# Chemical Interactions between Thorium(IV) and O2- in the FLiBe and FLiNaK Molten Salt Systems

Authors: Hao Peng; Nan Ji; Wei Huang; Yu Gong
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The chemical interactions between thorium(IV) and O2- in the LiF-BeF2 (2 : 1 mol%) (FLiBe) and LiF-NaF-KF (46.5 : 11.5 : 42 mol%) (FLiNaK) molten salt systems were studied at 873 K. The precipitation-dissolution behavior of Th(IV) oxide/oxyfluoride species, as well as the speciation law of these species and their relationship with oxide concentration in fluoride melts, were clarified. The results showed that the interactions between Th(IV) and O2- in the FLiBe melt would only produce ThO2. The saturated solubility of ThO2 in the FLiBe melt was 0.057 mol/kg, and its corresponding apparent solubility product (Ksp′) was 8.8±0.1×10−4 mol3/kg3 and 8.6±0.7×10−4 mol3/kg3 by dissolution and oxide titration methods, respectively. However, in the FLiNaK melt, the interactions between Th(IV) and O2- were more complex than that in FLiBe, and Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2) were produced. Th2OF104- was initially formed with the ratio of r≤0.5 (r=O2−added/ThIVinitial), and the Th2OF104- was converted to ThOF2 precipitate when r>0.5. Therefore, the soluble Th2OF104- is the important intermediate between Th(IV), O2-, and ThOF2 precipitation. The difference in results of the FLiBe and FLiNaK melts was attributed to the difference of free F- content in the two systems. The present work provided important reference and fundamental data for understanding the nature of thorium fuel precipitation and how to control it in the molten fluoride media, which is of significant importance for the safe operation of molten salt reactor (MSR).


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## Body

## 1. Introduction

Molten salt reactor (MSR) is one of the six recommended advanced nuclear fission reactors by Generation IV International Forum (GIF) [1–3]. Different from other reactors, MSR uses liquid fuel when operating, that is, the fissile material of uranium tetrafluoride (in the form of 233UF4 or 235UF4) and the fertile material of thorium tetrafluoride (in the form of 232ThF4) are dissolved in the carrier medium, i.e., the molten lithium-beryllium fluoride salt mixture (2LiF-BeF2, FLiBe) [4–6]. MSR is generally considered to be the most ideal reactor type for the utilization of thorium resources, in which the nuclear fuel breeding can be realized under the 232Th-233U cycle [7–13],and thus to maximize the utilization of thorium resources. The Oak Ridge National Laboratory (ORNL) developed a conceptual design of the molten salt breeder reactor (MSBR) using LiF-BeF2-ThF4-UF4 fuel salt from late 1960s, and it was terminated in 1976 [14]. Recently, the thorium-based molten salt reactor (TMSR) project of China, which was also powered as a 232Th–233U breeding reactor with the fuel of LiF-BeF2-ThF4-UF4 or LiF-BeF2-ZrF4-ThF4-UF4, was being launched by the Shanghai Institute of Applied Physics, Chinese Academy of Sciences (SINAP@CAS).Fluoride salts have inherent hygroscopic properties [15]. The water adsorbed by these fluorides will inevitably be introduced into the reactor and is prone to hydrolysis at high operating temperatures to generate reactive oxide ions O2- [16], as shown in Equation (1). Given its high affinity with oxide, the nuclear fuels would combine with the formed impurity O2- to produce insoluble oxide compounds or oxyfluoride species [17] (Equations. (2) and (3)). For example, as reported by Paul [18] and our previous studies [19–21], the high sensitivity of UF4 toward oxide in molten fluorides probably causes UO2(s), UOF2(s), and U2O2F4(s). Chamelot et al. [22] found the formation of ThO2 and ThOF2 compounds in the ThF4-containing fluoride melts with oxide additions. Generally, these compounds have low solubility, a high melting point, and a relatively high density, so they mostly exist in the form of precipitates in molten fluoride salts. The continuous accumulation of these nuclear fuel precipitates in the reactor will lead to the formation of localized overheating regions and ultimately lead to uncontrolled criticality risk and operation accident risk of the reactor [14, 19, 23, 24]. Therefore, the dissolution behavior of the fuel precipitate in molten fluorides should play an important role in the safe operation of MSR.
(1)H2O+2F−=2HF+O2−,(2)MIV+2O2−=MO2sM=U,Th,(3)nMIV+nO2−+2nF−=MOF2nsM=U,Th.The solubility of UO2 in molten fluorides has been extensively studied. Early studies at ORNL showed that the solubility product (Ksp) of UO2 in the fuel of MSBR (LiF-BeF2-ThF4-UF4) at 773 K was 2.6×10−7 mol3/kg3 [14]. Meanwhile, ORNL also obtained that the Ksp data of UO2 in the molten salt reactor experiment (MSRE) fuel salt system (LiF-BeF2-ZrF4-UF4) at 873 K was 1.2 ×10-5 mol3/kg3 [25]. That means with the substitution of ThF4 by ZrF4, the oxygen tolerance (i.e., the maximum oxygen concentration allowed in a molten salt system without nuclear fuel precipitate) of the fuel salt increases from 30 ppm level of MSBR to more than 100 ppm level of MSRE. In addition, Haas and Copper [18] and Varwig [26] studied the solubility of UO2 in the LiF-BaF2-UF4, LiF-UF4 and BaF2-UF4 systems at 1473 K. Alangi et al. [27] also reported the results on the dissolution behavior of UO2 in the LiF-BaF2 (1 : 1)-LaF3 system. Moreover, by combining electrochemical and chemical analysis methods, Peng et al. [19, 20, 28] studied the precipitation and dissolution behavior of UO2 in the LiF-NaF-KF (FLiNaK) and FLiBe molten salts. The results showed that the chemical interactions between U(IV) and O2- in both systems would produce UO2(s), and the solubility of UO2 in the FLiNaK and FLiBe melts corresponded to 9.15×10−3 mol/kg and 2.37×10−3 mol/kg, respectively; while the Ksp of UO2 in these two systems were 3.07×10−6 mol3/kg3 and 1.33×10−5 mol3/kg3, respectively. Peng et al. [28] also found that the maximum solubility of UO2 in the FLiNaK-ZrF4 melt was 5.76 times greater than that in the FLiNaK melt due to the formation of oxyfluoride species, i.e., UOF2 and ZrOF2.Unfortunately, the chemical interactions between thorium and oxide, e.g., the dissolution and precipitation behavior of thorium oxide/oxyfluoride species, have been rarely studied in molten fluorides. Wang et al. [29] studied the structures of thorium oxyfluoride anions in molten FLiBe and FLiNaK by Raman spectroscopy and found a stable Th2OF104- anion species with a linear Th−O−Th geometry existed in the two systems when containing oxide. Chamelot et al. [22] investigated the speciation in the LiF-CaF2-ThF4 melt with oxide addition and reported the formation of a stable oxide compound ThO2 with the intermediate product of ThOF2 in this system. Besides, they also found that the complex ThOF2 can be converted to ThO2 and ThO in molten fluorides. Bessada et al. [30] confirmed the existence of ThOF2 compound in fluoride media containing oxide through nuclear magnetic resonance (NMR) measurements. However, these studies mainly focused on the formation of Th(IV) oxide/oxyfluoride species rather than their speciation processes. That means the relationship between the chemical evolution law and the oxide content of these species, as well as their precipitation-dissolution behavior, is still required to be resolved. Therefore, as the important chemical properties of thorium fuel in the molten salt reactor, the key dissolution parameters (including solubility and Ksp) and the precipitation-dissolution equilibrium of thorium oxide or oxyfluoride compounds in molten fluoride salts are urgent topics to be studied.This study investigated the precipitation-dissolution behavior and equilibrium of thorium oxide/oxyfluoride compounds in the FLiBe and FLiNaK molten salts. The speciation between Th(IV) and O2- and the specific interaction mechanism or the chemical pathway were clarified. Besides, the key parameters, including solubility and the apparent solubility product (Ksp′) of these thorium oxide/oxyfluoride compounds (e.g., ThO2) in the two molten salt systems, were also obtained. The present work provided important reference and fundamental data for the nature and control of thorium fuel precipitation in molten fluoride media, which is of significant importance for the safe operation of MSR.


