Decontamination of Chloride Salt Solvent from Spent Chloride Salt Fuel and Pyro–Electrometallurgical Processing Salt for Recycling—A Review

Abstract

Alkaline and alkaline earth metal chloride salts are used in molten chloride fast reactors (MCFRs) and pyro–electrometallurgical (or –electrochemical) recovering of uranium and transuranic elements (PERUT) from spent nuclear fuel. Reprocessing of MCFR spent fuel with the PERUT process, after recovery of U and transuranic elements (Np, Pu, Am, Cm), results in a chloride salt solvent waste stream containing fission and activation product chlorides. Recycling the chloride salt solvent by separation of fission and light element activation products (FPs and LEAPs) is highly desired because of the low chloride loading in the available glass and ceramic waste forms. This paper reviews the status of chloride salt waste management, chloride salt recycling studies, and potential FP and LEAP chlorides sequestration approaches. The chloride salt solvent recycling studies are represented by chemical precipitation of rare earth (RE) fission product chlorides with carbonate, O₂ gas and phosphate in LiCl and eutectic LiCl-KCl salt solvent, which is then followed by separation of Cs and Sr with distillation or crystallization. More than 99% removal efficiencies are attained for RE FP chlorides, and distillation removes more than 99% of Sr and Ba from the salt solvent. Volatile species released from the operation of MCFRs need to be sequestered. Minor chlorides species, such as SnCl₃, FeCl₃, CrCl₃, and ZrCl_2, will be present in the waste stream, and the separation of these species will be required for salt solvent recycling. Bromine and iodine can form bromides and iodides with metal elements such as alkaline and alkaline earth metal elements, which behave chemically similarly to their chloride counterparts. The presence of these compounds in the salt solvent waste may complexify the recycling process, for which more experimental studies are required.

1. Introduction

Alkaline and alkaline earth metal chloride salts are used in molten salt reactors that are designed to operate with a fast neutron spectrum and burn transuranic (TRU) elements, such as Np, Pu, Am, Cm, etc., in a matrix consisting of fertile materials to fully close the fuel cycle, while generating energy at the same time. Because a fast neutron spectrum enables the production of heavy elements with high fissile Pu isotopic ratios, the spent fuel of fast spectrum reactors such as PHÉNIX contains high fissile isotopic ratios that merit reprocessing and recycling [1,2]. Similar spent fuel characteristics are expected to be seen in the spent fuel from molten chloride fast reactors (MCFRs). Secondly, it is advantageous to use enriched ³⁷Cl in the chloride salt fuel to minimize the generation of radioactive waste ³⁶Cl, a beta emitter with average energy of 0.251 MeV and a long half-life of 3.01 × 10⁵ years, from ³⁵Cl(n,γ)³⁶Cl reactions, and the recycling of enriched ³⁷Cl is desired [3,4].

Alkaline and alkaline earth metal chloride salts are also used as a solvent in the pyro–electrometallurgical (or –electrochemical) recovering of uranium and transuranic elements (PERUT) from spent nuclear fuels, where TRU elements are mainly the major/minor actinides Np, Pu, Am, and Cm. This technology has been demonstrated at a pilot scale, being second only to the commercial-scale aqueous plutonium uranium extraction (PUREX)-based reprocessing technologies. For spent chloride salt fuel reprocessing with the PERUT process, the use of enriched ³⁷Cl chloride salts in the PERUT process is desired to avoid the dilution of ³⁷Cl by natural chlorine when spent chloride salt fuel is reprocessed. The use of enriched ³⁷Cl merits the recycling of ³⁷Cl chloride salts in reducing the ³⁷Cl enrichment cost.

Since both MCFRs and PERUT employ chloride salts, the spent chloride salt fuel from the MCFRs can be reprocessed in principle more easily with the PERUT technology than with PUREX-based technologies, forming a closed fuel cycle [5]. The same principle would also apply to processing of molten fluoride salt reactor spent fuel with the fluoride volatility (FV) method [5]. However, this type of closed fuel cycle approach has been less researched as MCFRs are only in a design stage with no experimental or operating data available.

The introduction of spent chloride salt fuel into the PERUT process would complexify the electrochemical reactions and generate a combined waste stream of the PERUT and MCFR chloride salts. This combined waste steam will be more complex to manage than the salt waste from reprocessing spent UO₂ or metallic fuel, including the salt decontamination, salt recycling, and final waste conditioning. This paper reviews the status of PERUT chloride salt waste management technologies, explores the chemical and physical properties of key fission products (FPs) and light element activation products (LEAPs) relevant to salt decontamination, and discusses, in the above combined waste stream scenario, an integrated approach of potential salt waste cleanup and minimization methods.

The review is conducted against the backdrop of increasing efforts toward the development of GenIV reactor technologies, including MCFRs for actinide waste transmutation, and of increasing interest in proliferation-resistant pyroprocessing of spent fuel for recycling, which is considered as a promising alternative to the PUREX technology in the context of small modular reactor development [6,7]. MCSRs operate at a coolant outlet temperature of greater than 600 °C, higher than that for water-cooled reactors, and deliver higher temperature, greenhouse-gas-lean process heat. These features are important and can be exploited in the broader decarbonization scheme, e.g., supporting replacement of coal-fired power plants [6].

2. Potential Chloride Salts in Nuclear Applications

For the MCFR designs, a wide range of salt mixtures consisting of NaCl, KCl, RbCl, MgCl₂, CaCl₂, SrCl₂, etc., are considered as salt solvents for UCl₃ and (TRU)Cl₃ [5]. TRU usually consists of 90 wt% Pu.

The LiCl salt (melting point 610 °C) for electro-reduction of oxide fuel and a eutectic salt mixture of 3LiCl–2KCl (eutectic melting point 348 °C) for electrorefining and electrowinning of U and TRU metals have primarily been employed in the PERUT process development, because of the lower melting points, higher lanthanide (Ln) chloride solubilities, and higher electrical current efficiencies of the LiCl and LiCl–KCl salts than those of systems consisting of

• CaCl₂ salt (melting point 775 °C) for electroreduction [8,9], or
• a mixture of equal molar CaCl₂–NaCl (eutectic melting point 499.2 °C [10,11]) or NaCl–2CsCl (eutectic melting point 486 °C [12]) for electrorefining and electrowinning [13].

LiCl salt is spiked with 1 to 3 wt% Li₂O in the electro-reduction of spent oxide fuel. Li₂O acts as a medium in uranium oxide reduction, in which Li metal produced from Li₂O reduction at the cathode is used to convert uranium oxide to uranium metal.

The compositions of several salt mixtures studied for nuclear applications including pyro–electrometallurgical process and MCFRs are summarized in

Table 1. Salt mixtures in nuclear applications.

Application	Salt Mixture	Melting Point (m.p.)	Ref.
PERUT: Electrometallurgical reduction for oxide fuel (not for salt fuel)	mol%: 98.6LiCl–1.4Li2O wt%: 99LiCl–1Li2O, LiCl–0.9Li2O–6.2SNF *	~610 °C	[14,15]
PERUT: Electrometallurgical refining and wining	mol%: 59LiCl–41KCl (eutectic mix) wt%: 45LiCl–55KCl, 40LiCl–49KCl–11SNF *	~355 °C	[5,14]
MCFR: Molten chloride salt fast reactors for transuranic element burning and/or breeding (use of enriched 37Cl preferred)	mol%: 62.6NaCl–37.4(TRU)Cl3 wt%: 22NaCl–78(TRU)Cl3	~452 °C	[16]
	mol%: 41.5MgCl2–50NaCl–8.5(TRU)Cl3 wt%: 40MgCl2–30NaCl–30(TRU)Cl3	~445 °C	[16]
	mol%: 55NaCl–29.4UCl3–15.6PuCl3 wt%: 17NaCl–54UCl3–29PuCl3	~600 °C	[17]
	mol%: 42.5KCl–30.5SrCl2–27RbCl ** wt%: 28KCl–43SrCl2–29RbCl	~514 °C	[5]
	mol%: 48NaCl–52CaCl2 ** wt%: 33NaCl–67CaCl2	~507 °C	[5]

* SNF—spent nuclear fuel. **—additions of U and/or TRU will reduce the solvent salt concentrations.

.

