Separation of fluoride salt waste and immobilization into cermet, halmet, and glass-bonded composite waste forms

Abstract

Methods are disclosed for processing fluoride salt wastes from molten salt reactors. The methods include room temperature processes to separate fluoride salts into recyclable and radioactive waste components, a one-step process to trap wastes in metal halide perovskites (MHPs), and methods of trapping fluoride salts and MHPs in metal and glass matrices.

Description

105024-201 SEPARATION OF FLUORIDE SALT WASTE AND IMMOBILIZATION INTO CERMET, HALMET, AND GLASS-BONDED COMPOSITE WASTE FORMS GOVERNMENT LICENSE RIGHTS

[0001] This invention was made with government support under award number DE-AR0001615, awarded by the Advanced Research Projects Agency-Energy. The government has certain rights to the invention. CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims priority to U.S. Provisional Patent Application No.63 / 642,271, filed May 3, 2024, and 63 / 798,190, filed May 1, 2025, the disclosures of which are hereby incorporated by reference in their entireties. TECHNICAL FIELD

[0003] The present disclosure relates to structures, methods of recycling andsequestering 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 ofadvanced 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. However, for both thermal or fast molten salt reactors based on fluoride or chloride, 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. 31248341.1 1105024-201

[0005] Different approaches have been proposed to address complex salt-based wastestreams, including: (1) direct immobilization of unseparated fluoride salts into glass-bonded sodalite / apatite or fluoride-based glasses; and (2) salt separation from online processing to remove volatile off-gas or fission products. For direct disposal, single waste forms have been considered, e.g., sodalite through salt-occluded zeolite, titanate crystals and apatite. Apatite is considered a good option for fluoride-based salt waste due to its structural characteristics in incorporating halides, alkalis, and alkaline earth. Nonetheless, challenges exist in separating and immobilizing fluoride salt wastes into a single waste form. 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 TeO2glass. 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. However, 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., 1000oC). Glass binders with lower softening temperatures (e.g., < 750oC) have been developed for glass-bonded halide salt waste forms. However, the waste alkali elements and fission products in such glass matrices show low chemical durability, with high elemental release rates (e.g. several g / m2 / d when measured by both product-consistency tests and static leaching tests.

[0006] In brief, significant challenges exist with halide and salt waste disposal mainlydue to (1) complex waste streams, (2) highly volatile, and halide or salt waste loaded compounds typically highly water soluble, and chemically compatible with the baseline borosilicate glasses. It is essential to separate salt waste from complex salt waste stream and develop advanced waste forms that can securely immobilize halides and salt waste with high waste loadings. SUMMARY

[0007] According to some embodiments, a method is disclosed of treating a moltensalt 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 a31248341.12105024-201 phase within a metal matrix. For some embodiments, the metal powder includes copper, aluminum, or stainless steel.

[0008] Alternatively, if the sinterable composition includes a glass-forming material,then the sintering process provides a glass-bonded composite comprising the solid fluoride composition as a phase within a glass matrix. For some embodiments, the glass-forming material includes SiO2. For some embodiments, the glass-forming material includes SiO2and B2O3.

[0009] The sintering process can be a cold sintering process performed in thepresence of a transient solvent at less than about 100 °C. For such embodiments, the transient solvent can be added at between about 1 volume % to about 5 volume %. The transient solvent can be water or methanol.

[0010] For some embodiments, the sintering process can include field-assistedsintering 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).

[0011] For some embodiments, the solid fluoride composition and the sinterablecomposition are combined at a weight percentage of less than about 70% fluoride composition. For some embodiments the solid fluoride composition is combined at a weight percentage of between about 30% and about 70% fluoride composition.

[0012] For some embodiments, processing the molten salt reactor waste to form asolid 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.

[0013] According to other embodiments, the water-soluble metal fluorides includeCsF, 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. According to some such embodiments, the solid fluoride composition comprises CsF, and is formed by evaporating the organic solvent from the solution of CsF.

[0014] According to some embodiments, the solid fluoride composition comprises ametal fluoride salt. According to some embodiments, the solid fluoride composition comprises a metal halide perovskite (MHP), formed by mixing a metal fluoride salt with31248341.13105024-201 H2SiF6in the presence of SiO2and solvent to form the solid fluoride composition comprising the MHP as a metal salt of the hexafluorosilicate anion. According to some embodiments, the metal fluoride salt is selected from the group consisting of alkali metal halides, alkaline earth metal halides, and combinations thereof. According to some embodiments, the metal fluoride salt is CsCl or SrCl2. According to some embodiments, the solvent is water. According to some embodiments, the metal fluoride salt is CsF and the solvent is methanol, acetone, dimethylformamide, or mixtures thereof.

[0015] According to some such embodiments, the solid fluoride compositioncomprises a cesium halide perovskite, formed by mixing the solution of CsF with H2SiF6 in the presence of SiO2to form the solid fluoride composition comprising the cesium halide perovskite as a cesium salt of a hexafluorosilicate anion.

[0016] According to some embodiments, the organic solvent is selected from thegroup consisting of methanol, acetone, dimethylformamide, and combinations thereof.

[0017] According to some embodiments, a method is disclosed of removing andsequestering 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 H2SiF6and SiO2to 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 M2SiF6sequestered within a metal matrix, if the sinterable composition is a metal powder, or to obtain a glass- bonded composite comprising M2SiF6sequestered within a glass matrix, if the sinterable composition is a glass-forming material.

[0018] The sintering process can be a cold sintering process performed in thepresence of a transient solvent at less than about 100 °C. For such embodiments, the transient solvent can be added at between about 1 volume % to about 5 volume %. The transient solvent can be water or methanol.

[0019] For some embodiments, the sintering process can include field-assistedsintering 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).

[0020] For some embodiments, the fluoride composition and the sinterablecomposition are combined at a weight percentage of less than about 70% fluoride composition. For some embodiments, the fluoride composition and the sinterable31248341.14105024-201 composition are combined at a weight percentage of between about 30% and about 70% fluoride composition.

