according to which the reducing element oxoacid is aluminium, and is found in the form of Al, in the ionic form Al 3+< , in the form of salt, for example in the form of chloride AlCl 3 , Al 2 Cl 6 , KAlCl 4 , NaAlCl 4 , or LiAlCl 4 . Aspect 11. Use according to any one of aspects 1-8, whereby the oxoacid reducing element is beryllium and is in the form of salt (III): M x3 Be y3 X z3 (III) whereby: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x3 ≤ 10, preferably x3 = 0 or 1, 1 ≤ y3 ≤ 2, preferably y3 = 2, and 2 ≤ z3 ≤ 12, preferably 2 ≤ z3 ≤ 6. Aspect 12.Use according to any one of aspects 1-8 and 11, according to which the reducing oxoacid element is beryllium, and is found in the form of Be, in the ionic form Be 2+< , or in the form of chloride BeCl 2 , KBeCl 3 , NaBeCl 3 , or LiBeCl 3 . Aspect 13. Use according to any one of aspects 1-8, wherein the oxoacid reducing element is silicon and is in salt form, it may thus be in the form M x2 Si y2 X z2 (II) wherein: M is an alkali metal and / or an alkaline earth metal, preferably Na, K and / or Li, X is a halogen (i.e., Cl, F, I and Br), preferably Cl or F, 0 ≤ x2 ≤ 10, preferably x2 = 0 or 1, 1 ≤ y2 ≤ 2, preferably y2 = 1, and 4 ≤ z2 ≤ 14, preferably 4 ≤ z2 ≤ 10 or 4 ≤ z2 ≤ 8. Aspect 14. Use according to any one of the preceding aspects, wherein when it is in salt form, especially in the form of chloride, the reducing element aluminium oxoacid is in liquid, solid or gaseous form.Aspect 15. Use according to any one of the preceding aspects, wherein the oxoacid reducing element is used in combination with a reducing element other than the oxoacid reducing element. Aspect 16. Use according to the preceding aspect, wherein the reducing element other than the oxoacid reducing element is uranium metal. Aspect 17. Use according to aspect 15 or 16, wherein the oxoacid reducing element is aluminum and / or silicon and is used in combination with uranium metal. Aspect 18. Use according to any one of aspects 15-17, wherein the oxoacid reducing element is aluminum and / or silicon and is used in combination with uranium metal in alloy form. Aspect 19.A nuclear fission process within a molten salt nuclear fission reactor (MSFR), comprising the steps of: a) placing a fuel molten salt based on fissile or fissile heavy nuclei in the core of the MSFR; b) carrying out self-sustaining nuclear fission reactions; c) circulating the hot fuel salt through a primary heat exchanger so that it transfers heat to a heat transfer molten salt; said process being characterized in that it is carried out within the MSFR in which an oxo-acid reducing element is placed. Aspect 20. A process according to aspect 19, wherein the oxo-acid reducing element is in the form of a metallic component positioned within the MSFR, in contact with the salt. Aspect 21. A process according to aspect 19 or 20, wherein the oxoacid reducing element is selected from aluminium, silicon and beryllium, and may be used alone, in a mixture or as an alloy. Aspect 22.A process according to any one of aspects 19-21, wherein the oxoacid reducing element is used in combination with a separate reducing element. Aspect 23. A process according to aspect 22, wherein the separate reducing element is uranium metal. Aspect 24. A process according to any one of aspects 19-23, wherein the oxoacid reducing element is aluminum and / or silicon and is used in combination with uranium metal. Aspect 25. A process according to any one of aspects 19-24, wherein the oxoacid reducing element is aluminum and / or silicon and is used in combination with uranium metal in the form of an alloy. Aspect 26. A process according to any one of aspects 19-25, wherein the oxoacid reducing element is in the form of an alloy with a reducing element, preferably an aluminum-uranium alloy. Aspect 27.Use or process according to any of the preceding aspects, wherein the reducing oxoacid element is aluminium, for example Al (metal), Al 3+< , an aluminium chloride (solid, liquid, gas), in particular AlCl 3 , Al 2 Cl 6 , KAlCl 4 , NaAlCl 4 , or LiAlCl 4 , preferably Al (metal), Al 3+< or AlCl 3 .

Claims

1. Use of at least one oxoacid reducing element in a molten salt for a molten salt nuclear fission reactor (MSR) to control the oxoacidity of the molten salt.

2. Use according to claim 1, wherein the oxoacid reducing element is in the form of a metallic piece positioned in the RSF in contact with the salt.

3. Use according to claim 1, wherein the oxoacid reducing element is in ionic form or in salt form, preferably in chloride form, and is introduced into the reactor before or during the nuclear fission reaction.

4. Use according to claim 1, wherein the oxoacid reducing element is in ionic form or in salt form, preferably in chloride form, and is mixed with the fuel salt before the latter is introduced into the RSF reactor.

5. Use according to claim 1, wherein the oxoacid reducing element is in ionic form or in salt form, preferably in chloride form, and is mixed with the heat transfer salt.

6. Use according to any one of the preceding claims, wherein the oxoacid reducing element is selected from aluminium, silicon and beryllium, and can be used alone, in mixture or in alloy form.

7. Use according to any one of the preceding claims, wherein the oxoacid reducing element, preferably an aluminum and / or silicon salt, is used in combination with a reducing element different from the oxoacid reducing element, preferably uranium metal.

8. Use according to the preceding claim, wherein the reducing element differs from the oxoacid reducing element is uranium in alloy form.

9. A nuclear fission process within a molten salt nuclear fission reactor (MSFR), comprising the steps of: a) placing a fuel molten salt based on fissile or fissile heavy nuclei into the core of the MSFR; b) carrying out self-sustaining nuclear fission reactions; c) circulating the hot fuel salt through a primary heat exchanger so that it transfers heat to a heat transfer molten salt; said process being characterized in that It is carried out within the RSF reactor in which an oxoacid reducing element is placed.

10. A method according to claim 9, wherein the oxoacid reducing element is in the form of a metallic part positioned in the RSF reactor, in contact with the salt, and is in the form of an alloy with a reducing element, preferably an aluminum-uranium alloy.

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