The alkaline and alkaline earth chloride salts that can be used for nuclear applications include LiCl, NaCl, KCl, RbCl, MgCl₂, CaCl₂, and SrCl₂. The synergies are evident between the chloride salt-based pyro–electrometallurgical reprocessing method and spent chloride salt fuel because it is possible to use the salt solvent in the salt fuel as the PERUT process salt. This approach would simplify the salt purification process but may be challenging to the PERUT equipment and operation conditions because individual salt systems behave chemically differently in the PERUT process, such as electrical redox potentials, electrode material selection, and melting points that affect the optimal process temperatures. If the LiCl–KCl eutectic salt is used, more complex systems such as NaCl–LiCl–KCl, NaCl–MgCl₂–LiCl–KCl, NaCl–CaCl₂–LiCl–KCl, and SrCl₂–RbCl–LiCl–KCl will form during the PERUT processing of spent chloride fuel salts. For reprocessing of oxide fuel in the LiCl–KCl eutectic salt, the amount of spent UO₂ fuel per batch of reprocessing is approximately 10 wt% of the salt [14]. The quantity of spent chloride salt fuel to be introduced into a batch of the LiCl–KCl PERUT system will be determined by the uranium and transuranic metal chloride solubilities in the combined salt mixtures at the typical operating temperature of 450 °C.

After U and TRU metals are reprocessed and the FPs and LEAPs are accumulated to certain amounts in the salt that will significantly affect the operating chemistry, the FPs and LEAP are separated for salt recycling, instead of disposing the entire salt as waste. The efficiency of salt waste management will include recovery and recycling of LiCl, NaCl, KCl, RbCl, MgCl₂, CaCl₂, and SrCl₂ if they are used as solvents [14,18].

3. Salt Waste Management

The schemes of PERUT process steps required for various spent fuel types and volatile and gas release are shown in Figure 1.

MCFRs operate in a temperature range of 600 °C to 700 °C [19]. Significant volatile FP and LEAP chlorides and gases will be released during reactor operation [20,21]. The Molten-Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory (ORNL), the only molten fluoride salt fuel reactor, operated at approximately 700 °C in the past [22], and various gaseous and volatile materials were released [23,24]. In contrast, solid fuel retains most volatile FPs and LEAPs except for small fractions released from defective fuel elements, which are released later during the reprocessing stage [14]. This paper focuses on waste salt management associated with the MCFR chloride salt fuels and PERUT chloride salts.

Figure 1. Potential PERUT process steps for various fuel types. *—Voloxidation–volumetric oxidation involves heating oxide fuel with oxygen, sometimes with alternating oxidation and reduction, to oxidize UO₂ to a fine U₃O₈ powder with high surface area for dissolution [25].

Figure 1. Potential PERUT process steps for various fuel types. *—Voloxidation–volumetric oxidation involves heating oxide fuel with oxygen, sometimes with alternating oxidation and reduction, to oxidize UO₂ to a fine U₃O₈ powder with high surface area for dissolution [25].

The electrorefining of the spent metallic fuel from Experimental Breeder Reactor II (EBR–II), a sodium-cooled fast reactor operated from 1964 to 1994 at Argonne National Laboratory—West (ANL–W) has been developed and practiced for recovering enriched uranium and fissile materials from breeding blankets. Two electro-refiners, Mk–IV and Mk–V, have been operating with a LiCl–KCl eutectic salt mixture since 1996 and 1998 respectively. The Mk–IV and Mk–V are used to process enriched metallic uranium fuels and depleted U blanket fuels, respectively [26]. The capability and technology of transuranic element recovery with a liquid cadmium cathode were demonstrated at a bench scale in the 2000s but have not been practiced with the refiners. The process salt, after the accumulation of transuranic elements and fission and activation products to a certain level makes the processing operation unsustainable, is discharged and stored as waste [27,28]. The following approaches have been investigated for managing this salt waste stream.

3.1. Direct Immobilization

There are limited choices of waste form to immobilize chloride ions. The most reliable inorganic waste form matrix is sodalite (Na₈(CI₂Al₆Si₆O₂₄)), a natural mineral analogue, which can incorporate approximately 7 wt% of chloride or 12 wt% NaCl or 15 wt% KCl [29]. A higher waste loading of 73 wt% clinoptilolite containing 21 wt% Cs or 26.6 wt% CsCl is achieved in a monolithic glass composite material composed of clinoptilolite, sodalite, wollastonite and CsCl. However, in this system, the clinoptilolite and CsCl are not fully encapsulated in the glass matrix, leading to a considerable Cs leaching rate [30]. Engineering the glass composite to reduce the Cs leaching may be required in practical applications. The overall CsCl loading in the waste form is about 19 wt% CsCl, equivalent to a loading of 4 wt% chloride.

Alternatively, waste chloride salts can be converted to oxides, which are then immobilized in glass form such as aluminosilicate or phosphate glass having a waste oxide loading range from 15 wt% to 35 wt% with a very small tolerance for halides (or halide anions). The conversion process comes with other implications [31,32,33]:

• For natural chlorine, ³⁶Cl, an activation product of ³⁵Cl(n,γ)³⁶Cl reactions and a long-lived weak beta emitter in the used fuel, would be released as a gaseous species and must be captured for immobilization before releasing the conversion process off-gas to the environment.
• In the case of enriched ³⁷Cl used in the reactor and electrometallurgical processes, ³⁷Cl needs to be captured from the off-gas for recycling.

3.2. Direct Disposal

The electro-refiner salt could be an excellent candidate for disposal in a geologic salt formation—without any encapsulation or stabilization in a waste form [34,35,36]. Experiments were performed to assess the stability of surrogate electro-refiner salt in the presence of simulated brine that would be present in a salt dome [37,38]. The results indicated reduced solubility and rates of dissolution. This study was followed up by a detailed performance assessment calculation that indicated no appreciable impact to the surrounding environment from direct disposal of the electro-refiner salt into a generic salt repository.

3.3. Waste Minimization

Since the immobilization process significantly increases the final waste volume that requires disposal, waste salt minimization (or process salt decontamination and recycling) technologies are developed, such as chemical reactions and precipitation, physical distillation, and crystallization. In addition, since the use of enriched ³⁷Cl in the MCFR is recommended, it is sensible for the pyro–electrometallurgical process to use enriched ³⁷Cl process salt. Decontamination and recycling of enriched ³⁷Cl salts during the PERUT process can reduce the cost and thus the demand for ³⁷Cl enrichment [3]. For direct salt waste disposal, decontaminating and recycling a substantial fraction of used process-salts will also improve the direct disposal feasibility.

4. Salt Waste Decontamination for Recycling and Waste Minimization

4.1. Zeolite Ion Exchange

The early development of pyro–electrometallurgical processing in the 1990s identified zeolite 3A as showing preferentially extracting rare earth metal fission products over alkalis from the LiCl–KCl melt, and zeolite 4A preferentially extracted K and Cs over the alkaline earth metals Sr and Ba [29,39].

The heat-generating Cs and Sr could be loaded to zeolite A, in which Cs sorption occurred more rapidly than Sr and also showed a higher ion exchange capacity than Sr [40]. After the sorption process, the zeolite retained a significant “free salt” (or not occluded by the zeolite), accounting for 20 wt% of a post-sorption zeolite sample. Approximately 50 wt% of this “free salt”, which had negligible ion exchange with the zeolite due to a minimal contact with the zeolite, is removed by applying a vacuum pressure of 0.068 to 0.68 Bar (6.8 to 68 kPa) in a column setting, and additional zeolite must be added to immobilize the free salt in waste form preparation. Extra “free salt” removal measures are desired for further waste reduction [41].

A higher temperature would allow more fission products to be removed, but the temperature has to be controlled below 550 °C to avoid the zeolite degradation that was observed above 600 °C. A series of zeolite columns would be needed to clean up fission products in the used salt. Pre-treatment of used salt with physical crystallization or other means can be coupled with this sorption method in minimizing the salt waste to be treated with the zeolite columns. Fission-products-depleted salt can be recycled back to the electro-refiner. Fission product sorption onto zeolite A was also observed at temperatures below the salt melting point or without the presence of LiCl–KCl with the fission product chlorides, likely through diffusion in solid solution.

Zeolite A (3A and 4A) was primarily tested with the eutectic LiCl–KCl mixture at 400 to 450 °C. Experiments later confirmed that the ion exchange processes of zeolite A (4A and 5A) worked with the LiCl–LiO₂ system at 650 °C, but with the transformation of zeolite A into zeolite Li–Al (Li₂Al₂Si₂O₈) accompanied by the formation of a new phase of sodalite (Na₈(AlSiO₄)₆Cl₂) [42,43]. The sodalite formation was enhanced in the presence of CsCl or SrCl₂. This structural transformation was not reported in the tests with LiCl–KCl at 450 °C [39]. Ion exchange process results for Sr and Cs at 650 °C were similar to the results for the LiCl–KCl salt. Further studies suggested that maintaining the zeolite crystalline structure would be required to achieve a meaningful degree of ion exchange during the salt and zeolite contact process [44].

