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WO2025231330A1 - Séparation de déchets de sels fluorés et immobilisation sous forme de déchets en cermet, halmet et sous forme de composites liés au verre - Google Patents

Séparation de déchets de sels fluorés et immobilisation sous forme de déchets en cermet, halmet et sous forme de composites liés au verre

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Publication number
WO2025231330A1
WO2025231330A1 PCT/US2025/027437 US2025027437W WO2025231330A1 WO 2025231330 A1 WO2025231330 A1 WO 2025231330A1 US 2025027437 W US2025027437 W US 2025027437W WO 2025231330 A1 WO2025231330 A1 WO 2025231330A1
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Prior art keywords
composition
solid
metal
fluoride
glass
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English (en)
Inventor
Jie Lian
Saurabh Sharma
Junhua Shen
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Rensselaer Polytechnic Institute
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Rensselaer Polytechnic Institute
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
    • B09B—DISPOSAL OF SOLID WASTE NOT OTHERWISE PROVIDED FOR
    • B09B3/00—Destroying solid waste or transforming solid waste into something useful or harmless
    • B09B3/40—Destroying solid waste or transforming solid waste into something useful or harmless involving thermal treatment, e.g. evaporation
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C1/00—Reactor types
    • G21C1/04—Thermal reactors ; Epithermal reactors
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C1/00—Reactor types
    • G21C1/04—Thermal reactors ; Epithermal reactors
    • G21C1/24—Homogeneous reactors, i.e. in which the fuel and moderator present an effectively homogeneous medium to the neutrons
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C19/00—Arrangements for treating, for handling, or for facilitating the handling of, fuel or other materials which are used within the reactor, e.g. within its pressure vessel
    • G21C19/28—Arrangements for introducing fluent material into the reactor core; Arrangements for removing fluent material from the reactor core
    • G21C19/30—Arrangements for introducing fluent material into the reactor core; Arrangements for removing fluent material from the reactor core with continuous purification of circulating fluent material, e.g. by extraction of fission products deterioration or corrosion products, impurities, e.g. by cold traps
    • G21C19/307—Arrangements for introducing fluent material into the reactor core; Arrangements for removing fluent material from the reactor core with continuous purification of circulating fluent material, e.g. by extraction of fission products deterioration or corrosion products, impurities, e.g. by cold traps specially adapted for liquids

Definitions

  • TECHNICAL FIELD [ 0003] The present disclosure relates to structures, methods of recycling and sequestering waste products from fluoride containing molten salt compositions used in molten salt nuclear fission reactors.
  • BACKGROUND [ 0004] Effective salt waste management is important for the development of advanced fuel cycles and molten salt technologies for modern fission reactors.
  • a molten salt reactor (MSR) is a Gen IV fission reactor having unique safety and reliability, high energy efficiency and effective utilization of nuclear resources. The overall fuel and coolant chemistry of an MSR depends on different MSR designs and types.
  • complex waste streams are expected including volatile off-gas, salt-based waste components, separated salt streams, carbon- and metal-based waste streams, and wastes from operation and decommission.
  • the salt wastes cover a wide range of waste elements across the periodic table, which can be categorized into groups including alkali- and alkaline-earth halides, transition metal halides, fission products including Cs and Sr, and lanthanides, and actinides.
  • Such waste form materials typically display either low waste loadings, e.g., 3.8 wt% fluorine in apatite and/or low chemical durability, e.g., in TeO 2 glass.
  • Typical glasses cannot accommodate large amounts of fluorides or chlorides.
  • Iron-phosphate glass vitrified from spent salt wastes is considered as a durable matrix to immobilize salt wastes and mitigate the issue of immiscibility of halides in borosilicate glass.
  • the loss of volatile halides is inevitable due to their high volatility, particularly for fluoride salt-contained Fe-P glasses, as vitrification typically requires a high temperature (e.g., 1000 o C).
  • a method is disclosed of treating a molten salt reactor waste comprising metal fluorides.
  • the method includes processing the molten salt reactor waste to form a solid fluoride composition, mixing the solid fluoride composition with a sinterable composition, and sintering the solid fluoride composition with the sinterable composition by a sintering process to provide a metal composite if the sinterable composition includes a metal powder, the metal composite comprising the solid fluoride composition as a 31248341.1 2 105024-201 phase within a metal matrix.
  • the metal powder includes copper, aluminum, or stainless steel.
  • the sinterable composition includes a glass-forming material
  • the sintering process provides a glass-bonded composite comprising the solid fluoride composition as a phase within a glass matrix.
  • the glass-forming material includes SiO 2 .
  • the glass-forming material includes SiO 2 and B2O3.
  • the sintering process can be a cold sintering process performed in the presence of a transient solvent at less than about 100 °C.
  • the transient solvent can be added at between about 1 volume % to about 5 volume %.
  • the transient solvent can be water or methanol.