[0021] For some embodiments, a method is disclosed of treating a molten salt reactorwaste comprising water-soluble salts and water-insoluble salts, the water-insoluble salts comprising7LiF 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 SrI2so 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. mixing 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. For some such embodiments, the7LiF in the second solid is recycled for use in a molten salt composition for a molten salt reactor.

[0022] For some embodiments, metal halides from the salt reactors or as wasteproducts 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 ahalmet or in a glassy matrix.

[0024] Fig. 2. shows a general flow chart for the one step conversion of a fluoride saltto a hexafluorosilicate metal halide perovskite (MHP), followed by trapping the MHP in a cermet or in a glassy matrix.

[0025] Fig. 3 is a flow chart showing the separation of CsF from other fluoride saltsprior to sequestering CsF or a cesium halide perovskite in a cermet, halmet, or glassy matrix.31248341.15105024-201

[0026] Fig. 4 provides a flow chart for the separation of LiF from SrF2, andsubsequent sequestering or recycling of these salts or their perovskite derivatives.

[0027] Fig. 5 provides a flow chart of a process for handling multicomponent fluorideradioactive wastes from a molten salt reactor.

[0028] Fig. 6 provides XRD results for the MHP composition formed upon reacting amixture of NaF and KF with H2SiF6and SiO2.

[0029] Fig. 7 shows the result of multiphase refinement of the XRD results of Fig. 6.

[0030] Fig. 8 shows thermogravimetric results obtained for the MHP composition ofFig.6.

[0031] Fig. 9 shows an SEM image of the MHP composition of Fig. 6.

[0032] Fig. 10 shows a lower magnification image of the MHP composition of Fig. 6.

[0033] Fig. 11 shows XRD results for Cs2SiF6 synthesized in methanol from H2SiF6and SiO2.

[0034] Fig. 12 shows thermogravimetric analysis results for the Cs2SiF6 of Fig. 11.

[0035] Fig. 13 shows an SEM image for the Cs2SiF6 of Fig. 11.

[0036] Fig. 14 shows an XRD results for a cermet of 30 wt. % Cs2SiF6 and 70 wt. %copper.

[0037] Fig. 15 shows an SEM image of the cermet of Fig. 14.

[0038] Fig. 16 shows an SEM image of the cermet of Fig. 14 at a highermagnification.

[0039] Fig. 17 shows the thermal diffusivity of the cermet of Fig. 14 as a function oftemperature.

[0040] Fig. 18 shows SEM results for a 60:40 wt. % MHP:Al cermet of Cs2SiF6 in analuminum matrix.

[0041] Fig. 19 shows SEM results for a sample of 30 wt. % Cs2SiF6 and 70 wt. %copper.

[0042] Fig. 20 shows a higher magnification SEM result for the sample of Fig. 19.DETAILED DESCRIPTION

[0043] Definitions. As used in this description and the accompanying claims, thefollowing terms shall have the meanings indicated, unless the context otherwise requires:

[0044] Sintering is a process of compacting particles of different materials to form asolid mass of fused materials without melting the materials to the point of liquefaction. The31248341.16105024-201 atoms / molecules in the sintered materials diffuse across particle boundaries, fusing the particles together.

[0045] As used herein, 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.

[0046] A “cermet” is a ceramic-metal composite material formed by a sinteringprocess. Such materials include composites of metals with perovskite materials.

[0047] A “halmet” is a composite of a halide salt and a metal formed by a sinteringprocess.

[0048] As used herein, a “solid fluoride composition” is a composition havingfluorine present in the -1 oxidation state. Such a composition includes simple fluoride salts and more complex salts, including fluoride containing perovskites.

[0049] A “perovskite” as used herein refers to a class of crystalline material with acrystal structure similar to that of the mineral perovskite. Perovskites include alkali metal and alkaline earth salts of the hexafluorosilicate anion.

[0050] A “transient solvent” as used herein is a solvent added to a sinterable materialprior to sintering. By partially dissolving particles in the sinterable material, the transient solvent allows stronger particle bonding at lower temperatures. In the examples provide below, 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.

[0051] Some embodiments of this technology provide methods to separate fissionproducts from a molten salt reactor (MSR), including CsF and SrF2from complex salt waste streams and immobilize such fission products within matrices having high chemical durability and waste loadings. In some embodiments, 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.

[0052] Embodiments of the current application include methods of separating metalhalides 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+B2+F3, A+3B3+2F9, A+2 / 3 / 4B4 / 3 / 2+F6, A2B+B3+F631248341.17105024-201 and A+2Na+Ln3+F6,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 hereincan enable very high loading, of up to 70 wt% perovskite. In contrast, state-of-the-art sodalite or apatite-based waste forms allow for a halide loading of only 4 to 7 wt%.

[0054] A low temperature (room temperature) wet chemistry process is disclosedwhich in some embodiments provides effectively separation of NaF, KF, CsF and SrF2 into desired composites with high kinetics, low temperature below 200oC 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.

[0055] Waste loaded metal and glass composites can be fabricated by conventionalsintering, hot pressing, or advanced sintering technologies, e.g., spark plasma sintering (SPS) or hot isostatic pressing (HIP). However, such conventional methods require high temperatures, which can lead to phase decompositions. The low temperature SPS and cold sintering approaches of the present application enable the consolidation of the densified composite waste forms at temperature as low as 100oC 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). The 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.

[0056] Metal halides, after recovery from salt reactors, or as waste products uponchemical reprocessing can be directly incorporated into metal and glass matrices, to form halmets, or glass composites. As illustrated in Fig.1, a fluoride salt 105 of a radioactive metal, including CsF and SrF2can 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 SiO2, and can further include B2O3and other components so that the glassy matrix is a borosilicate gas. Using the methods disclosed in this application, such31248341.18105024-201 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.