Zeolite 4A or 4A molecular sieve adsorption was also used to clean up residual erbium in the range of 500 °C to 575 °C after an electrochemical extraction of Er in the mixture of LiCl-KCl-ErCl₃ with a liquid gadolinium cathode to form Gd-Er alloy Ga₆Er [45]. The electrolysis recovery rate of Er is 92.6%, and the zeolite took out 99.93% of the residual Er in the electrolyzed melt, achieving an overall Er removal efficiency of 99.99% from the combined electrochemical extraction or reduction and zeolite 4A adsorption processes.

Zeolite (or molecular sieve) 5A was tested for removal of residual Tb(III) species in a LiCl-KCl-NiCl₂-TbCl₃ system, with a Tb removal efficiency of 99.6% at 500 °C, after 98.7% Tb was first electrochemically reduced to form Ni-Tb alloy on a tungsten electrode within a testing temperature range of 500 °C to 575 °C [46]. The overall removal efficiency from the two-step process was 99.98%. The same author also used zeolite 5A to separate residual ytterbium from LiCl-KCl-YbCl₃ at 500 °C, to increase the total Yb separation efficiency after an electrochemical reduction process to reduce Yb(III) to form Tb-Gd alloy on the liquid gadolinium cathode [47]. The zeolite 5A removed 99.95% of residual Yb while the electrochemical reduction removal efficiency was 93.78% of initial Yb. The total Yb removal from the LiCl-KCl-YbCl₃ system reached 99.99%.

Two studies similar to the above were performed. He et al. tested the electrolysis and adsorption method with lanthanum in the LiCl-KCl-LaCl₃ system, where La replaced Tb, to form La-Ni alloy LaNi₅ on the nickel cathode and to remove residual La by zeolite 5A. The overall La removal efficiency was 99.81% [48]. Wang et al. used electrochemical reduction to separate Tm from LiCl-KCl-TmCl₃ melt, followed by zeolite 5A adsorption [49]. Tm³⁺ was reduced to form In₃Tm alloy on the liquid indium electrode in LiCl-KCl-TmCl₃ melt with a Tm recovery of 93.8%, and the residual Tm in the salt melt was removed by zeolite 5A adsorption with 99.5% Tm removal. The overall efficiency was estimated to be 99.97%.

4.2. Electrochemical Extraction of Fission Products

The electrochemical extraction or reduction used before the zeolite adsorption was mostly employed to extract uranium and transuranic heavy metals in the electrochemical pyroprocessing technology [50]. This extraction technique has been demonstrated separately by reducing fission products in salt waste to a metallic form on a cathode without the presence of U and transuranic metals:

• The electroreduction of Gd on a liquid Pb electrode was tested within LiCl-KCl melt in a temperature range of 450 °C to 600 °C by Han et al. [51]. A recovery rate of 82.2% of the Gd was attained.
• Vanadium was electrochemically recovered on a tungsten electrode from LiCl-KCl-VCl₃ melt at 450 °C in exploratory experiments, where no recovery or removal rates were reported [52].
• Chernyshev et al. [53] used pulsed electrolysis to reduce Mo in NaCl–KCl–MoCl₃ melt to form metallic Mo with a glassy carbon cathode at 780 °C. The Mo concentration in the melt was maintained by using a metallic Mo anode.
• Dysprosium and gadolinium in LiCl-KCl-DyCl₃-GdCl₃ were reduced with electrolysis techniques to form alloys on the magnesium and molybdenum electrodes at 500 °C, with removal efficiencies of 83.5% to 95.2% and 91.9% to 95.2% for Dy³⁺ and Gd³⁺, respectively [54].
• Samarium in the LiCl–KCl system was reduced to form Sm-Pb alloy on the liquid lead electrode at 500 °C with an Sm recovery efficiency of 94.2% [55]. The authors, Wang et al., also summarized electrodes previously tested for lanthanide recovery. These electrodes included Mg, Al, Ni, Cu, Zn, Ga, Sn, Pb, and Bi. The performance of the electrodes calculated based on thermodynamic data by Lebedev followed a ranking as follows: Al > Ga > Pb > Zn > Bi > Sn > In > Cd > Tl [56]. Electrodes made from Al were ranked high because of its low reduction potential or high resistance to reduction.
• Lead electrodes benefit from a low cost, high thermal conductivity, and a low melting point and were used in more studies than other electrodes.
• A solid boron electrode was tested for recovery of Sr, Cs, La, Nd, Sm, and Ce from waste molten salt NaCl-KCl at 720 °C [57]. These fission products formed stable metal borides on the boron electrode. A high efficiency of over 99% was attained for La, Nd, Sm, and Ce, with a removal efficiency of about 90% for Sr. However, Cs removal efficiency was only 47.7% due to a week bonding between B and Cs.

The electrochemical extraction method was effective for removing the fission products from waste molten salt. Adding a zeolite adsorption step further increased the overall separation efficiency to over 99% in recent studies [45,46,47,48,49], as summarized in Section 4.1. Therefore, a combination of electrolysis and zeolite adsorption might be a promising technology for purifying waste salt for reuse.

4.3. Precipitation

Several reagents have been used in precipitating rare earth (RE) elements (Y and lanthanides) in the LiCl–KCl salt: oxygen gas (O₂), Li₂O, lithium and/or potassium carbonates (Li₂CO₃, K₂CO₃), and lithium and/or potassium phosphates (Li₃PO₄, K₃PO₄).

4.3.1. Additions of LiO₂ and CO₃²⁻ and Crystallization

LiO₂ and Li₂CO₃ are used to introduce displacement reactions, like acid–base reactions, converting RECl₃ to oxides and/or oxychlorides that are insoluble in the LiCl–KCl or LiCl salt melt. In the LiCl melt above the melting point of 610 °C or LiCl–KCl melt above the melting point of 350 °C, the RE chloride reactions with LiO₂ or Li₂CO₃ produce RE oxides and oxychlorides. LiO₂, with a boiling point of 2563 °C [58], and Li₂CO₃, with a decomposition temperature of 1310 °C [59], are stable and dissolve in the melt, while RE carbonates start to decompose to form oxides around 200 °C [60].

Early salt cleanup studies showed that additions of 1 or 3 mol% Li₂O into the mixture of the LiCl–KCl salt with 1 or 3 mol% of a single RE metal chloride at 450 °C produced stable oxides and oxychlorides Y₂O₃, LaOCl, GdOCl, and Gd₂O₃ [61].

RE³⁺ + O²⁻ + Cl⁻ = REOCl

(1)

2 REOCl + O²⁻ = RE₂O₃ + 2 Cl⁻

(2)

2 RE³⁺ + 3 O²⁻ = RE₂O₃

(3)

The quantity of Li₂O added first is stoichiometrically necessary to form REOCl, but below the quantity at which a complete conversion to form RE₂O₃ or REO₂ occurs. An addition of 0.5 mol% Li₂O to the melt with 1 mol% GdCl₃ resulted in GdOCl, and increasing the Li₂O quantity to 1 mol% produced both GdOCl and G₂O₃. K₃RECI₆ (RE = Gd, Y, Yb) formed in the LiCl–KCl melt with 3 mol% of RECl₃ before Li₂O additions.

At a higher temperature of 500 °C, the X–ray diffraction (XRD) patterns of the salt precipitate samples showed that additions of Li₂O formed only oxides Ce₂O_3, Sm₂O₃, Eu₂O₃, and Gd₂O₃, while addition of Li₂CO₃ primarily produced precipitates REOCl (RE = Ce, Sm, Eu), Sm₂O₂CO₃ and Gd₂O₂CO₃ [62].

RE³⁺ + Cl⁻ + CO₃²⁻ = REOCl + CO₂

(4)

2 REOCl + CO₃²⁻ = RE₂O₃ + 2 Cl⁻ + CO₂

(5)

2 RE³⁺ + 3 CO₃²⁻ = RE₂O₃ + 3 CO₂

(6)

The products with Li₂O as a reactant were consistent with the previous studies at 450 °C in reference [61]. Approximately 2.0 or 2.8 mol% of a single RECl₃ compound was mixed in the LiCl–KCl eutectic melt, where 3.8 or 5.1 mol% Li₂O, or 2.6 or 3.9 mol% Li₂CO₃ were added, as a reactant, respectively. The quantities of LiO₂ were more than 20 mol% higher than the stoichiometric requirements for a complete conversion of RE³⁺ to RE₂O₃, while the quantities of Li₂CO₃ were 10 to 20 mol% lower than the requirement for the same conversion. Distillation was able to remove completely the salt adhering to the precipitates at a vacuum pressure of 1.0 Pa and a distillation time of 1 h at 1000 °C.