  • the sintering process can include field-assisted sintering technologies such as spark plasma sintering. Spark plasma sintering can be performed at a temperature of 400 °C or less. Other sintering technologies can also be applied, including hot pressing and hot isostatic pressing (HIP).
  • the solid fluoride composition and the sinterable composition are combined at a weight percentage of less than about 70% fluoride composition.
  • the solid fluoride composition is combined at a weight percentage of between about 30% and about 70% fluoride composition.
  • processing the molten salt reactor waste to form a solid fluoride composition includes mixing the molten salt waste with water to form a first solid enriched in water-insoluble metal fluorides, and a first solution enriched in water- soluble metal ion fluorides, separating the first solution from the first solid, and processing the first solution to form the solid fluoride composition.
  • the water-soluble metal fluorides include CsF, wherein Cs is present as a radioactive isotope, and the solid fluoride composition is formed by a procedure which includes evaporating the first solution to form a second solid, mixing the second solid with an organic solvent to form a solution of CsF and a third solid, depleted in CsF, and processing the solution of CsF to form the solid fluoride composition.
  • the solid fluoride composition comprises CsF, and is formed by evaporating the organic solvent from the solution of CsF.
  • the solid fluoride composition comprises a metal fluoride salt.
  • the solid fluoride composition comprises a metal halide perovskite (MHP), formed by mixing a metal fluoride salt with 31248341.1 3 105024-201 H 2 SiF 6 in the presence of SiO 2 and solvent to form the solid fluoride composition comprising the MHP as a metal salt of the hexafluorosilicate anion.
  • MHP metal halide perovskite
  • the metal fluoride salt is selected from the group consisting of alkali metal halides, alkaline earth metal halides, and combinations thereof.
  • the metal fluoride salt is CsCl or SrCl2.
  • the solvent is water.
  • the metal fluoride salt is CsF and the solvent is methanol, acetone, dimethylformamide, or mixtures thereof.
  • the solid fluoride composition comprises a cesium halide perovskite, formed by mixing the solution of CsF with H2SiF6 in the presence of SiO 2 to form the solid fluoride composition comprising the cesium halide perovskite as a cesium salt of a hexafluorosilicate anion.
  • the organic solvent is selected from the group consisting of methanol, acetone, dimethylformamide, and combinations thereof.
  • a method is disclosed of removing and sequestering an alkali metal M from a composition comprising a fluoride salt MF of the alkali metal, the method comprising: mixing the composition with a solvent to form a solution comprising dissolved MF, adding H 2 SiF 6 and SiO 2 to the solution with mixing, thereby forming a precipitate of M2SiF6, mixing the precipitate of M2SiF6 with a sinterable composition to obtain a ceramic metal composite (cermet) comprising M 2 SiF 6 sequestered within a metal matrix, if the sinterable composition is a metal powder, or to obtain a glass- bonded composite comprising M 2 SiF 6 sequestered within a glass matrix, if the sinterable composition is a glass-forming material.
  • cermet ceramic metal composite
  • the sintering process can be a cold sintering process performed in the presence of a transient solvent at less than about 100 °C.
  • the transient solvent can be added at between about 1 volume % to about 5 volume %.
  • the transient solvent can be water or methanol.
  • the sintering process can include field-assisted sintering technologies such as spark plasma sintering. Spark plasma sintering can be performed at a temperature of 400 °C or less. Other sintering technologies can also be applied, including hot pressing and hot isostatic pressing (HIP).
  • the fluoride composition and the sinterable composition are combined at a weight percentage of less than about 70% fluoride composition.
  • the fluoride composition and the sinterable 31248341.1 4 105024-201 composition are combined at a weight percentage of between about 30% and about 70% fluoride composition.
  • a method is disclosed of treating a molten salt reactor waste comprising water-soluble salts and water-insoluble salts, the water-insoluble salts comprising 7 LiF and radioactive strontium in the form of SrF2, the method comprising: mixing the molten salt reactor waste with water to form a first solid enriched in the water- insoluble salts, and a first solution enriched in the water-soluble metal salts, separating the first solution from the first solid, mixing the first solid with a solution of SrI 2 so as to obtain a second solid enriched in LiF and depleted of radioactive strontium, and a second solution having radioactive strontium ions dissolved therein, separating and drying the second solution to obtain a third solid enriched in radioactive strontium.
  • the third solid with a sinterable composition selected from the group consisting of a metal powder and a glass- forming material comprising SiO2, sintering the third solid with the sinterable composition to obtain a halide metal composite (halmet) comprising the third solid as a phase within a metal matrix, if the sinterable composition is a metal powder, or to obtain a glass-bonded composite comprising the third solid as a phase within a glass matrix, if the sinterable composition is a glass-forming material.
  • the 7 LiF in the second solid is recycled for use in a molten salt composition for a molten salt reactor.