[0057] As disclosed in Fig. 2, a single-step reaction can be used to separate alkali andalkaline-earth fluorides such as NaF, KF, CsF and SrF2from complex waste streams and incorporate them into metal halide perovskites. As discussed further below, the method involves reacting an alkali or alkaline earth fluoride salt and SiO2with hexafluorosilicic acid (H2SiF6) in a solvent 205 to form a MHP 210 as a metal salt of the SiF62-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.

[0058] As disclosed in Fig. 3, the ability of methanol to effectively solubilize CsF butnot 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. According to the method, 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.

[0059] The CsF can then be dried 325 and sintered in a halmet or glassy matrix 335.Alternatively, the CsF can be reacted with H2SiF6and SiO2to form a precipitate of the perovskite Cs2SiF6330, and then sintered in a cermet or glassy matrix 340.

[0060] Trapping waste in halmets, cermets, and glassy matrices, according to themethods of this application, enables very high loading, of up to 70 wt% perovskite. Therefore, the waste loading can be drastically improved over that of the state-of-the-art sodalite or apatite-based waste forms, which allow for a halide loading of only 4 to 7 wt%.

[0061] LiF and SrF2 are water-insoluble lithium salts and by-products of molten saltreactors (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 of7Li, in order to reduce neutron absorption. As an expensive product,7Li 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. Consequently, it is desirable to have a method to separate LiF and SrF2 from MSR waste streams so that the7LiF can be recycled and the SrF2can be safely sequestered and stored in compact form.

[0062] Fig. 4 discloses such a method. A fluoride molten salt composition 400 ismixed with water, forming a solution of the soluble salts 405 and leaving undissolved the31248341.19105024-201 insoluble LiF and SrF2salts 410. Following filtration and separation, the insoluble salts are then mixed with an aqueous solution of SrI2415, thereby extracting Sr2+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. Following solvent evaporation 425, 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.

[0063] As summarized in Fig. 5, combining these concepts provides a lowtemperature wet chemistry process to effectively separate NaF, KF, CsF and SrF2into desired composites with high kinetics, low temperature below 200oC or at room temperature, and high separation efficiency above 90 wt%. The wet-chemistry process offers unique merits than 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.

[0064] As embodied in the examples provided below, waste-loaded compounds canbe 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. However, 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 100oC 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.

[0065] Methods of the current application can be incorporated into schemata for thecomplete of recycling waste molten salt wastes from molten salt reactors. One such schema is provided in Fig.5. A 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 SrF2. The water-soluble salts can be separated based on their solubility in methanol 513. In this manner radioactive CsF can be dissolved in methanol 524, leaving the insoluble salts including non-radioactive31248341.110105024-201 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.

[0066] 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 the7Li 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.

[0067] While Fig. 5 provides a complete schema for nuclear waste recycling ofmolten salt reactor waste products, it is evident that other methods of separating wastes can incorporate key features of this disclosure, including the low temperature solubility separations, and the sequestration of waste products as halmets, cermets, and sintered material in glassy matrices by the sintering methods of the present application. Single-step SiO2assisted reaction of fluoride salts with H2SiF6to form hexafluorosilicate perovskites.

[0068] According to some embodiments, M2SiF6 can be synthesized in a single stepin the presence of SiO2. According to this method, SiO2 is mixed with the supernatant of soluble salts 112 in water, followed by the addition of H2SiF6, to give a precipitate of M2SiF6, 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.

[0069] The SiO2 assisted single step reaction has been used to successfullyimmobilize Na+, K+and F- ions into KNaSiF6 and K2SiF6 in aqueous medium.

[0070] In addition, Cs+ and F- ions can be immobilized into Cs2SiF6 MHP inmethanol. Because CsF is soluble in methanol but NaF and KF are not, a separation of CsF31248341.111105024-201 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. As discussed in more detail below, the Cs2SiF6can then be sequestered in a cermet or glassy matrix. Separation of SrF2 from LiF.

[0071] Effective management of fluoride salt wastes and efficient recycling ofvaluable Li from salt waste is critical for the sustainable development of advanced nuclear fuel cycles and future nuclear technologies. The main radionuclides fission products targeted for separation are I, Tc, Cs, and Sr. Removal of the latter two significantly reduces the heat load of the residual conditioned wastes.90Sr and137Cs, both short half-life fission products, have a substantial influence on determining the total toxicity and heat generation of the fission product nuclides (<300 yrs). The efficient separation of SrF2 from LiF has been a particular challenge for nuclear waste management. To realize the separation of SrF2 from LiF, embodiments of this disclosure include methods of converting SrF2 into water soluble SrFX (X: anions) compounds. In an embodiment, a separation processes is described for converting SrF2 into water soluble SrFX compounds, enabling effective separation of SrF2 from water insoluble LiF. In a particular embodiment of this method, 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).

[0072] Vitrification is not considered optimal for heat-conducting fission productssuch as137Cs and90Sr, 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.

[0073] Sintering processes with good densification and interparticle cohesion mayrequire 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. In the examples provided below we present two sintering methods that can be performed at temperatures below about 400 °C (SPS sintering) and below about 100 °C (cold sintering with a transient solvent). The sintered material can incorporate up to 70% sequestered salts31248341.112105024-201 (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 someembodiments of the methods described herein. Immobilization of an NaF and KF mixture into MHP KNaSiF6and K2SiF6.

[0075] To mimic typical ratios present in molten salts used in MSRs, 0.59 g of NaFand 2.96 g of KF were mixed in 30 mL water, and stirred at 400 rpm at RT for 5 minutes, following which the salts were completely dissolved, forming a salt solution. SiO2 (0.6509 g) at a 6:1 molar ratio was added. A stoichiometric amount of H2SiF6 (6:2 molar ratio of KF and NaF to H2SiF6) was poured into the salt solution, whereupon a white precipitate formed immediately. 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.