Li₂CO₃ was used to precipitate SrCl₂ as SrCO₃ in the LiCl melt at a higher temperature of 750 °C [63], as compared to [62]. A conversion ratio of 90.1% was observed at a molar ratio of Li₂CO₃:SrCl₂ = 2:1, and more than 99.5% conversion was achieved at a molar ratio of Li₂CO₃:SrCl₂ = 3:1 or greater. SrCl₂ and CsCl were also precipitated as SrSO₄ and Cs₂S₂O₆, respectively, by additions of Li₂SO₄, with a Li₂SO₄:(SrCl₂+CsCl) molar ratio of 1:2 at 650 °C to 750 °C. SrSO₄ and Cs₂S₂O₆ were the only species identified in the XRD pattern of the precipitate. The maximum sulphate conversion was 72% for CsCl and 95% for SrCl₂. Zone freezing tests were performed separately with 2.4 to 4.8 wt% of SrCl₂ and CsCl in the LiCl melt, and approximately 90% of SrCl₂ and CsCl ended up in the bottom 20% of the column after zone freezing. The time durations for the precipitation reactions and zone freezing tests were not specified.

4.3.2. Addition of O₂ Gas and Crystallization

Oxygen injection into the LiCl–KCl melt using an alumina sparger has been tested as an effective way to convert RE metal chlorides to oxides and/or oxychlorides [64]. Chlorine gas (Cl₂) is released during the conversion process with O₂ gas.

2 RECl₃ + O₂ = 2 REOCl + 2 Cl₂

(7)

2 CeCl₃ + 2 O₂ = 2 CeO₂ + 3 Cl₂

(8)

O₂ gas was sparged at a gas velocity of 0.05 m/s to form a dispersed bubble flow regime in the LiCl–KCl melt containing an RE chloride compound. While the conversion ratio of 1.4 mol% LaCl₃ to LaOCl was approximately 95%, 0.6 to 0.7 mol% of CeCl₃, NdCl₃, or GdCl₃ was converted to CeO₂, NdOCl, or GdOCl, respectively, with a conversion ratio of greater than 99% at 800 °C after 7 h. The solubility of oxygen gas in the molten salt was the limiting factor for the slower conversion of LaCl₃. A longer reaction time for LaCl₃ might be required to achieve a higher conversion ratio than 95%. In an oxygen atmosphere, LaOCl, NdOCl, and GdOCl started to decompose to La₂O₃, Nd₂O₃, and Gd₂O₃ at 860 °C, 1000 °C, and 1200 °C, respectively.

4 REOCl + O₂ = 2 RE₂O₃ + 2 Cl₂

(9)

The experiments conducted by the same authors in reference [64] showed that mixed Y, La, Ce, Pr, Nd, Sm, Eu, and Gd trichlorides were converted to LaOCl, PrOCl, NdOCl, SmOCl, EuOCl, GdOCl, and CeO₂, PrO₂, Y₂O₃ with over 99% conversion efficiency at a gas flow of 5 L/min O₂ after 6 h at 800 °C, based on the XRD patterns and scanning electron microscope–energy dispersive X–ray spectroscopy (SEM–EDS) analyses of the precipitates [65]. The same results were obtained at the same oxygen flow, a lower temperature of 750 °C, and a longer reaction time of 12 h, as compared to the above. At 700 °C and 12 h of reaction time with the same oxygen flow, only LaCl₃ was not completely precipitated, with approximately 73% conversion. As the temperature was further lowered to 650 °C with the same reaction time and oxygen flow, only CeCl₃ was completely precipitated after 3 h while the conversions for the other RE trichlorides were below 70%.

Longer reaction times and higher temperatures facilitated the formation of larger crystal grain sizes and particle settling. The RE trichloride conversion ranking in a decreasing order of the reaction time length and temperature used in the O₂ gas sparging process is as follows: La > Pr > Nd > Eu > Sm > Gd > Y > Ce. The ranking was determined by the thermodynamic properties of the reactants and crystal growth kinetics of the precipitates. At 750 °C, a settling time of approximately 7 h was needed for the precipitates to settle at the bottom section of the reaction vessel after the oxygen sparging operation ceased.

At 650 °C and 7 h of reaction time with unknown oxygen flow, oxygen injection into the molten salt containing mixed Ce, Pr, Nd, Eu trichlorides completely precipitated Ce, Pr, Nd, and Eu as CeO₂, Eu₂O₃, NdOCl, PrOCl, and PrO₂ in the salt melt, with over 99.9% conversion ratios [66]. The reaction time was shorter than the reaction time of 12 h used at 650 °C and 5 L/min O₂ in reference [65], where the complete conversion was not achieved for Pr, Nd, and Eu trichlorides. A higher oxygen flow than 5 L/min might have been used in achieving the high conversion for Pr, Nd, and Eu trichlorides at 650 °C and 7 h [66].

Sequential separations of rare earth metal chlorides YCl₃, CeCl₃, PrCl₃, and NdCl₃ and alkaline and alkaline earth metal chlorides SrCl₂ and CsCl from the Li–Cl–KCl salt were studied using the O₂ gas sparging and zone freezing processes [67]. YCl₃, CeCl₃, PrCl₃, and NdCl₃ were precipitated as oxides (CeO₂, PrO₂, Y₂O₃) and oxychlorides (PrOCl, NdOCl) at 800 °C, a gas flow rate of 1.5 L/min O₂, and a reaction time of 6 h, with a conversion rate of over 99.5%. After the precipitation, zone freezing was carried out by moving the crucible at 3.2 to 3.5 mm/h upward with the temperature gradually decreasing from the bottom to the top, while maintaining the LiCl–KCl in a melted state. Approximately 82% to 86% of the SrCl₂ and CsCl was separated when 80% of the salt volume was removed from the top of the zone freezing column for recycling. The separation efficiency for SrCl₂ and CsCl increased to 90% if 60% of the salt from the column top section was extracted for reuse. These zone-freezing separation efficiencies are lower than those reported in [63]. A similar vertical zone freezing technique was tested with a LiCl-SrCl₂ system, where a vertical temperature gradient, i.e., the highest temperature at the bottom and the lowest temperature at the top, enabled the concentration of Sr at the bottom [68]. The technique was then validated with LiCl₂-(La, Ce, or Nd)Cl₃, LiCl-KCl-SrCl₂, and LiCl-Li₂O-SrCl₂ systems. Sr separation was lower in the two binary systems of LiCl-KCl and LiCl-Li₂O than in the single LiCl system. A maximum separation of 98.3% SrCl₂ was achieved with the LiCl-KCl-SrCl₂ system, where 80% of the salt could be recovered for reuse, like the earlier experiments.

Shim et al. tested the zone freezing refining technique with LiCl-KCl-CsCl and LiCl-KCl-SrCl₂ systems by moving a heater horizontally at a predetermined speed away from the sample boat after the salt melted in a horizontal apparatus [69,70]. This action caused the Cs and Sr to migrate with the liquid phase or toward the higher temperature zone and concentrated at the sample boat end zone at the heater moving direction. The action was repeated as required to improve separation. A lower moving speed of the heater achieved a higher separation factor for Cs and Sr due to more time available for ions to migrate. A similar technique—a quartz tube containing a sample boat was moved within a horizontal tube furnace—was tested more recently with simulated pyroprocessing waste salts: LiCl-Li₂O-BaCl₂-RbCl-SrCl₂, LiCl-Li₂O-BaCl₂-CsCl-RbCl-SrCl₂, and LiCl-KCl-YCl₃-LaCl₃-CeCl₃-PrCl₃-NdCl₃-SmCl₃ [71]. These separation experiments also attained reuse of approximately 80% of the salt.

The separation method involving moving the sample boat appears to be more practicable than the one involving moving the heater.

The relative stability of REOCl and Re₂O₃ (RE = Gd, Y, and Yb) vs. the O²⁻ concentration and the temperature was determined in the eutectic LiCl–KCl salt melt containing Li₂O [61]. Rare earth oxide formation increased with increasing O²⁻ concentration and temperature. This observation is consistent with the experimental observations in [65]. REOCl formed favorably below the melting temperature in the solid state of the salt with the presence of oxygen gas. In the absence of oxygen in the melted salt, K₃RECI₆ formed at 450 °C.