  • metal halides from the salt reactors or as waste products upon chemical reprocessing can be directly incorporated into metal to form halmets and glass matrices to form glass-trapped halide compositions.
  • Such halmets and glass-trapped compositions can then be sintered by cold sintering at temperatures as low as 100 °C, or by low temperature field assisted sintering such as spark sintering for immobilization.
  • BRIEF DESCRIPTION OF THE DRAWINGS [ 0023] Fig. 1 provides a flow chart of the process for trapping fluoride salt wastes in a halmet or in a glassy matrix. [ 0024] Fig. 2.
  • Fig. 3 is a flow chart showing the separation of CsF from other fluoride salts prior to sequestering CsF or a cesium halide perovskite in a cermet, halmet, or glassy matrix. 31248341.1 5 105024-201 [ 0026] Fig. 4 provides a flow chart for the separation of LiF from SrF2, and subsequent sequestering or recycling of these salts or their perovskite derivatives.
  • Fig. 5 provides a flow chart of a process for handling multicomponent fluoride radioactive wastes from a molten salt reactor.
  • Fig. 6 provides XRD results for the MHP composition formed upon reacting a mixture of NaF and KF with H 2 SiF 6 and SiO 2 .
  • Fig. 7 shows the result of multiphase refinement of the XRD results of Fig. 6.
  • Fig. 8 shows thermogravimetric results obtained for the MHP composition of Fig.6.
  • Fig. 9 shows an SEM image of the MHP composition of Fig. 6.
  • Fig. 10 shows a lower magnification image of the MHP composition of Fig. 6.
  • Fig. 11 shows XRD results for Cs2SiF6 synthesized in methanol from H2SiF6 and SiO2.
  • Fig. 12 shows thermogravimetric analysis results for the Cs2SiF6 of Fig. 11.
  • Fig. 13 shows an SEM image for the Cs2SiF6 of Fig. 11.
  • Fig. 14 shows an XRD results for a cermet of 30 wt. % Cs2SiF6 and 70 wt. % copper.
  • Fig. 15 shows an SEM image of the cermet of Fig. 14.
  • Fig. 16 shows an SEM image of the cermet of Fig.
  • Fig. 17 shows the thermal diffusivity of the cermet of Fig. 14 as a function of temperature.
  • Fig. 18 shows SEM results for a 60:40 wt. % MHP:Al cermet of Cs2SiF6 in an aluminum matrix.
  • Fig. 19 shows SEM results for a sample of 30 wt. % Cs2SiF6 and 70 wt. % copper.
  • Fig. 20 shows a higher magnification SEM result for the sample of Fig. 19. DETAILED DESCRIPTION [ 0043] Definitions.
  • Sintering is a process of compacting particles of different materials to form a solid mass of fused materials without melting the materials to the point of liquefaction.
  • the 31248341.1 6 105024-201 atoms/molecules in the sintered materials diffuse across particle boundaries, fusing the particles together.
  • a “sinterable composition” is a metal powder or a glass- forming material which can be sintered with a particulate material in order to trap the particulate material in a metal or glassy matrix.
  • a “cermet” is a ceramic-metal composite material formed by a sintering process. Such materials include composites of metals with perovskite materials.
  • a “halmet” is a composite of a halide salt and a metal formed by a sintering process.
  • a “solid fluoride composition” is a composition having fluorine present in the -1 oxidation state. Such a composition includes simple fluoride salts and more complex salts, including fluoride containing perovskites.
  • a “perovskite” as used herein refers to a class of crystalline material with a crystal structure similar to that of the mineral perovskite.
  • Perovskites include alkali metal and alkaline earth salts of the hexafluorosilicate anion.
  • a “transient solvent” as used herein is a solvent added to a sinterable material prior to sintering. By partially dissolving particles in the sinterable material, the transient solvent allows stronger particle bonding at lower temperatures.
  • water or methanol are transient solvents added to a mixture of a solid fluoride composition and a sinterable composition in order to allow efficient sintering at temperatures below 100 °C.
  • Some embodiments of this technology provide methods to separate fission products from a molten salt reactor (MSR), including CsF and SrF 2 from complex salt waste streams and immobilize such fission products within matrices having high chemical durability and waste loadings.
  • MSR molten salt reactor
  • such matrices are formed through sintering processes of metal halide particles and fluoride-containing perovskite particles with sinterable materials including metals and glass-forming materials.
  • Embodiments of the current application include methods of separating metal halides including alkali metal fluorides and alkaline earth metal fluorides such as CsF, SrF2, and SrFI, although it would be understood by a person of ordinary skill in nuclear waste management that other fluoride-containing metal halides which occur as fission waste products could be treated by similar methods.