## 2. Experimental

### 2.1. Molten Salt Materials and Chemicals

The highly purified FLiBe (LiF-BeF2 (2 : 1 mol%); 99.97%; m.p. 733 K) and FLiNaK (LiF-NaF-KF (46.5 : 11.5 : 42 mol%); 99.98%; m.p. 727 K) eutectic salts were prepared and supplied by the SINAP@CAS. The salt was purified by sparging the H2(g)/HF(g) (10 : 1 mol ratio) gas mixture into the melt at 600°C for 40 hours. In this process, the oxide in the molten salt was effectively removed through the following reaction: O2- +2HF ⟶ H2O(g) +2F-; meanwhile, the metallic impurities and some oxygenated anions were removed through the following hydrogen reduction: (x/2)H2 + MFx ⟶ xHF + M↓ (M = Ni, Fe, Cr…) and 4H2 + SO42- ⟶4H2O(g) + S2- [15, 31]. Then, the melt was further purified by sparging the ultrapure H2 for 8 hours to remove the residual HF gas introduced by H2(g)/HF(g) purification method. The detailed preparation and purification process of these two salts have been described in our previous studies [19–21, 28]. However, due to the chemical equilibrium for H2(g)/HF(g) purification method, there is a purification limit, and impurities are inevitable. The impurity content of both salts is shown in Table 1. The trace impurities almost had no effect on the results of the experiments. The contents of the oxygenated anions and oxide ions would be analyzed in detail in Section 3.1.Table 1
Impurities of the FLiBe and FLiNaK molten salts used in this study (in ppm).
FLiBeFLiNaKFe16Cr13Ni1710Al104Mn31Mg29W—1Ti—1Ba18—Na14—K25—Si12—Ca11—NO3−2729SO42−1214PO43−86O2-150114The prepared salts were then hermetically transferred and stored in a glovebox before use. In this duration, there still exists a potential risk for the received salts to absorb water due to its hygroscopic nature, even if this negative influence can be ignored. So, in order to ensure the reliability of the experiments, the received FLiBe and FLiNaK eutectic salts stored in the glovebox were further dehydrated by heating under vacuum at their melting point for 72 h before use. In this process, the moisture absorbed by the salts can be effectively removed.ThF4 (99.99%; China North Nuclear Fuel Co., Ltd.), ThO2 (99.99%; China North Nuclear Fuel Co., Ltd.), and Li2O (99.99%; Sigma-Aldrich) were thus added to the prepared FLiBe and FLiNaK melts to form various experimental systems.


### 2.2. Experimental Method Design

#### 2.2.1. Dissolution Experiment

Firstly, an excess of ThO2 powder, ca. 5 g, was made into flake shape with a tablet press. Then, the prepared ThO2 sheet was fixed in a graphite holder that connected with a lifting device, making it convenient for the ThO2 sheet to be immersed in and removed from the molten salt. The detailed illustration of the schematic diagram of this experimental device was found in our previous studies [20, 21]. During the experiment, the ThO2 sheet was immersed and dissolved in 50 g of the FLiBe or FLiNaK melt, and the saturated solution of FLiBe-ThO2 or FLiNaK-ThO2 was formed after equilibrium time. The melt was sampled by dipping a high-purity nickel rod into the molten salt. All the thorium in the sample was considered to come from ThO2 dissolution, which could reflect the real solubility of ThO2.