A crystallization setup with the two hollow metal plates immersed in the salt melt, which was different than the one in [63], was also tested for separation of 0.1 mol% CsCl, 0.05 mol% SrCl₂, and 0.08 mol% BaCl₂ after the precipitation of 0.17 mol% NdCl₃ and 0.16 mol% EuCl₃ from the LiCl salt with Li₂CO₃ [72]. Approximately 1.52 mol% of Li₂CO₃, higher than the stoichiometric requirements of 0.67 mol% Li₂CO₃ for the complete conversion of the fission products to oxides, was added to the LiCl melt at 700 °C, followed by the injection of argon cooling gas at 25 L/min into the hollow metal plates to crystallize CsCl, SrCl₂, and BaCl₂ on the surfaces of the two hollow metal plates while maintaining the melt at 640 °C. The precipitation step converted 3.56% Cs, 18.90% Sr, 67.05% Ba, and 98.12% (Nd+Eu) of the total quantity of each element in the melt into the precipitates. The overall separation efficiency of the precipitation and crystallization for Cs, Sr, Ba, Nd, and Eu was 99.26%, 98.92%, 98.81%, 99.99%, and 99.92%, respectively. A scaled-up version, a 10 kg crystallizer consisting of multiple hollow plates, was tested for recovering LiCl from a mixture of LiCl-0.03wt%CsCl-1.11wt%SrCl₂-2.31wt%BaCl₂ [73]. Fission product separations efficiencies were over 90%.

Versey et al. (2013) used a small cold-finger crystallization apparatus to crystallize CsCl from a LiCl salt containing 1, 3, 5, and 7.5 wt% CsCl [74], achieving separation efficiencies similar to those reported in [67]. Williams et al. (2015) applied the method to separate CsCl and SrCl₂ from CsCl–LiCl–KCl, SrCl₂–LiCl–KCl, and CsCl–SrCl₂–LiCl–KCl, and identified that CsCl and SrCl₂ crystallized simultaneously from the quaternary system [75].

It is interesting that a better Cs separation was achieved with a conical- or flat-bottom vessel than with a round-bottom or hemisphere-bottom vessel in a crystallization or freezing modeling study of a LiCl-KCl-CsCl system, where the crystallization in a vessel was simulated with a temperature gradient from the cold wall to the warm center [76]. The maximum Cs separation efficiency was 94% to 95%. An experiment with a flat-bottom crucible demonstrated a Cs separation efficiency of 92.8% in a CsCl-NaCl-LiCl-KCl system [77]. These modelling research and experiments are mainly to demonstrate the crystallization mechanisms, instead of developing a practical means for salt purification.

4.3.3. Addition of PO₄^3–

A mixture of Y, La, Nd, Ce, Pr, Sm, Eu, and Gd trichlorides or a mixture of La, Nd, Ce, and Pr trichlorides was precipitated from the LiCl–KCl eutectic salt by adding a phosphate mixture consisting of 59.2 mol% Li₃PO₄ and 40.8 mol% K₃PO₄ at temperatures from 550 °C to 750 °C [78]. Li₃PO₄ and K₃PO₄ were added to the salt melt at the prescribed ratio to maintain the LiCl–KCl eutectic composition in the product.

RECl₃ + Li₃PO₄ (or K₃PO₄) = REPO₄ + 3 LiCl (or KCl)

(10)

Adding the stoichiometrically required quantity of the mixed Li₃PO₄ and K₃PO₄, i.e., the minimum quantity required for a complete conversion of the mixture of La, Nd, Ce, and Pr trichlorides to phosphates, resulted in more than 99.5% conversion after 1 h at 550 °C and with mixing by sparging 1.75 L/min Ar gas through each of four spargers.

While maintaining the same reaction conditions as the above, the addition of the Li₃PO₄ and K₃PO₄ mixture was then reduced to 85 mol% and to 95 mol% of the stoichiometrically required quantity in separate experiments with a mixture of Y and Ln (La, Nd, Ce, Pr, Sm, Eu, and Gd) trichlorides. After the precipitation step, O₂ gas was sparged for 4 to 8 h at 700 to 750 °C. The overall phosphate and oxide conversion efficiency of Y and Lns was greater than 99%, where adding 95 mol% of the Li₃PO₄ and K₃PO₄ mixture stoichiometrically required quantity achieved a higher overall conversion than adding only 85 mol% of the stoichiometrically required quantity. Y and Ln phosphates, oxides, and oxychlorides were identified in the precipitates in the combined phosphate precipitation and oxygen sparging process. The reactivity of the Lns with phosphate and oxygen followed a decreasing ranking of Y, Ce, Nd, Sm > Gd > Eu > Pr > La. When sufficient phosphate ions, i.e., >95 mol% of the stoichiometrically required quantity, were added, the conversion efficiencies were greater than 99.5% for all lanthanides. The final separation relied on the settling of the solid phosphates, oxides, and oxychlorides at the bottom of the reaction vessel. A higher temperature and longer time would enable crystals to grow to larger sizes, which facilitates the settling and separation of the precipitates in the molten salt.

A small amount of solid Li₃PO₄ was observed in the precipitates obtained by adding 3 or 4.5 times either the Li₃PO₄ or K₃PO₄ stoichiometrically required quantity to the LiCl–KCl eutectic melt containing SrCl₂, CsCl, and LaCl₃ at 520 °C or 600 °C [79]. However, Li₃PO₄ was not found in [78], where the phosphate quantity was equal to or lower than the stoichiometrically required quantity. No Sr and Cs phosphate precipitates formed according to the XRD patterns of the solids containing only Li₃PO₄ and LaPO₄.

Conversions of Sr, Cs, Ba, Y, La, Ce, Pr, or Eu trichlorides separately to phosphates in the LiCl–KCl eutectic melt were studied by adding the Li₃PO₄ quantities at 2 times the stoichiometrically required quantity at 500 °C [80]. La, Ce, Pr, Eu, and Y phosphate crystals were identified in the SEM images, as indicated by the SEM–EDS analyses. Crystalized Ba₃(PO₄)₂ precipitates were identified while Sr and Cs phosphates were not found, which is consistent with [79].

With the presence of a very small quantity of NaCl in the LiCl–KCl eutectic melt, double phosphates NaLn_x(PO₄)_y, such as NaCe(IV)₂(PO₄)₃, might be formed during the phosphate precipitation with Li₃PO₄ or K₃PO₄ [81]. The experiments used surrogate LiCl–KCl salt waste samples, each containing one fission product chloride listed as CsCl, SrCl₂, BaCl₂, YCl₃, LaCl₃, CeCl₃, PrCl₃, NdCl₃, SmCl₃, EuCl₃, and NaCl. Li₃PO₄ or K₃PO₄ was added at 2 times the stoichiometrically required quantity to a waste salt sample, which was heated to 520 °C or 600 °C, and the temperature maintained for 2 h. Li₃PO₄ formation was observed with large crystal sizes, and neither Sr nor Cs phosphate was observed, which was consistent with the previous experiments in [79]. LaCl₃ was completely converted to LaPO₄. The Li₃PO₄ reactions with YCl₃ and LnCl₃ produced larger particles than the K₃PO₄ reactions, indicating that the Li₃PO₄ is a better precipitating and separation reagent for the removal of Y and Lns from the LiCl–KCl salt. The absence of Sr₃(PO₄)₂ formation was not consistent with the paper’s thermodynamic modeling prediction, for which further studies were recommended. A different experiment tested only Cs removal from LiCl-KCl by adding K₃PO₄ to precipitate Cs as Cs₃PO₄ at 550 °C, showing a Cs removal of 94% [82]. The salt was then treated with zeolite 4A, followed by immobilizing zeolite 4A and Cs₃PO₄ in a borosilicate glass form.

Li₃PO₄ additions were used for separating rare earth metal and thorium fluorides from a LiCl-KCl system for the thorium fluoride fuel reprocessing [83]. The concentration of RE (CeF₃, NdF₃, LaF₃, EuF₃, SmF₃, YbF₃, YF₃) was 3 wt% in the ternary LiCl-KCl-ReF₃ mixture. The ratio of Li₃PO₄ to ReF₃ ranged from 1 to 3, and the maximum removal by precipitation was 98.1% to 98.6% for Y³⁺, La³⁺, Ce³⁺, Sm³⁺, Dy³⁺, and Yb³⁺; and 96.2% for Nd³⁺ at the Li₃PO₄ to ReF₃ ratio of 3 and at 550 °C. A relatively lower maximum Eu³⁺ removal of 54.7% was observed at the Li₃PO₄ to ReF₃ ratio of 1.0. This indicates that Eu requires additional stages of treatment to achieve a higher separation efficiency. The impact of rare earth metal fluorides on thorium fluoride separation was tested with a LiCl-KCl-1wt%NdF₃-1wt%SmF₃-10wt%ThF₄ mixture, where an excess of Li₃PO₄ ranging from 30 mol% to 60 mol% was needed to achieve a Th precipitation efficiency of more than 95%. The precipitates are identified by XRD as LaPO₄, EuPO₄, Eu₃(PO₄)₂, ThO₂, NdPO₄, SmPO₄, Th₃(PO₄)₄, and KTh₂(PO₄)₃. Although the XRD data of Y, Ce, Dy, and Yb were not presented, the higher precipitation efficiency suggests that their form is REPO₄, like LaPO₄.