  • Fluoride-containing perovskite particles amenable to the methods describe in this application include compounds with a general formula chosen from the group consisting of A + B 2+ F 3 , A + 3 B 3+ 2 F 9 , A + 2/3/4 B 4/3/2+ F 6 , A 2 B + B 3+ F 6 31248341.1 7 105024-201 and A + 2 Na + Ln 3+ F 6, where A is Na, K, Cs or Sr and B is a multivalent metal or lanthanide (Sn, Si, Fe, Al etc.), and Ln are lanthanides. [ 0053] Trapping waste in metal and glass matrices by the methods described herein can enable very high loading, of up to 70 wt% perovskite.
  • a low temperature (room temperature) wet chemistry process is disclosed which in some embodiments provides effectively separation of NaF, KF, CsF and SrF2 into desired composites with high kinetics, low temperature below 200 o C or at room temperature, and high separation efficiency above 90 wt%.
  • the wet-chemistry process offers unique merits compared to other salt separation technologies such as oxidative precipitation, melt crystallization, dehalogenation, and phosphorylation, where high temperatures and multiple processing steps are required to separate the salt waste constituents.
  • Waste loaded metal and glass composites can be fabricated by conventional sintering, hot pressing, or advanced sintering technologies, e.g., spark plasma sintering (SPS) or hot isostatic pressing (HIP).
  • SPS spark plasma sintering
  • HIP hot isostatic pressing
  • SPS and cold sintering approaches of the present application enable the consolidation of the densified composite waste forms at temperature as low as 100 o C without any phase decompositions, reducing the energy cost typically associated with high temperature sintering and mitigating potential phase decomposition/element loss.
  • These approaches and concepts can be applied for a wide range of the salt-contained materials and different metal and glass-bonded composites (e.g. borosilicate glass).
  • metal or glass matrices in the composites act as barriers to prevent the ingress of water and the release of waste elements from the crystalline phases, and thus greatly improve the chemical durability of sequestered waste materials.
  • Metal halides, after recovery from salt reactors, or as waste products upon chemical reprocessing can be directly incorporated into metal and glass matrices, to form halmets, or glass composites.
  • a fluoride salt 105 of a radioactive metal, including CsF and SrF 2 can be directly sequestered by sintering such a fluoride salt with a metal to form a halmet material or with a glass-forming composition to form a glassy matrix having the fluoride salt trapped therein 110.
  • Suitable metals include but are not limited to copper, aluminum, stainless steel, and combinations thereof.
  • Suitable glass-forming compositions include SiO 2 , and can further include B 2 O 3 and other components so that the glassy matrix is a borosilicate gas.
  • 31248341.1 8 105024-201 sintering can be performed at low temperatures, below about 400 °C using SPS and below about 100 °C using cold sintering in the presence of a transient solvent.
  • a single-step reaction can be used to separate alkali and alkaline-earth fluorides such as NaF, KF, CsF and SrF 2 from complex waste streams and incorporate them into metal halide perovskites.
  • the method involves reacting an alkali or alkaline earth fluoride salt and SiO 2 with hexafluorosilicic acid (H2SiF6) in a solvent 205 to form a MHP 210 as a metal salt of the SiF6 2- anion.
  • the solvent can be water or, for CsF, can be a polar organic solvent such as methanol, acetone, or dimethylformamide.
  • the resultant MHP can then be effectively sequestered in a cermet or in a glassy matrix 215.
  • the ability of methanol to effectively solubilize CsF but not other alkali and alkaline earth metal halides provides a method of separating CsF from other metal fluorides, prior to subsequent sequestration in a halmet, cermet, or glassy matrix.
  • fluoride salts including NaF, KF, and CsF are mixed with methanol 305.
  • the CsF dissolves in the methanol forming a solution of CsF in methanol 320.
  • the remaining salts, including NaF and KF can then be separated as a filtered solid 310 to be further recycled or sequestered 315.
  • the CsF can then be dried 325 and sintered in a halmet or glassy matrix 335.
  • the CsF can be reacted with H 2 SiF 6 and SiO 2 to form a precipitate of the perovskite Cs2SiF6330, and then sintered in a cermet or glassy matrix 340.
  • Trapping waste in halmets, cermets, and glassy matrices, according to the methods of this application, enables very high loading, of up to 70 wt% perovskite.
  • LiF and SrF2 are water-insoluble lithium salts and by-products of molten salt reactors (MSRs). Lithium fluoride salts are used in molten salt compositions in MSRs to reduce the generation of tritium, and such lithium salts have very high fractions of 7 Li, in order to reduce neutron absorption. As an expensive product, 7 Li recycling is highly desirable. Radioactive SrF2 on the other hand is a waste product of MSRs which it is desirable to sequester in compact form to avoid release into the environment.
  • Fig. 4 discloses such a method.
  • a fluoride molten salt composition 400 is mixed with water, forming a solution of the soluble salts 405 and leaving undissolved the 31248341.1 9 105024-201 insoluble LiF and SrF 2 salts 410.
  • the insoluble salts are then mixed with an aqueous solution of SrI 2 415, thereby extracting Sr 2+ from the insoluble salts as SrFI, and leaving behind undissolved LiF 420.