#### 2.2.2. Oxide Titration Experiment

Firstly, the Th(IV) in the form of ThF4 was added to the FLiBe and FLiNaK melts to prepare the Th(IV)-containing systems, i.e., FLiBe-ThF4 and FLiNaK-ThF4. Then, the oxide ions O2- in the form of Li2O was accurately weighed and then successively added to the prepared FLiBe-ThF4 and FLiNaK-ThF4 systems. In order to make Li2O dissolve more sufficient and release O2- into the melt faster, it was added in powdered state. In this way, the amount of O2- introduced into the molten salt can be regulated by weighing Li2O, which was in accordance with our previous studies [19–21, 28]. The chemical interactions between Th(IV) and O2- would produce (i) insoluble oxide/oxyfluoride compounds (e.g., ThO2 and ThOF2) or (ii) soluble oxyfluoride species (e.g., Th2OF104-). The former will form a precipitate and deposit at the bottom of the bath (ρ≈2 g/cm3) due to its higher density (ρThO2=9.86 g/cm3), while the latter will dissolve in the melt as the form of soluble complex ions. The melt was sampled when the system reached equilibrium.It should be noted that, for the case (i) with the formation of sediment, the supernatant of the molten salt was sampled. However, there still might be a small amount of solid particles suspended in the supernatant, which would contaminate the sampler and further cause test errors. Therefore, to eliminate the negative effect of precipitation and ensure the accuracy of the experimental results, the sintered nickel filter was used in the sampling process, which prevented the formed precipitates from sticking to the sampler as proposed by the ORNL [32].


### 2.3. Experimental Apparatus

The schematic drawing of the experimental apparatus is shown in Figure1. The molten salts were weighed and loaded in a vitreous carbon crucible, which was further placed in a stainless steel cell and transferred to an electric furnace. Then, the molten salts were heated and melted at the target temperature of 873 K. The temperature was measured by a nickel–chromium thermocouple positioned just outside the crucible. The entire experimental assembly was placed inside a glove box protected by a dry argon atmosphere (99.999%), and the typical concentrations of moisture and oxygen in the glove box were both generally below 0.5 ppm. The experimental apparatus used has been described in detail in our previous studies [19–21, 28, 33].Figure 1
Schematic drawing of the experimental apparatus used in this study.


### 2.4. Analytical Technology

The total oxide content of the molten salt was determined by using the LECO oxygen detector (RO600, LECO Co., Ltd.) based on the carbothermal reduction mechanism, which was in accordance with the work of Mediaas et al. [34, 35] and our previous studies [19–21, 28]. The fluoride salt sample (0.1-0.15 g) was first sealed in the tin capsule in the glove box, and then transferred for LECO analysis. The use of a tin capsule can effectively avoid the risk of moisture absorption in a salt sample. Then, the tin capsule with the salt sample was heated in a graphite crucible to 2700°C over a short period of time (~400 s). During the heating process, the tin capsule (m.p. of 232°C) and salt sample (m.p. of 460°C) would firstly both melt, and the sample began to directly contact the graphite crucible at this stage. When the temperature was up to 2700°C, the salt sample would boil off, and the oxide contained in salt was completely released and fully reacted with carbon at this high temperature. After a period of time (up to 400 s) at 2700°C, the oxides in the sample completely react with the graphite crucible to form CO (g) and CO2 (g), which can be further detected in an infrared radiation (IR) cell. To minimize systematic error, five samples were chosen from different locations of the melting salts, and each sample was analyzed. The mean value was reported as the final result. Herein, the result measured by LECO was the total oxide content, including free oxide ions (O2-) and oxide in oxygenated anions.The contents of the oxygenated anions in the melts were determined by using ICE ion chromatography (IC, ICS-2100, Dionex Co., Ltd.) with a valve-switching technology. In this method, approximately 0.2 g of the salt sample (in powder state) was initially dissolved in 20 g of deionized water for enough long time (>24 h) to fully release oxygenated anions to the solution. In this way, all the oxygenated anions in the sample solution can be considered to come from the salt sample. Then, the sample solution was analyzed after pretreatment of fluorine ion removal. IonPac AG 11 HC was used as the concentrator column to trap the oxygenated anions and to exclude the matrix at the same time. The trapped oxygenated anions were separated with the IonPac AS18 column by valve switching and detected by using a suppressed conductive detector. The real contents of the oxygenated anions in the salt sample were thus obtained through the analysis of the sample solution. With the obtained LECO and ICE results, the practical O2- content was thus determined by the difference between total oxide and the oxide in oxygenated anions.The thorium concentration in melt was determined by the inductively coupled plasma optical emission spectroscopy (ICP-OES, Arcos, Spectro Co., Ltd.) with a detection limit of ~1 ppm (4.3×10−6 mol/kg). The precipitates formed in melts and the salt phase were analyzed by X-ray diffraction (XRD, DY3614, PANzlytical Co., Ltd.). The specific sample preparations and testing procedures for LECO, IC, ICP-OES, and XRD techniques have been reported previously [19–21, 28, 33].