4.4. Distillation

Physical distillation was studied for recovering LiCl–KCl for recycling by Eun et al. [84], and a precipitation treatment usually preceded the distillation. A eutectic salt mixture of LiCl–KCl containing selected lanthanide fission products CeCl₃, PrCl₃, NdCl₃, and EuCl₃ was first oxidized by O₂ gas to precipitate CeO₂, PrO₂, Eu₂O₃, PrOCl, and NdOCl. The product from the oxidation consisted of approximately 90 wt% LiCl–KCl salt and 10 wt% precipitates. The recovery rate of the LiCl–KCl salt increased with increasing the distillation temperature from 900 to 1100 °C, and approximately 99.3% of the LiCl–KCl salt was recovered by distillation for 9 h at 1100 °C, −200 Torr (−26.7 kPa) of gauge pressure (or 560 Torr (74.7 kPa) of vacuum pressure) and the highest evaporation surface area used in the test. The distillation of a fraction of the salt adsorbed on the lanthanide precipitate fine particles was slower than the bulk salt distillation. The salt recovery rate increased by increasing the evaporation surface area and heating time but was not significantly by the reduced gauge pressure tested in a range of −50 Torr (−6.7 kPa) (vacuum pressure = 710 Torr (94.7 kPa)) to −200 Torr (−26.7 kPa) (vacuum pressure = 560 Torr (74.7 kPa)). The testing temperatures were much lower than the boiling points of LiCl (1383 °C) and KCl (1485 °C), and the melting points of the lanthanide oxides (>2000 °C). PrOCl and NdOCl were observed in the solid residue at the testing temperatures.

Follow-up studies by Eun et al. (2018) [85] indicated that the salt evaporation started at two pairs of temperature and gauge pressure of 825 °C, −710 Torr (−94.7 kPa) (vacuum pressure = 50 Torr (6.7 kPa)) and 700 °C, −759.5 Torr (−101.3 kPa) (vacuum pressure = 0.5 Torr (67 Pa)), in which the temperatures and pressures were much lower than those in the previous studies [84]. These studies confirmed that lowering the vacuum pressure significantly reduced the distillation temperature and time in recovering the salt. In the absence of the LiCl–KCl salt, PrOCl and NdOCl started to decompose to PrO₂ and Nd₂O₃ at 1070 °C at the presence of O₂ gas and the decomposition completed at approximately 1290 °C.

Eun et al. then identified that the optimum conditions for more than 99 wt% salt recovery by distillation were 900 °C, 5 Torr (0.7 kPa) of vacuum pressure, and 5 h of distillation time [86,87]. This high salt recovery rate was partly attributed to an optimized design of the experimental distillation apparatus where the temperature profiles were controlled to facilitate the salt vapor transport from the evaporation chamber to the condensation chamber. The authors also conducted studies on the distillation process at the optimum conditions by adding Li₂CO₃ and K₂CO₃ as reagents in converting rare earth lanthanide (La, Nd, Ce, Pr) chlorides to oxides and oxychlorides [88], in which the reactions are as follows.

2 RECl₃ + 3 K₂CO₃ (or Li₂CO₃) = RE₂O₃ + 6 KCl (or LiCl) + 3 CO₂

(11)

RECl₃ + K₂CO₃ (or Li₂CO₃) = REOCl + 2 KCl (or LiCl) + CO₂

(12)

The above are displacement reactions instead of redox reactions, which could be advantageous over the O₂ injection in reducing equipment corrosion. The total amounts of Li₂CO₃ or K₂CO₃ were added at a molar ratio of 1.5 of total carbonates to total RE trichlorides, which is a stoichiometrically required quantity for Equations (11) and (12).

CO₂ gas is released from the conversion process. As the REOCl formed, there would be residual Li₂CO₃ or K₂CO₃ after the reactions. However, the XRD patterns of the distillation residue samples showed no peaks for lithium and potassium chlorides and carbonates, indicating that the carbonates were either below the detection limits of XRD in the precipitates and/or collected in the LiCl–KCl distillate.

Further demonstrations of reactive distillation with K₂CO₃ as a precipitating reagent were performed to verify the process flowsheet proposed by KAERI with rare earth fission products Y, Ce, Pr, Na, and Sm in LiCl-KCl [89]. The precipitation reactions were controlled at 550 °C and the temperature was increased to 850 °C for distillation under vacuum. The results were similar to those in [86,87].

Distillation was also used to recover LiCl–KCl from 20wt%ThF₄-1wt%SmF₃-1wt% NdF₃-LiCl–KCl for uranium–thorium fuel cycle applications [90]. The decontamination factors reached 1720, 123, and 106 for Th, Nd, and Sm, respectively, and over 90% of LiCl-KCl was recovered.

The success of the distillation is due to the significant differences in the vapor pressure (or in the melting and boiling points) between the LiCl–KCl and the RE oxides or carbonates. In the real system of pyro–electrometallurgically used salt, there will be more elements present in the used salt, which needs to be considered in the salt purification process.

4.5. Centrifuge—Mechanical Separation

A centrifuge method was also evaluated to separate precipitates from a LiNO₃ surrogate sample melt [91]. However, this method might be able to separate particles or pellets with sizes greater than the pore size of the metallic filter, approximately 2 mm in diameter.

4.6. Summaries of Literature Salt Decontamination Data

A summary of FP precipitates from the literature is shown in

Table 2. Chemical forms of FP precipitates observed in the experimental studies reviewed above.

Y	La	Ce	Pr	Nd	Sm	Eu	Gd
Y2O3		Ce2O3 CeO2	PrO2		Sm2O3	Eu2O3	Gd2O3
	LaOCl		PrOCl	NdOCl	SmOCl	EuOCl	GdOCl
					Sm2O2CO3		Gd2O2CO3
YPO4	LaPO4	CePO4	PrPO4	NdPO4	SmPO4	EuPO4
Cs	Sr	Ba
	SrCO3
Cs2S2O6	SrSO4
		Ba3(PO4)2

. These precipitates are stable in the LiCl or LiCl–KCl eutectic salts up to 1000 °C.

• Chemical precipitation methods with carbonates or phosphates require overdoses of carbonates or phosphates to achieve conversions of greater than 99%. Settling and distillation are used to separate the precipitates from the bulk salts. The data for the removal of overdosed carbonates and phosphates are not available in the reviewed literature.
• Sparging O₂ gas in the LiCl–KCl melt appears to be a simpler method than the chemical additions, in which O₂ gas is continuously injected until lanthanides are completely precipitated as oxides and/or oxychlorides. Settling or distillation is then used to separate the precipitate from the salt. Chlorine gas is released and needs to be managed, e.g., using a chlorine scrubber.
• Distillation at a temperature of above 900 °C and a vacuum pressure of 5 Torr (0.7 kPa) enables nearly 100% salt recovery after the precipitation step.
• Crystallization is primarily used to separate CsCl, SrCl₂, and BaCl₂, which cannot be effectively precipitated by carbonates and phosphates. The setup with hollow metal plates or cold fingers immersed in the salt melt and containing coolant gas flowing inside appears to enable cleaner salts to be recovered than the zone freezing method, as the CsCl melting point is 645 °C, only slightly higher than the LiCl melting point of 610 °C.

Among the techniques tested for the purification of LiCl-KCl and LiCl salts, oxygen gas sparging for precipitation, electrolysis, distillation, and crystallization appear to generate the least secondary waste, thus being cleaner than other effective, chemical precipitation methods with carbonate and phosphate additions. Future development may need to focus on combinations of oxygen gas sparging, electrolysis, distillation, and crystallization.

5. Holistic Sequestration of FPs and LEAPs from Salt Solvent

The above experimental literature study review focuses on key alkaline and alkaline earth metals and rare earth metals, as they represent the major FPs that can form stable chloride salts. The salt purification studies employ differences in vapor pressure and solubility to separate the FP compounds in the LiCl or eutectic LiCl–KCl salt melt. There are other salt systems available for the PERUT process, such as eutectic NaCl–2CsCl (melting point of 486 °C [92]), and eutectic 55LiCl–45RbCl (melting point of 307 °C [93]).