  • the insoluble LiF 420 can then be recycled for reuse in an MSR.
  • the SrFI can then be sequestered directly in a halmet or glassy matrix 430, or further processed to form an MHP, which can be sequestered in a cermet or glassy matrix 440.
  • combining these concepts provides a low temperature wet chemistry process to effectively separate NaF, KF, CsF and SrF 2 into desired composites with high kinetics, low temperature below 200 o C or at room temperature, and high separation efficiency above 90 wt%.
  • waste-loaded compounds can be sequestered into metal and glass matrices to form composite waste forms by low temperature spark plasma sintering and an innovative cold-sintering process.
  • the salt-waste loaded composites can be fabricated by conventional sintering, hot pressing, or advanced sintering technologies, e.g., SPS or HIP.
  • the low temperature SPS and cold sintering approaches are clearly advantageous in enabling the consolidation of the densified composite waste forms at temperature as low as 100 o C without any phase decompositions, greatly reducing the energy cost typically associated with high temperature sintering and mitigating potential phase decomposition/element loss.
  • These approaches and concepts can be applied for a wider range of the salt-contained materials and different metal matrix for cermet composites, and also glass-bonded composites (including borosilicate glass).
  • the metal or glass matrix in the composites provides barriers to prevent the ingress of water and release of waste elements from the crystalline phases, and thus greatly improve the chemical durability of waste form materials.
  • molten salt nuclear waste product comprising a fluoride molten salt composition is mixed with water 501 to separate the waste stream into water-soluble salts 512 such as NaF, KF, and CsF and insoluble salts 522 such as LiF and SrF 2 .
  • water-soluble salts can be separated based on their solubility in methanol 513.
  • radioactive CsF can be dissolved in methanol 524, leaving the insoluble salts including non-radioactive 31248341.1 10 105024-201 NaF and KF as insoluble solids 514.
  • the methanol solution of radioactive CsF can be dried 535 for direct sequestration into a halmet or glassy matrix 562.
  • the insoluble salts such as NaF and KF can be directly recycled for use in a molten salt reactor 515, or if too contaminated with radioactive products, can be processed to form a perovskite structure and trapped in a cermet or glassy matrix 560.
  • the water insoluble fluoride salts 522 can be reacted with a solution of SrI2, resulting in the reactive dissolution of SrFI into solution 523, leaving behind LiF salts in the solid phase.
  • the insoluble LiF salts which are typically enriched in the 7 Li isotope, can then be recycled 534 for reuse in a molten salt reactor.
  • the radioactive SrFI waste can be captured for use in commercial applications (e.g. to treat certain cancers) or can be sequestered directly into a metal or glass matrix as radioactive SrFI, or further processed to form a perovskite as a halmet prior to incorporation 566.
  • SiO2 is mixed with the supernatant of soluble salts 112 in water, followed by the addition of H 2 SiF 6 , to give a precipitate of M 2 SiF 6 , according to the equation: 6 ⁇ + 2 ⁇ 2 ⁇ 6 + ⁇ 2 ⁇ 3 ⁇ 2 ⁇ 6 ⁇ +2 ⁇ 2 ⁇ (1) where M is selected from the group consisting of Na, K, Cs, and combinations thereof. Because this single step chemistry does not generate any residual acid with high loading of F + , Na + , and K + , it is more environmentally benign than previous methods.
  • a separation of CsF 31248341.1 11 105024-201 from NaF and KF can be performed as shown in Fig.3 by mixing a composition of these salts with methanol 305, separating the undissolved solid 310 from the dissolved CsF 320, and reacting the CsF in methanol according to equation (1) to form a precipitate of Cs2SiF6330.
  • the Cs 2 SiF 6 can then be sequestered in a cermet or glassy matrix. Separation of SrF2 from LiF.
  • embodiments of this disclosure include methods of converting SrF2 into water soluble SrFX (X: anions) compounds.
  • a separation processes is described for converting SrF2 into water soluble SrFX compounds, enabling effective separation of SrF2 from water insoluble LiF.
  • aqueous solutions of SrI2 can effectively extract strontium from a mixture of LiF and SrF2. Immobilization of fluoride salt waste into cermet, halmet, and glass-bonded composite waste forms by cold sintering and spark plasma sintering (SPS).
  • SPS spark plasma sintering
  • Vitrification is not considered optimal for heat-conducting fission products such as 137 Cs and 90 Sr, which necessitate a highly conductive waste form.
  • a waste form matrix with higher thermal conductivity facilitates efficient heat dissipation, reducing thermal gradients across the waste form. This equalizing of temperature across the sample minimizes hot spots and helps prevent reactions such as corrosion and phase segregation.
  • Sintering processes with good densification and interparticle cohesion may require temperatures that may volatilize or decompose the materials to be sequestered. In order to avoid such issues, sintering temperatures should be kept below about 500 °C.