## 2.1. Molten Salt Materials and Chemicals

The highly purified FLiBe (LiF-BeF2 (2 : 1 mol%); 99.97%; m.p. 733 K) and FLiNaK (LiF-NaF-KF (46.5 : 11.5 : 42 mol%); 99.98%; m.p. 727 K) eutectic salts were prepared and supplied by the SINAP@CAS. The salt was purified by sparging the H2(g)/HF(g) (10 : 1 mol ratio) gas mixture into the melt at 600°C for 40 hours. In this process, the oxide in the molten salt was effectively removed through the following reaction: O2- +2HF ⟶ H2O(g) +2F-; meanwhile, the metallic impurities and some oxygenated anions were removed through the following hydrogen reduction: (x/2)H2 + MFx ⟶ xHF + M↓ (M = Ni, Fe, Cr…) and 4H2 + SO42- ⟶4H2O(g) + S2- [15, 31]. Then, the melt was further purified by sparging the ultrapure H2 for 8 hours to remove the residual HF gas introduced by H2(g)/HF(g) purification method. The detailed preparation and purification process of these two salts have been described in our previous studies [19–21, 28]. However, due to the chemical equilibrium for H2(g)/HF(g) purification method, there is a purification limit, and impurities are inevitable. The impurity content of both salts is shown in Table 1. The trace impurities almost had no effect on the results of the experiments. The contents of the oxygenated anions and oxide ions would be analyzed in detail in Section 3.1.Table 1
Impurities of the FLiBe and FLiNaK molten salts used in this study (in ppm).
FLiBeFLiNaKFe16Cr13Ni1710Al104Mn31Mg29W—1Ti—1Ba18—Na14—K25—Si12—Ca11—NO3−2729SO42−1214PO43−86O2-150114The prepared salts were then hermetically transferred and stored in a glovebox before use. In this duration, there still exists a potential risk for the received salts to absorb water due to its hygroscopic nature, even if this negative influence can be ignored. So, in order to ensure the reliability of the experiments, the received FLiBe and FLiNaK eutectic salts stored in the glovebox were further dehydrated by heating under vacuum at their melting point for 72 h before use. In this process, the moisture absorbed by the salts can be effectively removed.ThF4 (99.99%; China North Nuclear Fuel Co., Ltd.), ThO2 (99.99%; China North Nuclear Fuel Co., Ltd.), and Li2O (99.99%; Sigma-Aldrich) were thus added to the prepared FLiBe and FLiNaK melts to form various experimental systems.


## 2.2. Experimental Method Design

### 2.2.1. Dissolution Experiment

Firstly, an excess of ThO2 powder, ca. 5 g, was made into flake shape with a tablet press. Then, the prepared ThO2 sheet was fixed in a graphite holder that connected with a lifting device, making it convenient for the ThO2 sheet to be immersed in and removed from the molten salt. The detailed illustration of the schematic diagram of this experimental device was found in our previous studies [20, 21]. During the experiment, the ThO2 sheet was immersed and dissolved in 50 g of the FLiBe or FLiNaK melt, and the saturated solution of FLiBe-ThO2 or FLiNaK-ThO2 was formed after equilibrium time. The melt was sampled by dipping a high-purity nickel rod into the molten salt. All the thorium in the sample was considered to come from ThO2 dissolution, which could reflect the real solubility of ThO2.


### 2.2.2. Oxide Titration Experiment

Firstly, the Th(IV) in the form of ThF4 was added to the FLiBe and FLiNaK melts to prepare the Th(IV)-containing systems, i.e., FLiBe-ThF4 and FLiNaK-ThF4. Then, the oxide ions O2- in the form of Li2O was accurately weighed and then successively added to the prepared FLiBe-ThF4 and FLiNaK-ThF4 systems. In order to make Li2O dissolve more sufficient and release O2- into the melt faster, it was added in powdered state. In this way, the amount of O2- introduced into the molten salt can be regulated by weighing Li2O, which was in accordance with our previous studies [19–21, 28]. The chemical interactions between Th(IV) and O2- would produce (i) insoluble oxide/oxyfluoride compounds (e.g., ThO2 and ThOF2) or (ii) soluble oxyfluoride species (e.g., Th2OF104-). The former will form a precipitate and deposit at the bottom of the bath (ρ≈2 g/cm3) due to its higher density (ρThO2=9.86 g/cm3), while the latter will dissolve in the melt as the form of soluble complex ions. The melt was sampled when the system reached equilibrium.It should be noted that, for the case (i) with the formation of sediment, the supernatant of the molten salt was sampled. However, there still might be a small amount of solid particles suspended in the supernatant, which would contaminate the sampler and further cause test errors. Therefore, to eliminate the negative effect of precipitation and ensure the accuracy of the experimental results, the sintered nickel filter was used in the sampling process, which prevented the formed precipitates from sticking to the sampler as proposed by the ORNL [32].


## 2.2.1. Dissolution Experiment

Firstly, an excess of ThO2 powder, ca. 5 g, was made into flake shape with a tablet press. Then, the prepared ThO2 sheet was fixed in a graphite holder that connected with a lifting device, making it convenient for the ThO2 sheet to be immersed in and removed from the molten salt. The detailed illustration of the schematic diagram of this experimental device was found in our previous studies [20, 21]. During the experiment, the ThO2 sheet was immersed and dissolved in 50 g of the FLiBe or FLiNaK melt, and the saturated solution of FLiBe-ThO2 or FLiNaK-ThO2 was formed after equilibrium time. The melt was sampled by dipping a high-purity nickel rod into the molten salt. All the thorium in the sample was considered to come from ThO2 dissolution, which could reflect the real solubility of ThO2.


## 2.2.2. Oxide Titration Experiment

Firstly, the Th(IV) in the form of ThF4 was added to the FLiBe and FLiNaK melts to prepare the Th(IV)-containing systems, i.e., FLiBe-ThF4 and FLiNaK-ThF4. Then, the oxide ions O2- in the form of Li2O was accurately weighed and then successively added to the prepared FLiBe-ThF4 and FLiNaK-ThF4 systems. In order to make Li2O dissolve more sufficient and release O2- into the melt faster, it was added in powdered state. In this way, the amount of O2- introduced into the molten salt can be regulated by weighing Li2O, which was in accordance with our previous studies [19–21, 28]. The chemical interactions between Th(IV) and O2- would produce (i) insoluble oxide/oxyfluoride compounds (e.g., ThO2 and ThOF2) or (ii) soluble oxyfluoride species (e.g., Th2OF104-). The former will form a precipitate and deposit at the bottom of the bath (ρ≈2 g/cm3) due to its higher density (ρThO2=9.86 g/cm3), while the latter will dissolve in the melt as the form of soluble complex ions. The melt was sampled when the system reached equilibrium.It should be noted that, for the case (i) with the formation of sediment, the supernatant of the molten salt was sampled. However, there still might be a small amount of solid particles suspended in the supernatant, which would contaminate the sampler and further cause test errors. Therefore, to eliminate the negative effect of precipitation and ensure the accuracy of the experimental results, the sintered nickel filter was used in the sampling process, which prevented the formed precipitates from sticking to the sampler as proposed by the ORNL [32].