[0076] The resultant MHP was characterized by X-ray diffraction (XRD) forstructural 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 K2SiF6(ICDD: 00-007-0217) and KNaSiF6(ICDD: 00-043-1313). Acomparison of these spectra distinctly illustrates that the synthesized MHP exhibitsdiffraction patterns associated with both K2SiF6and KNaSiF6. The weight of the resultant MHP was 6.66 g. With a targeted yield of 6.92 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%.

[0077] The recorded diffraction pattern was further analyzed through Rietveldrefinement for qualitative and quantitative information of both crystallographic phases. TheK2SiF6 structure belongs to a cubic lattice framework according to ^^^^3̅^^, 225 space group(perovskite-type). The XRD pattern of the obtained white precipitate exhibited a perfect match with the published K2SiF6(^^^^3̅^^) fluorite structure with face-centered cubic phase. Moreover, the Bravais lattice of KNaSiF6indicates that this compound (Heklaite) adopts an31248341.113105024-201 orthorhombic structure with space group Pnma which distinguishes it from other similar hexafluorosilicates such as cubic K2SiF6 and hexagonal Na2SiF6. The primitive cell of KNaSiF6shows four formula units of KNaSiF6. In the orthorhombic structure, K+was coordinated with nine F- ions while Na+was bonded with nine F- ions. Si⁴⁺ was bonded in anoctahedral geometry to six F⁻ atoms i.e., SiF6 octahedra. Thus, K+, Na+, and Si4+ all showedsingle-single Wyckoff positions. F⁻ occupied four non-equivalent atomic positions. The refined parameters of the synthesized MHP are tabulated in Table 2. Table 2. Refined parameters for KNaSiF6 and K2SiF6. Parameters KNaSiF6 K2SiF6a (Å) 9.31938(65) 8.12681(4)

[0078] Fig. 7 shows as the upper curve an XRD pattern refined through multiphaserefinement 710. In this figure, the experimental results are shown as data points ( ). Thebottom curve shows the difference spectrum 730, and the middle curve 720 provides the Bragg positions.

[0079] For the targeted MHP yield of about 6.92 g, according to reaction (1), theweight of KNaSiF6 & K2SiF6 were estimated to be 2.85 g and 4.08 g, respectively. Rietveld 31248341.1 14105024-201 refinement provided the relative weight % of the KNaSiF6phase as 39% and the K2SiF6phase as 61%, in excellent agreement with the theoretic estimates of 41% and 59% for the KNaSiF6 phase and the K2SiF6 phase, respectively.

[0080] The calculated structural parameters estimated for synthesized MHP are inaccordance with published data. A TGA study was also performed to examine the thermal stability of the synthesized MHP, with the results shown in Fig.8. For the TGA testing, roughly 20 mg powders were heated to 850oC in an alumina crucible with a heating rate of 10 °C / minute. These TGA results show that the MHP experienced less than 1% weight loss up to 490 °C, and can be considered stable up to that point. At higher temperatures, the MHP showed an 8% weight loss as decomposition of first KNaSiF6and then K2SiF6.

[0081] Figs. 9 and 10 are SEM images of the synthesized samples observed underdifferent magnifications, showing that the sample powders have a good crystallinity and a homogeneous distribution of grain size (~1-20 μm). 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.

[0082] The separation of CsF from NaF and KF is based on the solubility differencesshown in Table 3. Large amounts of CsF can be dissolved in methanol whereas NaF and KF are largely insoluble in this solvent. remain undissolved because of a large difference in their solubilities in methanol, enabling effective separation of CsF from NaF and KF. Accordingly, as shown in Fig.3 and discussed above, we have developed a waste stream for CsF separation from NaF and KF.

[0083] KF (2.96 g) and NaF (0.59 g) were taken in a weight ratio simulating arepresentative molten salt composition. These fluorides, along with 0.5 g CsF, were added in 10 ml methanol and mixed via a magnetic stir bar 500 rpm for almost 10 minutes. After 10 minutes, the solution was filtered through a filter paper (11 μm) in vacuum filtration system. The filtered solid, mainly composed of NaF and KF, was dried at room temperature. Table 3. A list of solubilities in methanol and other physical properties of CsF, NaF, and KF.31248341.115105024-201 Salt WeightMolar Molar Density Solubility Mass Separation ratio ratio mass 3 (g / cm ) (g / 10 mL) (g) efficiency )mL water, a slightly larger quantity than required for their complete dissolution. Then, 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. Once the salts were dissolved, SiO2 (0.6509 g) at a 6:1 molar ratio was added to the mixture. A stoichiometric amount of H2SiF6(6:2 molar ratio of KF and NaF to H2SiF6) was poured into the solution, immediately forming a white precipitate. The measured amount of H2SiF6was ~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. The damp precipitate was dried at room temperature.

[0085] A total of 4.05 g simulant FNaK-C3F salt (0.5 g CsF, 0.59 g NaF, and 2.96 gKF) was present prior to treatment with methanol. After dissolution in methanol, the filtered solid was measured as 3.325 g. In the initial mixture, 3.55 gram of KF-NaF were present and due to partial solubility of KF (~5%) in methanol, the expected separation efficiency is 95%. In the present experiment, the filtered undissolved KF-NaF was separated with 94% efficiency and around 6% KF was dissolved along with CsF in the methanol. XRD patterns clearly showed predominantly KF and NaF in the solid and CsF in the methanol (data not shown).

[0086] The MHP obtained as a result of immobilization of separated KF-NaF intoaqueous solution was weighed as 5.2551 g against the targeted MHP (~6.66 g). The driedpowder was also analyzed through XRD. The observed diffraction maxima belonged toK2SiF6 (ICDD: 00-007-0217) and KNaSiF6 (ICDD: 00-043-1313). There were no impurities peaks as KF-NaF successfully separated from the salt mixture. The decreased reaction yield can be explained based on ~5% dissolved KF into methanol.31248341.116105024-201

[0087] To the supernatant of CsF in methanol, a stoichiometric amount of SiO2 andH2SiF6(6:2 molar ratio of CsF to H2SiF6) were added, resulting in the immediate formation of a white precipitate. The measured amount of H2SiF6 was 1 mmol ~0.1441 g which is equivalent of 0.4117 g acidic solution of H2SiF6(35 weight%).