  • the sintered material can incorporate up to 70% sequestered salts 31248341.1 12 105024-201 (e.g. nuclear waste products). Surprisingly, increasing the weight percent of sequestered salts can result in increasingly stable densified structures. Examples: [ 0074] The following examples provide experiments which demonstrate some embodiments of the methods described herein. Immobilization of an NaF and KF mixture into MHP KNaSiF 6 and K 2 SiF 6 .
  • the measured amount of H2SiF6 was ⁇ 3.122 g, which is equivalent to 8.92 g acidic solution of H2SiF6 (with a concentration of 35 weight%).
  • the reaction proceeded for 30 minutes, after which the precipitate was separated out from water through gravity filtration.
  • the damp white precipitate was dried at room temperature.
  • the resultant MHP was characterized by X-ray diffraction (XRD) for structural confirmation as shown in Fig.6.
  • the upper spectrum (a) shows diffraction patterns for the sample as crystalized.
  • the middle and bottom spectra (b) and (c) show, respectively, diffraction patterns for K 2 SiF 6 (ICDD: 00-007-0217) and KNaSiF 6 (ICDD: 00-043-1313).
  • ICDD 00-007-0217
  • KNaSiF 6 ICDD: 00-043-1313
  • a comparison of these spectra distinctly illustrates that the synthesized MHP exhibits diffraction patterns associated with both K 2 SiF 6 and KNaSiF 6 .
  • the weight of the resultant MHP was 6.66 g.
  • the yield for this process at RT is 96.1 %, suggesting almost full immobilization of waste elements (3.55 g: NaF + KF) into MHPs (6.66 g: KNaSiF6 and K2SiF6) through the one-step aqueous solution with a very high yield of more than 95%.
  • the recorded diffraction pattern was further analyzed through Rietveld refinement for qualitative and quantitative information of both crystallographic phases.
  • the K2SiF6 structure belongs to a cubic lattice framework according to ⁇ 3 ⁇ , 225 space group (perovskite-type).
  • the XRD pattern of the obtained white precipitate exhibited a perfect match with the published K 2 SiF 6 ( ⁇ 3 ⁇ ) fluorite structure with face-centered cubic phase.
  • the Bravais lattice of KNaSiF 6 indicates that this compound (Heklaite) adopts an 31248341.1 13 105024-201 orthorhombic structure with space group Pnma which distinguishes it from other similar hexafluorosilicates such as cubic K2SiF6 and hexagonal Na2SiF6.
  • the primitive cell of KNaSiF 6 shows four formula units of KNaSiF 6 . In the orthorhombic structure, K + was coordinated with nine F- ions while Na + was bonded with nine F- ions.
  • the experimental results are shown as data points ( ).
  • the bottom curve shows the difference spectrum 730, and the middle curve 720 provides the Bragg positions.
  • the weight of KNaSiF6 & K2SiF6 were estimated to be 2.85 g and 4.08 g, respectively.
  • Rietveld 31248341.1 14 105024-201 refinement provided the relative weight % of the KNaSiF 6 phase as 39% and the K 2 SiF 6 phase as 61%, in excellent agreement with the theoretic estimates of 41% and 59% for the KNaSiF6 phase and the K2SiF6 phase, respectively.
  • FIGS 9 and 10 are SEM images of the synthesized samples observed under different magnifications, showing that the sample powders have a good crystallinity and a homogeneous distribution of grain size ( ⁇ 1-20 ⁇ m). Grains formed are spherical in shape and the majority of grains were agglomerated to form larger-sized clusters.
  • Energy Dispersive X- ray Spectroscopy (EDS) was used to determine the spatial distribution of elements in the images (data not shown). The EDS measurements showed that the sample consists of K, Na Si, and F. The Na distribution was less prominent compared to the K distribution, which further validates that the MHP has two different phases. CsF separation from a salt mixture with NaF and KF.
  • the mixed solution was stirred at 400 rpm at RT for 5 minutes, so that the KF and NaF salts were completely dissolved into DI water.
  • SiO2 (0.6509 g) at a 6:1 molar ratio was added to the mixture.
  • a stoichiometric amount of H 2 SiF 6 (6:2 molar ratio of KF and NaF to H2SiF6) was poured into the solution, immediately forming a white precipitate.
  • the measured amount of H 2 SiF 6 was ⁇ 3.122 g, which is equivalent to 8.92 g acidic solution of H2SiF6 (with a concentration of 35 weight%).
  • the reaction was performed for 30 minutes, and the precipitate was separated out from water through gravity filtration.
  • H 2 SiF 6 6:2 molar ratio of CsF to H2SiF6
  • the measured amount of H 2 SiF 6 was 1 mmol ⁇ 0.1441 g which is equivalent of 0.4117 g acidic solution of H2SiF6 (35 weight%).
  • the XRD result demonstrates that a complete immobilization of 0.4557 g CsF into Cs2SiF6 was achieved in accordance with Eq (2).