## 2.3. Experimental Apparatus

The schematic drawing of the experimental apparatus is shown in Figure1. The molten salts were weighed and loaded in a vitreous carbon crucible, which was further placed in a stainless steel cell and transferred to an electric furnace. Then, the molten salts were heated and melted at the target temperature of 873 K. The temperature was measured by a nickel–chromium thermocouple positioned just outside the crucible. The entire experimental assembly was placed inside a glove box protected by a dry argon atmosphere (99.999%), and the typical concentrations of moisture and oxygen in the glove box were both generally below 0.5 ppm. The experimental apparatus used has been described in detail in our previous studies [19–21, 28, 33].Figure 1
Schematic drawing of the experimental apparatus used in this study.


## 2.4. Analytical Technology

The total oxide content of the molten salt was determined by using the LECO oxygen detector (RO600, LECO Co., Ltd.) based on the carbothermal reduction mechanism, which was in accordance with the work of Mediaas et al. [34, 35] and our previous studies [19–21, 28]. The fluoride salt sample (0.1-0.15 g) was first sealed in the tin capsule in the glove box, and then transferred for LECO analysis. The use of a tin capsule can effectively avoid the risk of moisture absorption in a salt sample. Then, the tin capsule with the salt sample was heated in a graphite crucible to 2700°C over a short period of time (~400 s). During the heating process, the tin capsule (m.p. of 232°C) and salt sample (m.p. of 460°C) would firstly both melt, and the sample began to directly contact the graphite crucible at this stage. When the temperature was up to 2700°C, the salt sample would boil off, and the oxide contained in salt was completely released and fully reacted with carbon at this high temperature. After a period of time (up to 400 s) at 2700°C, the oxides in the sample completely react with the graphite crucible to form CO (g) and CO2 (g), which can be further detected in an infrared radiation (IR) cell. To minimize systematic error, five samples were chosen from different locations of the melting salts, and each sample was analyzed. The mean value was reported as the final result. Herein, the result measured by LECO was the total oxide content, including free oxide ions (O2-) and oxide in oxygenated anions.The contents of the oxygenated anions in the melts were determined by using ICE ion chromatography (IC, ICS-2100, Dionex Co., Ltd.) with a valve-switching technology. In this method, approximately 0.2 g of the salt sample (in powder state) was initially dissolved in 20 g of deionized water for enough long time (>24 h) to fully release oxygenated anions to the solution. In this way, all the oxygenated anions in the sample solution can be considered to come from the salt sample. Then, the sample solution was analyzed after pretreatment of fluorine ion removal. IonPac AG 11 HC was used as the concentrator column to trap the oxygenated anions and to exclude the matrix at the same time. The trapped oxygenated anions were separated with the IonPac AS18 column by valve switching and detected by using a suppressed conductive detector. The real contents of the oxygenated anions in the salt sample were thus obtained through the analysis of the sample solution. With the obtained LECO and ICE results, the practical O2- content was thus determined by the difference between total oxide and the oxide in oxygenated anions.The thorium concentration in melt was determined by the inductively coupled plasma optical emission spectroscopy (ICP-OES, Arcos, Spectro Co., Ltd.) with a detection limit of ~1 ppm (4.3×10−6 mol/kg). The precipitates formed in melts and the salt phase were analyzed by X-ray diffraction (XRD, DY3614, PANzlytical Co., Ltd.). The specific sample preparations and testing procedures for LECO, IC, ICP-OES, and XRD techniques have been reported previously [19–21, 28, 33].


## 3. Results and Discussion

### 3.1. Oxide Analysis of the Molten FLiBe and FLiNaK Salts

Before the dissolution and oxide titration experiments, the original oxide content of the FLiBe and FLiNaK salts was firstly determined, and the results are shown in Table2. Apparently, the total oxide content consisted of two parts, i.e., the oxide in oxygenated anions (including NO3−, SO42−, and PO43−) and the free oxide ions (O2-). The purpose of the oxide analysis was to obtain the practical content of the free oxide ions (O2-) in molten fluorides, which is the basis of the study of the interactions between Th(IV) and O2- in fluoride media.Table 2
Oxide analysis results of the FLiBe and FLiNaK melts used in the present study.
Salt typeTotal oxide concentration ([O]LECO)/ppmOxygenated anion concentration/ppmOxide in oxygenated anions ([O]IC)/ppmO2− concentration ([O2−])/ppmFLiBe184NO3−2734150 (9.375×10−3mol/kg)SO42−12PO43−8FLiNaK150NO3−2936114 (7.125×10−3 mol/kg)SO42−14PO43−6The total oxide concentrations ([O]LECO) of the FLiBe and FLiNaK salts used in this study were determined by the LECO instrument and were 184 ppm and 150 ppm, respectively. As shown in Table 2, the oxygenated anions in FLiBe and FLiNaK salts were mainly NO3−, SO42−, and PO43−, as determined by the IC technique. According to the mass percentage of oxygen in these three oxygenated anions, the sum of the oxide content in oxygenated anions ([O]IC) was calculated to be 34 ppm for FLiBe and 36 ppm for FLiNaK, respectively. Thus, the practical content of free oxide ions O2- ([O2-]) was obtained by subtracting the [O]IC from the [O]LECO (Equation (4)), which were 150 ppm in FLiBe and 114 ppm in FLiNaK, respectively.
(4)O2−=OLECO−OIC.