However, the LiCl–KCl eutectic system (3LiCl–2KCl, melting point of 350 °C) represents the most studied option as both LiCl and KCl are available at low costs. LiCl salt (melting point of 610 °C) is primarily used to reduce oxide fuels before electrorefining. The physical and chemical properties of FPs and LEAPs that are relevant to the separation of the FPs and LEAPs from the spent molten salt fuel, LiCl, and 3LiCl–2KCl melts are shown in

Table 3. Possible approaches for removing FPs and LEAPs in chloride salt fuel, LiCl or LiCl–KCl melts [95,96] *.

wt% of FP+LEAP		Compound & Removal	wt% of FP+LEAP		Compounds & Removal	wt% of FP+LEAP		Compound & Removal
Alkali Metal			Transition Metal—Period 5			Rare Earth (RE) Lanthanide (Ln)
Na	0.49%	NaCl, KCl, RbCl, CsCl Ion exchange with zeolite Crystallization, distillation No separation if present as part of solvent salt	Y	1.15%	YCl3, ZrCl2/ZrCl4 [97], MoCl3, TcCl4, RuCl3, RhCl3, AgCl, CdCl2 (or remain as metals in fuel [98]) YCl3 behaves like Ln chlorides ZrCl4, MoCl3, TcCl4, RuCl3, RhCl3, AgCl, CdCl2 decompose to metals or reduced to metal at the cathode [99,100,101] ZrCl4, sublime at 331 °C Tc2O7, boil at 310 °C TcCl4, sublime at 300 °C Form MoO2/MoO3 precipitates with O2 [102]	La	3.29%	LaCl3, CeCl3, PrCl3, NdCl3, PmCl3, SmCl3, EuCl3, GdCl3 Form oxide and/or oxychloride precipitates with O2 or CO32− [13,103] Form phosphate precipitates with PO43−
K	0.20%		Zr	9.39%		Ce	6.32%
Rb	0.90%		Mo	9.17%		Pr	3.02%
Cs	6.70%		Tc	2.38%		Nd	10.77%
Akali Earth Metal			Ru	6.13%		Pm	0.23%
Mg	0.10%	MgCl2, CaCl2, SrCl2, BaCl2 Ion exchange with zeolite Crystallization, distillation No separation if present as part of solvent salt Form MgO precipitate with O2 [104,105,106]	Rh	1.78%		Sm	2.19%
Ca	0.49%		Pd	3.92%		Eu	0.29%
Sr	2.09%		Ag	0.29%		Gd	0.22%
Ba	4.06%		Cd	0.21%		Reactive Non-Metal
Transition Metal—Period 4			Post-Transition Metal			C	2.02%	CCl4, boil at 76.7 °C, decomposes to C + Cl2 at 200 °C N2, release in gas
Cr	0.14%	CrCl2/CrCl3 [107], FeCl2/FeCl3 [108,109], NiCl2 [110], CuCl/CuCl2 [101], ZnCl2 [111] Form Fe2O3/Fe3O4 [112,113], NiO [114], Cr2O3 precipitates [115,116], e.g., at 550 °C with O2 Form FePO4 precipitates with PO43− [117] NiCl2, decompose at 740 °C [118,119] CrCl3, NiCl2 crystallization (m.p.: 1150 °C, 1001 °C) CrCl2/CrCl3 [107], CuCl/CuCl2 [120], ZnCl2 reduced to metal at cathode [121]	Al	0.24%	AlCl3, SnCl2 [122] AlCl3, sublime at 180 °C SnCl2, boil at 623 °C Form SnO2 with O2 [123]	N	0.14%
Fe	0.74%		Sn	0.18%		F	0.29%	F−, form volatiles with noble metals Mo, Tc, Ru, Rh [124,125]
Ni	0.20%		Metalloid			P	0.35%	PCl3, boil at 76.1 °C
Cu	0.10%		Si	0.29%	SiCl4, TeCl2/TeCl4 (Te4Cl16) [126,127] SiCl4, boil at 58 °C Form SiO2 with O2 TeCl4, boil at 387 °C TeCl2, boil at 328 °C	S	0.12%	S2Cl2, boil at 138 °C SeCl4, sublime at 191.4 °C, decompose to SeCl2 and Cl2 gases
Zn	0.13%		Te	1.41%		Se	0.17%
Noble Gas						Br	0.17%	Br−, Br2, release in gas I−, I2, release in gas
Kr	0.93%	Gas, decay and delayed release	Xe	15.64%	Gas, decay, and delayed release	I	0.59%

m.p.—melting point, LEAP—light element activation product. wt% of FP+LE—based on CANDU^® spent fuel data, 9.2 MWd/kg–U burnup, 5 years cooling [94]. *—Chemical thermodynamic properties are from CRC Handbook of Chemistry and Physics, 104th edition [95] and from Chemistry of the Elements [96], unless different references are provided in the table.

. CANDU^® spent natural uranium fuel with a burnup level of 9.2 MWd/KgU and a cooling time of 5 years was used to select the FPs and LEAPs in Table 3 with a fraction of greater than 0.1 wt% of the total FPs and LEAPs [94]. These selected FPs and LEAPs together account for 99.6% of the total FPs and LEAPs in the CANDU spent natural uranium fuel. The percentage for each element in Table 3 will be different in the spent fuel containing enriched ²³⁵U, such as pressurized water and boiling water reactor (PWR and BWR) fuels. However, the use of a low threshold of 0.1 wt% allows for selected FPs and LEAPS to be representative of major FPs and LEAPs in a wide range of spent fuels, including the spent fuels from small modular and advanced reactor fuels currently under development.

5.1. Evaporation

Voloxidation, the high temperature oxidation of spent UO₂ fuels for de–cladding at a temperature of greater than 1000 °C, showed a release of nearly 100% of the ³H, ¹⁴C, ⁸⁵Kr, ¹²⁹I, ⁹⁹Tc, Ru, and Cs before pyro–electrometallurgical processing of the melts of LiCl (reduction) and eutectic LiCl–KCl (refining and winning) [14]. However, for MCFR salt fuels, such pre-treatments are not necessary for spent salt fuel processing.

FP and LEAP chlorides with low boiling points will be evaporated during the reactor operation at a temperature of greater than 450 °C or at a typical operation temperature of 450 °C to 500 °C for the pyrometallurgical process with 3LiCl–2KCl eutectic salt. These FPs and LEAPs are mainly reactive non-metal elements, including:

• Carbon (C):

  ○

  Stable chloride CCl₄, low formation tendency with a standard formation free Gibbs energy of −57.4 kJ/mol, boils at 76.7 °C and starts to decompose at 227 °C [95,96].

  ○

  Likely present as elemental graphite carbon.

• Aluminum (Al):

  ○

  AlCl₃ sublimes at 180 °C, starts to decompose at 200 °C [95], and completely decomposes to Al₂O₃ in the presence of H₂O at approximately 400 °C to 600 °C [128].

  ○

  Likely present as Al₂O₃ particulates in off-gas after contact with O₂ and H₂O moisture in the air in the ventilation system.

• Silicon (Si):

  ○

  SiCl₄ has a boiling point of 58 °C [95].

  ○

  Likely present as SiO₂ after contact with O₂ and H₂O in off-gas in the ventilation system [96].

• Phosphorus (P):

  ○

  Reactive chlorides PCl₃(L) and PCl₅(g); PCl₃ boils at 76.1 °C; PCl₅ is formed as gas; both react with H₂O to form H₃PO₃ and H₃PO₄; decompose to Cl₂ gas and volatile phosphorus with a boiling point of 280.5 °C. Phosphorus reacts with O₂ to produce phosphorus trioxide and pentoxide that boil at 173 °C and 605 °C [95,96].

  ○

  Likely present as phosphorus oxides, accompanied with HCl and Cl₂ in off-gas after contact with O₂ and H₂O in the ventilation system.

• Sulphur (S):

  ○

  Stable chloride S₂Cl₂ evaporates at 138 °C and reacts with H₂O to form elemental S (boiling at 444.6 °C), HCl, and SO₂ [95,96].

  ○

  Likely present as SO₂ and elemental sulphur after contact with O₂ and H₂O in off-gas in the ventilation system.

• Selenium (Se):

  ○

  SeCl₄ sublimes at 191.4 °C and reacts with H₂O to form SeO₂ (subliming temperature 315 °C) and HCl [95,96].

  ○

  Likely present as SeO₂ after contact with O₂ and H₂O in off-gas in the ventilation system.

• Tin (Sn):

  ○

  SnCl₂ has a boiling point of 623 °C and can evaporate quickly at a temperature of 463 °C if the SnCl₂ vapor is removed as it evaporates [123]. SnCl₂ reacts with O₂ at 475 °C to form SnO₂ precipitates.

  ○

  Sn may be present as SnCl₂ in off-gas in the ventilation system.

• Tellurium (Te):

  ○

  TeCl₄ (sublimes at above 200 °C, boils at 387 °C) and TeCl₂ (boils at 328 °C), reacts with H₂O to form TeO₂ and HCl. TeCl₄ dissociates to TeCl₂ and Cl₂ as it vaporizes [129].