  • the weight of white precipitate obtained through SiO2 assisted RT solution chemistry was determined as 0.6078 g, compared to a theoretical yield of 0.6118 g. Consequently, the yield for this approach at room temperature is 99.3%.
  • this approach facilitates the immobilization of 0.4557 g CsF through aqueous solution into 0.6118 g Cs 2 SiF 6 in a single step with 99.3 % reaction yield at the cost of only 5 ml methanol.
  • Table 5 summarizes the amounts of reactants and product. Table 5.
  • TGA results also shows good thermal stability up to 650 o C by the observed negligible 0.6% weight loss as shown in Fig.12.
  • SEM micrographs as in Fig. 13 show homogeneous, spherical grains of synthesized powder of a few micrometers in diameter.
  • EDS analysis performed on the surface of Fig.13 demonstrate the homogeneous distribution of Cs, F, and Si.
  • the structural, microstructural, and thermal properties of Cs2SiF6 show a single phase MHP stable 31248341.1 18 105024-201 up to 650 o C.
  • the grinding process comprised 48 cycles, with each cycle lasting 30 minutes using zirconia balls as grinding material and methanol as a dispersing liquid. A 15-minute pause between consecutive cycles was implemented to prevent excessive heating (to avoid exceeding the thermal stability limit of 650 °C as shown in Fig.12).
  • spark plasma sintering SPS
  • cold sintering was performed at 100 °C in the presence of DI water since Cs2SiF6 is partially soluble in DI water through a dissolution and reprecipitation mechanism.
  • the ball-milled powder was poured in a 10 mm graphite die-set along with a few droplets of DI water. Sintering was conducted at 100 °C for 5 minutes, with a heating and cooling rate of 20°C/min. The process was executed under a constant hydrostatic pressure of 50 MPa, gradually applied and released in alignment with the heating and cooling protocol.
  • the prepared sample of consolidated cermet was polished with sandpaper in the presence of methanol as Cs2SiF6 is insoluble in methanol.
  • the density of prepared cermet pellet was estimated as ⁇ 92% of the maximum density through Archimedes’ principle.
  • the consolidated cermet sample was characterized by XRD and the results are shown in the top pattern shown in Fig.14.
  • EDS analysis determined that the lighter areas 810 correspond to Cs 2 SiF 6 and the darker areas 820 to metal binder.
  • Copper was uniformly dispersed within the material, occupying the interstitial cracks and voids among the Cs2SiF6 particles.
  • EDS mapping validates the elemental distribution of Cs, Cu, Si, and F.
  • SEM and EDS analyses conclusively demonstrate the absence of phase dissolution or reactive phase formation. Instead, the two distinct phases are independently maintained, exhibiting a homogenous integration of their respective microstructures.
  • the mechanical properties of the cermet sample were evaluated through microindentation using 1 kgf (9.8 Newton) for a duration of 15 seconds.
  • the calculated 31248341.1 21 105024-201 hardness and fracture toughness are tabulated in Table 8.
  • the prepared cermet sample was further subjected to nanoindentation to determine its elastic modulus and the obtained mechanical properties are also provided in Table 8.
  • pure Cs2SiF6 salt was similarly consolidated using cold sintering at 100°C for 5 minutes through SPS under a pressure of 50 MPa, with the aid of DI water.
  • the pellet of Cs2SiF6 was characterized through micro-indentation using 500 gf (4.9 Newton) for a duration of 15 seconds.
  • the cermet sample exhibits microhardness and nanohardness values that are 2.4 and 2.6 times greater, respectively, compared to those of the pure Cs 2 SiF 6 sample. Table 8.
  • the thermal diffusivity measurement utilized a Laser Flash Analysis system, requiring the preparation of an opaque sample achieved by applying a thin graphite coating to both faces of the pellet. The transient response of the sample on one face was recorded following the application of a brief laser pulse on the opposite face, delivered at equal temperature intervals in the range of 25 °C to 500 °C.
  • Fig.17 shows the thermal diffusivity of the materials as a function of temperature.
  • the thermal diffusivity exhibited a decline from 0.53 mm 2 /s to 0.17 mm 2 /s with a temperature increase from 25 °C to 500 °C. 31248341.1 22 105024-201
  • the cermet demonstrated an exponential rise from 7.7 mm 2 /s to 13.2 mm 2 /s over the same temperature range.
  • This larger thermal diffusivity of cermet shows better thermal properties provided by highly conducting Cu matrix.
  • the enhancement of thermal properties significantly reduces the temperature gradient between the centerline and the surface of the nuclear waste containment.
  • Cs 2 SiF 6 powders were mixed with 842.6 mg copper powders using agate mortar and pestle by hand for several minutes and formed into a cylindrical pellet measuring 10 mm in diameter and 2 mm in thickness. DI water was added during loading of the mixed powders into a graphite die-set during cold sintering to partially dissolve Cs2SiF6 surfaces and promote the transport behavior to reduce the sintering temperature. Cold sintering was performed at 100 o C for 1 minute.