### 3.2. Chemical Interactions between Th(IV) and O2- in the FLiBe Melt

#### 3.2.1. Saturated Dissolution Experiment: FLiBe-ThO2 System

(1) Solubility of ThO2. The ThO2 pellet was directly immersed into the FLiBe melt to determine the solubility of ThO2 in this melt. The melt was sampled at different dissolution times, and the thorium content in each sample was analyzed. Since the increase of thorium concentration came from the dissolution of ThO2, the maximum value of thorium concentration could thus reflect the solubility of ThO2, and this moment corresponds to the dissolution equilibrium time.Figure2 shows the evolution of the Th(IV) concentration in the FLiBe melt with ThO2 immersed. Apparently, no thorium was present in the initial FLiBe salt. Then, the Th(IV) concentration in the melt remarkably increased as dissolution proceeds. As the dissolution time reached 6 h, the Th(IV) concentration achieved the maximum value of 0.057 mol/kg and maintained stable, which corresponded to the solubility of ThO2 in the FLiBe melt. The dissolution equilibrium time corresponded to 6 h.Figure 2
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).The variation in oxide content of the samples taken from the FLiBe-ThO2 melt was also determined, as shown in Figure 2. Apparently, the oxide content of the system also increased with dissolution time and finally reached its maximum value of 0.124 mol/kg also at 6 h. It should be noted that the actual O2- caused by the dissolution of ThO2 should be 0.115 mol/kg, which is obtained by subtracting the initial O2- content of the FLiBe (9.375×10−3 mol/kg, Table 2) from the value of 0.124 mol/kg. Figure 3 shows the concentration ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiBe salt) during the entire dissolution process of ThO2, which was maintained at around 0.5. These results confirmed that the dissolution of ThO2 will simultaneously release O2- and Th(IV) into the melt with the stoichiometric ratio of Th(IV): O2- =1: 2. Therefore, the dissolution mechanism of ThO2 in the FLiBe melt can be described by Equation (5). The specific dissolution mechanism will be discussed in detail in Section 3.4.
(5)ThO2s⟶ThIV+2O2−.Figure 3
Invariable ratio of [Th(IV)]/[O2-] = 0.5 during the dissolution of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).(2)Ksp′ of ThO2. When the dissolution equilibrium of ThO2 was achieved, the species of ThO2 compound, Th(IV), and O2- ions coexisted and equilibrated (Equation (5)). Thus, the apparent solubility product (Ksp′) of ThO2 could be obtained by multiplying the equilibrium concentration of Th(IV) and O2-, as shown in Equation (6). According to Figure 2, the equilibrium concentration of Th(IV) and O2- were experimentally determined as 0.057 mol/kg and 0.124 mol/kg, respectively. Thus, a set of Ksp′ values could be calculated after an equilibrium time of 6 h, and the results are shown in Table 3. Apparently, the obtained Ksp′ of ThO2 showed good agreement and were located in the same order of 10-4. The slight fluctuation within the range of 8.69−8.91×10−4 may be due to the testing errors caused by ICP-OES, LECO, and IC techniques. Thus, the mean value of 8.8×10−4 with an uncertainty of ±0.1×10−4 was reported as the final result.
(6)Ksp′=ThIVO2−2.Table 3
TheKsp′ of ThO2 in the FLiBe melt at 873 K obtained by dissolution method.
Dissolution time (h)[Th(IV)]eq (mol/kg)[O2-]eq (mol/kg)Ksp′(ThO2) (mol3/kg3)AverageKsp′(ThO2) (mol3/kg3)65.70×10−20.1248.81×10−48.8±0.1×10−475.73×10−20.1248.81×10−485.64×10−20.1248.70×10−4105.75×10−20.1258.91×10−4155.67×10−20.1248.69×10−4225.72×10−20.1248.75×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported averageKsp′(ThO2) is ±0.1×10−4 mol3/kg3.


#### 3.2.2. Oxide Titration Experiment: FLiBe-ThF4-Li2O System

(1) Precipitation Behavior of ThO2. The interactions between oxide and thorium were investigated by artificially introducing the oxide ions O2- (in the form of Li2O) to the Th(IV)-containing FLiBe melt. When the system reached equilibrium, the supernatant of the molten salt was sampled, and variation of the equilibrium concentration of Th(IV) in the melt with oxide addition were then clarified. Table 4 shows the residual Th(IV) concentration in the FLiBe-ThF4 (0.143 mol/kg) melt after O2- additions from 0.112 to 0.286 mol/kg. Obviously, the concentration of Th(IV) in the melt gradually decreased with oxide addition, indicating the precipitation of Th(IV).Table 4
Variation of the equilibrium concentration of Th(IV) and O2- in the FLiBe-ThF4 melt with different oxide additions at 873 K.
Added [O2-] ([O2-]added) (Mol/kg)Equilibrium [O2-] ([O2-]eq) (Mol/kg)Δ[O2-] ([O2-]added-[O2-]eq) (mol/kg)Equilibrium [Th(IV)] ([Th(IV)]eq) (mol/kg)K’sp (ThO2) ([Th(IV)]eq[O2-]eq2) (mol3/kg3)StatisticalK’sp (ThO2) (mol3/kg3)0000.140—8.6±0.7×10−40.1128.54×10−22.62×10−20.1269.19×10−40.1559.22×10−26.25×10−20.1089.15×10−40.2099.84×10−20.1118.28×10−28.03×10−40.2550.1060.1496.84×10−27.61×10−40.2860.1170.1695.64×10−27.67×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported statistical result ofKsp′(ThO2) is ±0.7×10−4 mol3/kg3.To further confirm the precipitated species, the equilibrium concentration of O2- after each oxide addition was also analyzed, and this led to the calculation of the consumption of O2- (Δ[O2-]) during the precipitation reaction, which could be obtained by the difference between the added oxide and equilibrium oxide, i.e., ΔO2−=O2−added−O2−eq. Then, a linear relationship between the [Th(IV)]eq and Δ[O2-] was evidenced by Figure 4. The slope of this straight line was 0.495, which approximately corresponded to 0.5, indicating the formation of ThO2 or ThxO2xFy compound. Moreover, the FLiBe-ThF4 melt presented as colorless before oxide addition (Figure 5(a)), and a white precipitate layer was found at the bottom of the melting bath (ρ≈2 g/cm3) during the oxide titration experiment, as shown in Figure 5(b). This layer corresponds to ThO2 precipitate (ρ≈9.86 g/cm3) as further proved through XRD analysis (Figure 5(c)). Therefore, the interactions between Th(IV) and O2- in the FLiBe melt would produce ThO2 through the following reaction (Equation (7)).
(7)ThF4+2O2−⟶ThO2s+4F−.Figure 4
Linear relationship between the equilibrium [Th(IV)] and theΔ[O2-] in the FLiBe-ThF4 melt with oxide additions at 873 K.Figure 5
Photography of the FLiBe-ThF4 (0.143 mol/kg) melt before (a) and after (b) O2- addition, observed at high temperature: Upper transparent liquid corresponding to FLiBe-ThF4 melts and the bottom white layer corresponding to solid precipitate. (c) XRD patterns of the bottom white precipitate after distillation treatment to remove the residual molten salt substrate.
(a)(b)(c)(2)Ksp′ of ThO2. After oxide addition, ThO2 precipitate was formed, and its precipitation-dissolution equilibrium would be established in the system. Thus, a set of Ksp′(ThO2) data can be calculated in accordance with Equation (6), and the results are shown in Table 4. The obtained Ksp′(ThO2) exhibited a good consistency and located within 7.61−9.19×10−4 mol3/kg3 except for some slight fluctuations, which could be due to the testing errors of the used analytical methods. A linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 was plotted in Figure 6, and the slope of this straight line corresponded to the statistical result of Ksp′(ThO2) because Eq. (6) can also be written as
(8)ThIV=Ksp′·1O2−2.Figure 6
Linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 in the FLiBe-ThF4 melt with oxide additions at 873 K.Thus, the resultingKsp′(ThO2) was finally determined to be 8.6±0.7×10−4 mol3/kg3. As an equilibrium constant, the value of solubility product theoretically only depends on temperature. The present result by oxide titration method was highly consistent with that obtained by the dissolution method in the FLiBe-ThO2 system at 873 K (8.8±0.1×10−4 mol3/kg3), as stated in Section (2) Ksp′ of ThO2 of Section 3.2.1.