[0088] The obtained solid after immobilization of dissolved CsF weighed 0.8825 g,which was larger than the targeted MHP (Cs2SiF6~0.6714 g) due to the presence of some dissolved KF as impurities. The resultant XRD pattern (not shown) was primarily dominated by the Cs2SiF6phase from the XRD database ICDD: 00-007-0006. Table 4 summarizes the amount of products and reactants for the MHPs of the separated Cs and Na / K salts. Table 4. MHPs from the separated Cs (Cs2SiF6) and Na / K (KNaSiF6 & K2SiF6) salts. Synthesis Product Product Reaction route Reactants (Targeted) (Obtained) YieldCsF immobilization into Cs2SiF6 in methanol.

[0089] CsF was directly immobilized into Cs2SiF6 in methanol through SiO2 assistedRT solution chemistry. CsF (3 mmol or 0.4557 g) and SiO2 (0.5 mmol or 0.0301 g) at a 6:1 molar ratio was added into an appropriate amount of methanol (5 ml or 3.5 g) at room temperature. The volume of the methanol added is slightly higher than the solubility of CsF. The mixed solution was then stirred at 250 rpm at room temperature for only 1 min, at which point CsF was completely dissolved in methanol. Then, a stoichiometric amount of H2SiF6(6:2 molar ratio of CsF to H2SiF6) was poured into the CsF aqueous solution, at which point a white precipitate formed immediately. The measured amount of H2SiF6was 1 mmol ~0.1441 g which is equivalent of 0.4117 g acidic solution of H2SiF6 (35 weight%). The synthesis followed equation (2):31248341.117105024-201 6^^^^^^ + 2^^2^^^^^^6 + ^^^^^^2 → 3^^^^2^^^^^^6 ↓ +2^^2^^ (2)

[0090] The reaction was performed for 30 min and the precipitate was separated outthrough gravity filtration. At room temperature, the damp precipitate was dried. XRD confirmed the phase of the precipitated solid. Fig.11 shows the representative diffraction patterns recorded for as-synthesized Cs2SiF6. The XRD pattern of obtained white precipitate (upper curve) exhibited a perfect match with the published Cs2SiF6 (Fm-3m) cubic structure (ICDD: 00-007-0006) shown in the lower curve. 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%. Thus, this approach facilitates the immobilization of 0.4557 g CsF through aqueous solution into 0.6118 g Cs2SiF6in 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. Summary of MHP (Cs2SiF6) synthesized in methanol at room temperature. Synthesis Reactants Product Product Reaction route (Targeted) (Obtained) Yield (%)

[0091] As with Cs2SiF6 synthesized in acetone and DI water (data not shown), theRietveld refinement of this obtained Cs2SiF6powder was perfectly matched with the Fm-3m cubic structure and therefore, the composition was refined as Cs2Si1.01F6.36 from the Wyckoff positions of these elements and tabulated in Table 6. The obtained variation in Cs, Si, and F ions with the respect to standard composition is only 0%, -1.1%, and +6%, respectively. TGA results also shows good thermal stability up to 650oC by the observed negligible 0.6% weight loss as shown in Fig.12.

[0092] SEM micrographs as in Fig. 13 show homogeneous, spherical grains ofsynthesized powder of a few micrometers in diameter. EDS analysis performed on the surface of Fig.13 (data not shown) demonstrate the homogeneous distribution of Cs, F, and Si. The structural, microstructural, and thermal properties of Cs2SiF6 show a single phase MHP stable31248341.118105024-201 up to 650oC. These results validate the single step immobilization of CsF into Cs2SiF6as a promising MHP waste form. Table 6. Refined parameters for Cs2SiF6 synthesized through SiO2-assisted RT solution chemistry in methanol. Parameters Cs2SiF6 (In methanol) a = b= c(Å) 8.9184(0).

[0093] To demonstrate the effective separation of SrF2 from LiF, a model sample isprepared with LiF is mixed with 10 wt% SrF2. Note that the actual content of SrF2 from reactor waste will likely be significantly lower than the model sample, depending on the burnup of nuclear fuels during reactor operation. As such, the results and yields reported here are conservative.

[0094] To compare the solubility of SrFI with LiF and SrF2, after dispersing LiF (1g)and SrF2(100 mg) in 500 ml DI water, SrI2(400 mg) was added to the solution mixture and heated at 100 °C for 12 hrs to form an initial mixture. After redispersing the precipitates into DI water, SrFI will be dissolved along with minor amount of SrF2 and LiF below their solubility. The filtered solution was dried, and the obtained solid mixture was characterized by XRD. As a comparison, the initial mixture was evaporated. Both the initial mixture and the filtered and dried solution XRD results reveal the formation of SrFI phase through this wet solution approach. Moreover, after filtration, the concentration of the SrFI phase is much enhanced, and the amount of SrF2is greatly reduced. These results testify to our designing principles and calculated results, showing that SrFI solubility is much higher than LiF and SrF2. By converting SrF2into SrFI, then the majority of radioactive SrF2waste can be separated out from LiF. Since SrFI and SrI2 have relatively high solubility in DI water, after dissolution and filtration of the solvent (DI water), only LiF and unreacted SrF2will be left in final precipitates. The refined XRD data demonstrate that the final product consists primarily of LiF and a small amount of SrF2 (2.67 wt%) residual. Compared with the initial unreacted31248341.119105024-201 LiF and 10 wt% SrF2mixture, more than 73% of SrF2was removed and the efficiency of recycling LiF could be enhanced significantly after removal of SrF2. Low temperature sintering to form cermets and halmets.