  ○

  Likely present as TeO₂ in off-gas after reacting with O₂ and H₂O moisture in the ventilation system.

• Bromine (Br) and Iodine (I) [95,96,130,131]:

  ○

  No stable chlorides.

  ○

  Alkaline and alkaline earth metal bromides and iodides are thermodynamically stable and thus will likely be in the 3LiCl–2KCl melt and molten chloride salt fuel. Their physical and chemical behaviors are similar to their chloride counterparts. The melting and boiling points of alkaline and alkaline earth metal bromides and iodides are summarized with those of their chloride counterparts in

  Table 4. Melting and boiling points of alkaline and alkaline earth chlorides [95].

  Salt	Melting Point (°C)	Boiling Point (°C)	Salt	Melting Point (°C)	Boiling Point (°C)
  LiCl	610	1383
  NaCl	802	1465	MgCl2	714	1412
  KCl	771	1485	CaCl2	775	1735
  RbCl	718	1390	SrCl2	874	1250
  CsCl	645	1297	BaCl2	961	1560
  LiBr	550	1300
  NaBr	747	1390	MgBr2	711.0	n.a.
  KBr	734	1435	CaBr2	742	1815
  RbBr	692	1340	SrBr2	657	n.a.
  CsBr	636	1300	BaBr2	857	1835
  LiI	469	1171
  NaI	661	1304	MgI2	634.0	n.a.
  KI	681	1323	CaI2	783	1100
  RbI	656	1300	SrI2	538	1773 (d.t.)
  CsI	632	1280	BaI2	711	n.a.

  n.a.—not available. d.t.—decomposition temperature.

  .

  ○

  Br⁻ and I⁻ may be oxidized to form Br₂ and I₂ electrochemically or by using Cl₂ gas an oxidant, because alkaline and alkaline earth metal chlorides are thermodynamically more stable.

Non-metal reactive elements are most likely to be present in the off-gas system during the MCFR operation stage and may not be found in the spent chloride salt fuels and in the pyrometallurgical 3LiCl–2KCl salt waste. Evaporated FP and LEAP compounds can be captured on solid filters as solid particles or by condensing as liquids and solids, as compiled in [132]. While current nuclear power plants have managed volatiles and gases from conventional reactors, the characteristics of the volatiles and gases from molten chloride and fluoride salt reactors are largely unknown [23]. Safe management of the off-gas waste streams from the molten salt reactors including MCFRs requires more experimental studies.

5.2. Fission Products in Metallic State

Noble metal fission product chlorides (Zr, Mo, Tc, Ru, Rh, Pd, Ag, Cd) are chemically less stable than heavy actinide metal chlorides (U and transuranic metals Np, Pu, Am, Cm) according to their free formation Gibbs energy values at 500 °C [133,134]. These noble metal FPs are in a metallic state in the spent metallic fuel. Experiments on irradiated metallic U–Zr fuel from the EBR–II in the eutectic LiCl–KCl at 500 °C show [135,136,137]:

• There is incomplete anodic dissolution of Zr, Tc, Mo, Ru in the LiCl–KCl melt, while U is 100% dissolved.
• Noble metal chlorides are reduced to metals in reductive and inert environments.
• The electro-refiner for the EBR–II driver fuel was able to separate 99.9% of noble metals from the U product at the cathode with the electro-refiner electrode design and configuration that minimize the fine noble metal particles (<10 μm) in the uranium products on the cathode [135].

Noble metal FPs are predicted, based on both their chemical and thermodynamic properties and experiments, to be partially in metallic forms in the molten fluoride salt fuel reactors operating at 750 °C and may be removed as metals by online reprocessing during the reactor operation, where the remaining parts that form volatile fluorides are captured in the off-gas [138]. Both metallic and chloride forms of noble metal FPs can be predicted in the spent fuel from fast spectrum MCFRs, based on the incomplete anodic dissolution of noble metals in the LiCl–KCl melt [136]. The noble metal FP chlorides are not volatile at the reactor operating temperature of 600 °C to 700 °C, and will be reduced, precipitated, and collected during electrorefining of spent MCFR fuel salt, as shown in [135], which is then removed from the LiCl–KCl melt. These processes take place prior to the salt purification step.

• Nickel (Ni):

  ○

  Stable NiCl₂ is likely present in the spent MCFR salt fuel and in the PERUT salt.

  ○

  NiCl₂ decomposes at 740 °C and may behave similarly to noble metal chlorides.

  ○

  Due to oxidation during the salt purification step [115], NiCl₂ may also precipitate as NiO, which is insoluble in LiCl–KCl at 700 °C [114].

• Copper (Cu):

  ○

  CuCl₂ and CuCl are unstable and will behave like the noble metals discussed above.

• Zinc (Zn):

  ○

  Stable ZnCl₂ is likely to be present in the spent MCFR salt fuel and in the PERUT salt [139].

  ○

  ZnCl₂ may behave similarly to the noble metal chlorides during pyrometallurgical electrorefining.

  ○

  ZnO may be formed by oxidation during salt purification and has a solubility of 0.8 g/kg–LiCl–KCl in Li–KCl melt at 700 °C [114].

  ○

  ZnCl₂ has a high vapor pressure at the melted state and will likely be present in the gas phase [140,141,142].

5.3. Precipitation

FP and LEAP chlorides, if forming precipitates due to the introduction of O₂, CO₃²⁻, PO₄³⁻, or SO₄²⁻ into the 3LiCl–2KCl eutectic salt, can be separated from the pyro–metallurgical reprocessing 3LiCl–2KCl salt waste. The precipitates may be in the form of oxides, oxychloride, phosphates, and sulphates.

• Lanthanide (Ln) and Yttrium (Y):

  ○

  RECl₃ are stable at the MCFR and pyro–electrometallurgical operating temperature range of 450 °C to 700 °C.

  ○

  RE oxides and/or oxychlorides are formed by oxidation of RECl₃ with O₂ and decomposition of RE carbonates that are formed by reactions of RECl₃ with Li₂CO₃ and/or K₂CO₃ in the used pyro–electrometallurgical reprocessing salt.

  ○

  RE phosphate precipitates are formed by reactions of RECl₃ with Li₃PO₄ and/or K₃PO₄.

• Iron (Fe):

  ○

  FeCl₃ decomposes at a temperature of 195 °C in the presence of O₂ [112].

  ○

  Stable FeCl₂ and FeCl₃ are likely to be present in the spent MCFR salt fuel and in the PERUT salt produced under inert and reducing environments.

  ○

  Fe₂O₃ precipitates can be formed during pyro–electrometallurgical reprocessing [113], indicating that Fe may be removed by oxidation during the salt purification.

  ○

  FePO₄ may form during the phosphate precipitation process [117].

  ○

  FeCl₂ has a high vapor pressure at temperatures greater than 550 °C and will be likely present in the gas phase [141,142].

• Chromium (Cr):

  ○

  Stable CrCl₂/CrCl₃ are likely to be present in the spent MCFR salt fuel and in the PERUT salt [107,143].

  ○

  Cr₂O₃ precipitates can be formed during pyro–metallurgical reprocessing [115], indicating that Fe may be removed by oxidation during the salt purification.

5.4. Crystallization and Distillation

Chloride salts with a melting point higher than the solvent chloride salts can be crystallized during the salt decontamination process. However, this technique is primarily applied to separate alkaline earth metal chlorides after the other FPs are precipitated or removed from the LiCl–KCl or LiCl melt. The melting and boiling points of alkaline and alkaline earth metal chlorides, which may be present as solvents and/or FPs and LEAPs, are summarized in Table 4.

Distillation is primarily used to evaporate salt solvents such LiCl–KCl from the precipitates at a temperature greater than or equal to 900 °C, typically below the boiling points of the salts and assisted by vacuum operation [87]. Distillation may obtain higher purity products but consumes more energy as compared to crystallization.

6. Conclusions

The major FPs and LEAPs can be separated from the spent chloride salt fuel and pyro–metallurgical electrorefining salt LiCl–KCl according to the chemical and physical properties of the individual elements and compounds. Volatile FP and LEAP compounds, primarily in chloride and/or oxide forms, will likely be released during the MCFR operation. The removal of the volatile compounds alleviates the complexity of the PERUT process including salt purification. The experimental studies so far have largely concentrated on alkaline and alkaline earth metal FPs and RE FPs, because their chlorides are the most stable ones and major fractions of their chlorides will stay in the used LiCl or LiCl–KCl salt. Limited experimental data are available for noble metal and noble-metal-like FPs. Experimental data on the behaviors of volatile FP and LEAP compounds from molten salt reactors including MCFRs are also limited. Such data will be needed for managing the off-gas from the MCFR operation and the PERUT process.