  • the pellet formed from this cold sintering process was compared to two additional pellets for which the cold sintering was followed by SPS sintering at a temperature of either 300 o C, or 400 o C, with a dwelling time for the low temperature SPS sintering of 1 minute, and with heating and cooling rates of 25 °C/min.
  • the process was executed under a constant hydrostatic pressure of 50 MPa, gradually applied and released in alignment with the heating and cooling protocols.
  • the three cermet pellets (a) prepared only with the cold sintering process at 100 o C, (b) prepared by 31248341.1 23 105024-201 cold sintering at 100 o C, followed by SPS at 300 o C, and (c) prepared by cold sintering at 100C, followed by SPS at 400 o C, were mechanically polished with sandpapers in the presence of methanol as Cs2SiF6 is insoluble in methanol.
  • the microstructural analysis was carried out via SEM to evaluate distribution of densified pellets.
  • EDS analysis was performed to validate micro-chemistry in cermet samples.
  • the phase of the cermet composite was analyzed by x-ray diffraction (XRD).
  • the sintering process can be selected from the group consisting of cold sintering performed in the presence of a small quantity of transient solvent (1 to 5 volume %) at less than about 100 °C, spark plasma sintering at a temperature of less than about 400 °C, and combinations thereof.
  • Suitable metal matrices include but are not limited to copper, aluminum, stainless steel, and combinations thereof.
  • low temperature SPS of a sample of of 33 wt% Cs2SiF6 and 67 wt% SS-316 stainless steel gave a densification of 80% TD, whereas treatment of the same sample by cold sintering with transient solvent resulted in a densification of 93% TD.
  • Fig.18 shows SEM results for a 60:40 wt.
  • silica gel was mixed with the MHP at a 25:75 mass ratio of MHP to glass.
  • 0.05 g of MHP were mixed with 0.15 g silica gel, in order to obtain a cylindrical pellet measuring 10 mm in diameter and 2 mm in thickness. Water was added during loading of mixed powders into graphite die-set during cold sintering to partially dissolve Cs 2 SiF 6 surfaces and promote the transport behavior to reduce the sintering temperature.
  • the cold sintering was performed at 300 o C and the dwell time for the low temperature SPS sintering was 1 minute with heating and cooling rates of 25 °C/min.
  • the process was executed under a constant hydrostatic pressure of 50 MPa, gradually applied and released in alignment with the heating and cooling protocols.
  • the consolidated cermet pellets were mechanically polished with sandpaper in the presence of methanol as Cs2SiF6 is insoluble in methanol.
  • a microstructural analysis was carried out via SEM to evaluate the distribution of MHP and glass in the densified pellets, and EDS analysis was performed to validate the distribution of elements.

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  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Thermal Sciences (AREA)
  • Environmental & Geological Engineering (AREA)
  • Processing Of Solid Wastes (AREA)

Abstract

L'invention concerne des procédés de traitement de déchets de sels fluorés provenant de réacteurs à sels fondus. Les procédés comprennent des processus à température ambiante pour séparer les sels fluorés en composants recyclables et déchets radioactifs, un processus en une seule étape pour piéger les déchets dans des pérovskites d'halogénures métalliques (MHP) et des procédés pour piéger les sels fluorés et les MHP dans des matrices métalliques et vitreuses.
PCT/US2025/027437 2024-05-03 2025-05-02 Séparation de déchets de sels fluorés et immobilisation sous forme de déchets en cermet, halmet et sous forme de composites liés au verre Pending WO2025231330A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2013249235A (ja) * 2012-06-01 2013-12-12 Mitsubishi Chemicals Corp フッ化物塩の製造方法及びフッ化物塩、並びに該フッ化物塩を用いたシリコンの製造方法
CN112875730A (zh) * 2021-03-30 2021-06-01 中国科学院上海有机化学研究所 一种核纯氟化锂的提纯方法
CN114388166A (zh) * 2021-12-31 2022-04-22 中国科学院上海应用物理研究所 一种通过玻璃陶瓷固化含氯和/或氟放射性废物的方法以及由此得到的玻璃陶瓷固化体

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2013249235A (ja) * 2012-06-01 2013-12-12 Mitsubishi Chemicals Corp フッ化物塩の製造方法及びフッ化物塩、並びに該フッ化物塩を用いたシリコンの製造方法
CN112875730A (zh) * 2021-03-30 2021-06-01 中国科学院上海有机化学研究所 一种核纯氟化锂的提纯方法
CN114388166A (zh) * 2021-12-31 2022-04-22 中国科学院上海应用物理研究所 一种通过玻璃陶瓷固化含氯和/或氟放射性废物的方法以及由此得到的玻璃陶瓷固化体

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