[0095] To consolidate Cs2SiF6 into copper matrix, a mixture of 50% Cs2SiF6 and 50%Cu powder by volume ratio was mixed by ball milling in a zirconia jar. A 50% volume loading of Cs2SiF6 in copper is equivalent to a 30% weight loading of Cs2SiF6 in copper. 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).

[0096] To avoid possible decompensation of the contained Cs2SiF6, spark plasmasintering (SPS) was performed at 300 °C and 400 °C. In addition, 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.

[0097] The consolidated cermet sample was characterized by XRD and the results areshown in the top pattern shown in Fig.14. For comparison, the cubic phase of Cs2SiF6 (ICDD: 00-004-0836) is shown as the middle pattern and the cubic phase of Cu (ICDD: 00- 004-0836) is shown as the lower pattern. The obtained diffraction pattern perfectly matched with a mixture of these two patterns. For ease of visualization, the three peaks representing copper are highlighted with black triangles. Consequently, XRD analysis shows that the prepared pellet consists of a mixture of two phases: copper and Cs2SiF6. A Rietveld refinement was carried out to quantitatively analyze both phases in the cermet pellet as well as in the ball-milled powders. Table 7 summarizes the structural parameters for both refined patterns.31248341.120105024-201 Table 7. The structural parameters obtained from the Rietveld refinement of diffraction patterns of ball-milled powder and cermet pellet. Sample a (Å) Volume PhaseWyckoff positions Rp, Rwp, Re(Å)3fraction and ^27, d 0, d

[0098] Based on XRD, the phase fraction of Cs2SiF6 and Cu in the as-prepared ball-milled powder was estimated to be 31.5% and 68.5%, respectively. Nearly identical phase fractions of both Cs2SiF6and Cu were observed in the sintered pellet. Structural refinement confirms that there is no loss of Cs2SiF6 during consolidation through low temperature sintering or SPS. The refined phase fraction of Cu and Cs2SiF6were equal to the weight % loading. It is deducible from the XRD studies with structural refinement that the consolidation of Cs2SiF6within the Cu matrix occurred without any discernible loss of the salt or phase transformation.

[0099] The low melting temperature of copper and the partial solubility of Cs2SiF6 inwater facilitated the consolidation by cold sintering at 100°C, preserving the integrity of the ceramic structure. A microstructural analysis was carried out via SEM to evaluate distribution of both phases. Moreover, EDS analysis was performed to validate the micro-chemistry in the sample. Figs.15 and 16 show SEM images taken at different magnifications. EDS analysis determined that the lighter areas 810 correspond to Cs2SiF6and the darker areas 820 to metal binder.

[0100] Copper was uniformly dispersed within the material, occupying the interstitialcracks and voids among the Cs2SiF6 particles. EDS mapping validates the elemental distribution of Cs, Cu, Si, and F. Hence, 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.

[0101] The mechanical properties of the cermet sample were evaluated throughmicroindentation using 1 kgf (9.8 Newton) for a duration of 15 seconds. The calculated31248341.121105024-201 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. For comparison, 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 Cs2SiF6sample. Table 8. A summary of the mechanical properties of a copper cermet of Cs2SiF6with pure Cs2SiF6. Mechanical Properties Cs2SiF6 CERMET Hardness (GPa) 0.43 ± 0.03 1.1 ± 0.09

[0000] o ab y, e cerme samp e s ows a .9 mes arger e as c modu us an pureCs2SiF6., demonstrating that the consolidation of Cs2SiF6 into the Cu matrix significantly improved the mechanical properties compared to pure Cs2SiF6.

[0103] An investigation into the cermet’s thermal properties was conducted,predicated on the hypothesis that the continuous, highly conductive copper matrix would enhance its thermal conductivity. Given that a substantial volume fraction (50%) of highly conductive Cu is uniformly distributed within the cermet, it is expected to form continuous conductive pathways that serve as effective heat carriers. 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. For the pure salt sample (hollow circles, left y-axis), the thermal diffusivity exhibited a decline from 0.53 mm2 / s to 0.17 mm2 / s with a temperature increase from 25 °C to 500 °C.31248341.122105024-201 Conversely, the cermet’s thermal diffusivity (solid circles, right y-axis) demonstrated an exponential rise from 7.7 mm2 / s to 13.2 mm2 / s over the same temperature range.

[0104] This larger thermal diffusivity of cermet shows better thermal propertiesprovided by highly conducting Cu matrix. Thus, the enhancement of thermal properties significantly reduces the temperature gradient between the centerline and the surface of the nuclear waste containment. The immobilization of Cs into MHP with larger loading, coupled with the consolidation of Cs-based MHP within a copper matrix through cold sintering at 100°C, significantly advances cermet technology for nuclear waste immobilization.

[0105] These results demonstrate successful immobilization of CsF into Cs2SiF6using a one step SiO2assisted wet chemistry process at room temperature, and the encapsulation of Cs2SiF6 into a copper metal-bonded composite at a 50:50 volume ratio (30:70 weight ratio) of MHP to metal. Low temperature SPS sintering was used to sinter the cermet composite at 300~400oC for 5 minutes to mitigate possible phase decomposition or loss of highly volatile Cs and halide. To further improve the manufacturing viability and improve the cost effectiveness for manufacturing, an innovative cold sintering process using distilled water as a sintering agent was demonstrated to fabricate the cermet composites at temperatures as low as 100oC and achieve 92% theoretical density (TD) for the densified composite pellets. Moreover, the MHP is encapsulated into the copper matrix without any loss of salt due to very low temperatures for sintering and consolidation.