### 3.3. Chemical Interactions between Th(IV) and O2- in the FLiNaK Melt

#### 3.3.1. Solubility of ThO2 in the FLiNaK Melt

Figure7 shows the variation of Th(IV) and O2- concentrations during the dissolution of ThO2 in the FLiNaK melt at 873 K. The results showed that the concentrations of Th(IV) and O2- remarkably increased with dissolution time and were finally maintained at the maximum value of 0.048 mol/kg and 0.121 mol/kg (with initialO2−subtractionof7.125×10−3 mol/kg in FLiNaK, Table 2), respectively, and an equilibration time of 8 h was obtained. The dissolution mechanism of ThO2 in FLiNaK was probably due to the following reactions (Equations. (9)–(12)):
(9)ThO2s⟶Th4++2O2−ThIVeqO2−eq=1:2,(10)ThO2s+2F−⟶ThOF2s+O2−ThIVeqO2−eq=0,(11)2ThO2s+10F−⟶Th2OF104−+3O2−ThIVeqO2−eq=1:2,(12)3ThO2s+12F−⟶ThOF2s+Th2OF104−+4O2−ThIVeqO2−eq=2:5.Figure 7
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).FLiNaK is a typical strong basic molten salt and contains abundant free F- ions, which will participate in the dissolution process of ThO2 to form oxyfluoride species, such as ThOF2(s) and Th2OF104-, as suggested by Wang et al. [29]. Therefore, as a simple dissolution process without yielding oxyfluoride species, Equation (9) was obviously not the mechanism. To further confirm the dissolution mechanism, the ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiNaK salt) during ThO2 dissolution was determined, as shown in Figure 8. The ratio of [Th(IV)]/[O2-] maintained at around 0.4, indicating the stoichiometric relation of Th(IV) and O2- ions caused by ThO2 dissolution is 2:5, which is in good agreement with the situation of Equation (12). Therefore, the dissolution pathway of ThO2 in the FLiNaK melt could be best described by Equation (12), with the ratio of 2:5 of the dissolved Th(IV) to O2-. The specific dissolution mechanism will be discussed in detail in Section 3.4.Figure 8
Invariable ratio of [Th(IV)]/[O2-] =0.4 during the dissolution of ThO2 in the FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).