[0106] Further experiments were performed to investigate the effect of different wasteloadings on microstructure and mechanical properties. Lower loading pellets were sintered with a 40:60 volume ratio of Cs2SiF6 to copper, corresponding to a 22.7% weight loading of Cs2SiF6in copper. Specifically, 247.6 mg Cs2SiF6powders 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 100oC 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 300oC, or 400oC, 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 100oC, (b) prepared by31248341.123105024-201 cold sintering at 100oC, followed by SPS at 300oC, and (c) prepared by cold sintering at 100C, followed by SPS at 400oC, 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. Moreover, EDS analysis was performed to validate micro-chemistry in cermet samples. The phase of the cermet composite was analyzed by x-ray diffraction (XRD).

[0107] The physical density for the 40 vol% Cs2SiF6 cermet composite pelletsdensified at 100, 300 and 400oC were measured as ~84%, 86%, and 91.8% TD through Archimedes’ principle, indicating that for this lower % MHP sample, a high degree of densification is not achieved by cold sintering at 100oC alone. This surprising result is in contrast with our previous result for a 50 vol% Cs2SiF6 cermet achieving a 93% TD by cold sintering at 100oC for 5 minutes. Without being bound by theory, it is hypothesized that the higher physical density for the pellet achieved with a higher amount of MHP fraction could be attributed to the dissolution-reprecipitation mechanism responsible for cold sintering process. In particular, Cs2SiF6 can be partially dissolved in DI water to enhance the elemental transport and atomic diffusion in facilitating densification within transient solvent. By adding a few water droplets during the sintering body, the consolidation of the cermet pellets can be greatly enhanced at a temperature as low as 100 °C, preserving the integrity of the ceramic structure. However, with larger amount of metal phases in the composite, the effects of dynamic dissolution and transport behavior could be suppressed as a result of high resistance of Cu against water dissolution. This result implies, counterintuitively, that a greater amount of Cs2SiF6 can improve the sinterability and physical density of the composite and increase the waste loading simultaneously.

[0108] The phases of the consolidated cermet pellets were characterized by XRD asdiscussed previously for the 50:50 MHP:copper sample. Similar to the pattern shown in Fig. 14 for the 50:50 sample, the XRD patterns of all three pellets have diffraction maxima belonging to that of copper and MHP, and no measurable loss of Cs2SiF6 during consolidation, consistent with our expectation based on the sintering temperature being lower than the phase decomposition temperature of the MHP (~600oC, as shown in Fig.12).

[0109] The microstructures of the MHP-Cu cermet composites sintered by coldsintering followed by low temperature SPS was characterized by SEM as. As previously discussed, the SEM results for the 50:50 cermet composite show that the copper is uniformly dispersed within the material matrix, interlacing with the Cs2SiF6 particles.31248341.124105024-201

[0110] For the 40-60 MHP-Cu cermet composites, the waste-loaded MHP phase wasalso well preserved in the Cu matrix. Additionally, sintering at higher temperature improved particle bonding, with copper more effectively encapsulating the MHP regions at 300 °C and at 400 °C, and fewer cracks and voids as the temperature increased for 300 C to 400 C. As was found for the 50% by volume MHP sample, EDS mapping again validates the expected elemental distribution of Cs, Cu, Si, and F. Hence, SEM and EDS analyses conclusively demonstrate the absence of elemental mixing or reactive phase formation. Instead, the two distinct phases are independently maintained, exhibiting a homogenous integration of their respective microstructure. The composite cermet of Cs2SiF6 with a copper matrix comprising 40:60 volume ratio of MHP to metal shows better microstructure than the 50:50 volume ratio of MHP to metal despite its lower physical density.

[0111] Further experiments were performed to examine higher MHP loadings, andthe use of other metal matrices, including Al and SS-316 stainless steel. Based on these experiments, it is found that loadings of up to 70% MHP may be effectively integrated within metal matrices using cold sintering at 100 °C, and that such cermets can be further densified by SPS at temperatures of 300 °C or less, or at 400 °C or less. Accordingly, 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. Such sintering approaches allow for rapid densification of cermets at lower temperatures than are conventionally used, effectively confining processing and confining highly volatile fluoride waste products without significant loss or decomposition. Suitable metal matrices include but are not limited to copper, aluminum, stainless steel, and combinations thereof.

[0112] As an example, low temperature SPS of a sample of of 33 wt% Cs2SiF6 and67 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. As another example, Fig.18 shows SEM results for a 60:40 wt. % MHP:Al cermet of Cs2SiF6 in an aluminum matrix, treated by cold sintering at 100 °C and then consolidated by SPS at 300 C for one minute. For each of these sintering processes, good cohesion and mechanical properties were obtained.

[0113] Cold sintering experiments using a transient solvent also demonstrates thedensification not only of cermets, but also of halmets, including SrF2and CsF in copper and aluminum matrices.31248341.125105024-201 Low temperature sintering in glass matrices.

[0114] Glass matrices provide another effective option to immobilize MHPs andmetal halides. To immobilize the MHP Cs2SiF6 in a glass (amorphous SiO2) matrix, silica gel was mixed with the MHP at a 25:75 mass ratio of MHP to glass. To obtain the 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 Cs2SiF6surfaces and promote the transport behavior to reduce the sintering temperature. The cold sintering was performed at 300oC 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.

[0115] The microstructure of the MHP-SiO2 composites sintered by cold sinteringand followed by low temperature SPS was characterized by SEM as shown at two different magnifications in Figs.19 and 20. From these figures it can be seen that SiO2is uniformly dispersed within the material matrix, interlacing with the Cs2SiF6 particles. The glassy composite showed a densification of 97% TD. EDS mapping (not shown) validates the elemental distribution of Cs, F, Si, and O. Hence, SEM and EDS analyses conclusively demonstrate the absence of elemental mixing or reactive phase formation. Instead, the two distinct phases are independently maintained, exhibiting a homogenous integration of their respective microstructure.

[0116] The microstructural data clearly demonstrates that in both glass-ceramic andmetal-ceramic composites, the MHP or metal halide can be effectively integrated into the glass or metal matrices using cold sintering in the presence of transient solvent at temperatures of less than about 100 °C, by means of SPS sintering at temperatures of less than about 400 °C (in some cases less than about 300 °C), or by a combination of cold and SPS sintering.

[0117] What has been described and illustrated herein is an example along with someof its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the31248341.126105024-201 spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.31248341.127

Claims

105024-201 What is claimed is:

1. A method of treating a molten salt reactor waste comprising metal fluorides, the methodcomprising: processing the molten salt reactor waste to form a solid fluoride composition; mixing the solid fluoride composition with a sinterable composition selected from the group consisting of a metal powder and a glass-forming material comprising SiO2; sintering the solid fluoride composition with the sinterable composition by a sintering process to obtain: a) if the sinterable composition includes a metal powder, a metal compositecomprising the solid fluoride composition as a phase within a metal matrix, b) if the sinterable composition includes a glass-forming material, a glass-bonded composite comprising the solid fluoride composition as a phase within a glass matrix, wherein the sintering process is selected from the group consisting of cold sintering performed in the presence of a transient solvent at less than about 100 °C, spark plasma sintering at a temperature of less than about 400 °C, and combinations thereof.

2. The method of claim 1, wherein the solid fluoride composition and the sinterablecomposition are combined at a weight percentage of less than about 70% fluoride composition.

3. The method of claim 1 or claim 2, wherein processing the molten salt reactor waste toform a solid fluoride composition comprises: 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; processing the first solution to form the solid fluoride composition.

4. The method of any one of claims 1 to 3, wherein the sinterable composition is the metalpowder.

5. The method of any one of claims 1 to 3, wherein the sinterable composition is the glass-forming material, and wherein the glass-forming material further comprises B2O3.31248341.128105024-2016. The method of claim 4 wherein the metal powder includes a metal selected from thegroup consisting of copper, aluminum, stainless steel, and combinations thereof.

7. The method of any one of claims 1 to 6, wherein the solid fluoride composition and thesinterable composition are combined at a weight percentage of between about 30% and about 70% fluoride composition.

8. The method of any one of claims 1 to 7, wherein the sintering process comprises sparkplasma sintering.

9. The method of any one of claims 1 to 7, wherein the sintering process is cold sinteringperformed in the presence of a transient solvent added at between about 1 volume % to about 5 volume %.

10. The method of claim 9, wherein the transient solvent is water or methanol.

11. The method of any of claims 1 to 10, wherein the solid fluoride composition comprises ametal fluoride salt.

12. The method of any one of claims 1 to 10, wherein the solid fluoride compositioncomprises a metal halide perovskite (MHP), formed by mixing a metal fluoride salt with H2SiF6 in the presence of SiO2 and solvent to form the solid fluoride composition comprising the MHP as a metal salt of the hexafluorosilicate anion.

13. The method of any one of claim 11 or claim 12, wherein the metal fluoride salt is selectedfrom the group consisting of alkali metal halides, alkaline earth metal halides, and combinations thereof.

14. The method of claim 13, wherein the metal fluoride salt is CsCl or SrCl2.

15. The method of claim 12, wherein the solvent is water.31248341.129105024-20116. The method of claim 12, wherein the metal fluoride salt is CsF and the solvent is selectedfrom the group consisting of methanol, acetone, dimethylformamide, and combinations thereof.

17. The method of claim 16, wherein the solvent is methanol.

18. The method of claim 3, wherein the water-soluble metal fluorides include CsF, whereinCs is present as a radioactive isotope, and wherein the solid fluoride composition is formed by a procedure comprising: 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; processing the solution of CsF to form the solid fluoride composition.

19. The method of claim 18, wherein the solid fluoride composition comprises CsF, and isformed by evaporating the organic solvent from the solution of CsF.

20. The method of claim 18, wherein the solid fluoride composition comprises a cesiumhalide perovskite, formed by mixing the solution of CsF with H2SiF6in the presence of SiO2to form the solid fluoride composition comprising the cesium halide perovskite as a cesium salt of a hexafluorosilicate anion.

21. The method of any one of claims 18 to 20, wherein the organic solvent is selected fromthe group consisting of methanol, acetone, dimethylformamide, and combinations thereof.

22. The method of claim 21, wherein the organic solvent is methanol.

23. A method of removing and sequestering an alkali metal M from a compositioncomprising 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 H2SiF6 and SiO2 to the solution with mixing, thereby forming a precipitate of M2SiF6; mixing the precipitate of M2SiF6 with a sinterable composition selected from the group consisting of a metal powder and a glass-forming material comprising SiO2;31248341.130105024-201 sintering the precipitate of M2SiF6with the sinterable composition to obtain a) if the sinterable composition is a metal powder, a ceramic metal composite(cermet) comprising M2SiF6 sequestered within a metal matrix, b) if the sinterable composition is a glass-forming material, a glass-bondedcomposite comprising M2SiF6 sequestered within a glass matrix, wherein the solid fluoride composition and the sinterable composition are combined at a weight percentage of less than about 70% fluoride composition, and wherein the sintering process is selected from the group consisting of spark plasma sintering at a temperature of less than about 400 °C and cold sintering performed in the presence of a transient solvent at less than about 100 °C.

24. The method of claim 23 wherein the solvent is water.

25. The method of claim 23, wherein the solvent is a polar organic solvent and MF is CsF.

26. The method of claim 25, wherein the polar organic solvent is methanol.

27. A method of treating a molten salt reactor waste comprising water-soluble salts andwater-insoluble salts, the water-insoluble salts comprising7LiF 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 SrI2 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; mixing 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) if the sinterable composition includes a metal powder, a halide metalcomposite (halmet) comprising the third solid as a phase within a metal matrix,31248341.131105024-201 b) if the sinterable composition includes a glass-forming material, a glass-bonded composite comprising the third solid as a phase within a glass matrix.

28. The method of claim 27, wherein the 7LiF is recycled for use in a molten salt compositionfor a molten salt reactor.31248341.132

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