#### 3.3.2. Oxide Titration Experiment: FLiNaK-ThF4-Li2O System

The chemical interactions between Th(IV) and O2- in the FLiNaK melt showed significant difference from that in the FLiBe melt. As shown in Figure 9 and Table 5, the evolution of the equilibrium concentration of Th(IV) and O2- in the FLiNaK-ThF4 melt with oxide additions could be regarded as three stages, indicating a more complicated speciation in this system.Figure 9
Evolution of the equilibrium [Th(IV)] and [O2-] in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K.Table 5
Summary of speciation and chemical interaction between Th(IV) and O2- in the FLiNaK molten salt at different r ([O2-]added/[Th(IV)]0) at 873 K.
Reaction/equilibriumSpeciation[O2-]eq/[Th(IV)]eqStage 1r=0—ThF4(ThFx4-x)00<r<0.52ThF4excessive+2F−+O2−⟶Th2OF104−Th2OF104-, ThF4(ThFx4-x)<0.5r=0.52ThF4+2F−+O2−⟶Th2OF104−Th2OF104-0.5Stage 20.5<r<1Th2OF104−excessive+O2−⟶2ThOF2s+6F−Th2OF104-, ThOF2(s)0.5r=1∗Th2OF104−+O2−⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and O2- (1 : 1)1Stage 3r>1Th2OF104−+O2−excessive⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and excessive O2->1(∗denotes the establishment of precipitation-dissolution equilibrium.).Stage 1. 0≤O2−added≤0.0721 mol/kg. 0≤O2−added/ThIV0r≤0.5.With oxide additions less than 0.0721 mol/kg, the equilibrium Th(IV) concentration in the melt remained invariable and maintained at the initial concentration of 0.1433 mol/kg ([Th(IV)]0), while the equilibrium O2- in the melt linearly increased with oxide addition with the slope of 1 (k1=0.99), as shown in Figure 9. These results indicated that all the added O2- exists in a soluble form in the melt without precipitation of Th(IV), probably forming soluble Th(IV) oxyfluoride ions. Therefore, the ratio of [O2-]eq/[Th(IV)]eq at this stage gradually increases with oxide addition, as shown in Figure 10. When the added O2- was increased to be 0.0721 mol/kg, the equilibrium [O2-] reached its maximum value of 0.0715 mol/kg, which was the upper limit of the formed oxyfluoride species dissolved in the melt. At this point, the ratio of [O2-]added/[Th(IV)]0 (defined as r) corresponded to 0.5 (Figure 9), indicating that the stoichiometry of the oxide and thorium in this formed soluble Th(IV) oxyfluoride species was 1 : 2, presumably Th2OF104-. This hypothesis was also comforted by the recent observation by Raman measurements of a stable Th2OF104- cluster structure in fluoride media [29]. Therefore, the ThF4 would combine with the added oxide to form Th2OF104- in this stage, and all the initial ThF4 in the melt were converted to the Th2OF104- at the end of this stage (r=0.5). At the same time, the ratio of [O2-]eq/[Th(IV)]eq increased to 0.5 due to the single species of Th2OF104- existing in the melt (Figure 10). Therefore, the reaction pathway of the process in this stage can be described by Equation (13). The soluble and colorless nature of the Th2OF104- species caused no obvious change in the appearance of the molten salt, as shown in Figures 11(a) and 11(b).
(13)2ThIV+10F−+O2−⟶Th2OF104−.Figure 10
Evolution of the ratio of [O2-]eq/[Th(IV)]eq in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K. (The slopes in stage 1 and stage 3 were obtained due to [O2-]eq = [O2-]added).Figure 11
Photography of the FLiNaK-ThF4 (0.1433 mol/kg) melt (a) with different oxide additions at 873 K: (b) 0.0354 mol/kg (r=0.247); (c) 0.0918 mol/kg (r=0.641); (d) 0.1436 mol/kg (r=1.002); (e) 0.1863 mol/kg (r=1.300).
(a)(b)(c)(d)(e)Stage 2: 0.0721<O2−added≤0.1436 mol/kg. 0.5<O2−added/ThIV0r≤1.When the added O2- was more than 0.0721 mol/kg (r>0.5), both the equilibrium [Th(IV)] and [O2-] in the melt began to decrease. This result suggested that a precipitation reaction occurred between the formed Th2OF104- and added O2-, resulting in a solid compound that simultaneously containing thorium and oxygen elements. With the added O2- further increased to 0.1436 mol/kg (r=1), both the [Th(IV)]eq and [O2-]eq almost decreased linearly and finally decreased to their minimum value of 0.0047 mol/kg and 0.0048 mol/kg, respectively. The slopes of the two fitted straight lines were approximately 2 (k4=1.94) and 1 (k2=0.93), respectively. This result suggested that the Th(IV) consumed by precipitation in molten salt is twice that of oxide, and the stoichiometric ratio of the reactants Th2OF104- and added O2- was 1 : 1. Besides, the r corresponded to 1 at O2−added=0.1436 mol/kg, indicating an equimolar of thorium and oxygen atoms contained in this formed precipitate, presumably ThOF2. During the experiment, a kind of white precipitate was observed at the bottom of the bath (Figure 11(c)), and this precipitate was proved to be ThOF2 by XRD (Figure 12). Related literature also reported that the ThOF2 compound can stably exist in fluoride salt media [30]. Therefore, Reaction (14) was speculated to occur during this stage, with a stable ratio of O2−eq/ThIVeq=0.5 due to the unreacted Th2OF104- dissolving in the melt (Figure 10).
(14)Th2OF104−+O2−⟶2ThOF2s+6F−.Figure 12
XRD patterns of the precipitate formed in stage 2 of the FLiNaK-ThF4 melt with oxide addition.It should be noted that the ratio [O2-]eq/[Th(IV)]eq suddenly increases from 0.5 to 1 at r=1 (Figure 10), which was caused by the dissolution equilibrium of ThOF2. At the point of r=1, almost all Th2OF104- was converted into ThOF2(s), and the ThOF2(s) precipitate at the bottom of the bath reached the maximum amount and formed a white insoluble layer (Figure 11(d)). Then, the precipitation-dissolution equilibrium of the ThOF2(s) was established, as shown in Equation (15). Thus, the residual [Th(IV)]eq and [O2-]eq remaining in the melt came from the dissolution of ThOF2(s), resulting in O2−eq/ThIVeq=1 with equimolar Th2OF104- and O2- dissolved.
(15)2ThOF2s+6F−⟷Th2OF104−+O2−.Based on the above analysis, Th2OF104- was the intermediate species in the overall chemical pathway of precipitation of ThF4 into ThOF2, as shown in Equations (13), (14), and (16). Thus, it could be concluded that the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2) due to the abundant F- in this system.
(16)ThIV+O2−+2F−↔ThOF2s.Stage 3: O2−added>0.1436 mol/kg. O2−added/ThIV0r>1When the added oxide was more than 0.1436 mol/kg (r>1), the O2- ions in the melt become excessive. The equilibrium O2- in the melt increased linearly with oxide addition and the slope was k3=1 (Figure 9). This result indicated that all the added Li2O has been dissolved to release O2- ions, resulting in extra oxide in the melt. The layer of the ThOF2 precipitate at the bottom did not change significantly (Figure 11(e)). Meanwhile, the equilibrium concentration of Th(IV) remains invariable (0.0047 mol/kg) due to the dissolution equilibrium establishment of ThOF2(s). Table 5 summarizes the speciation law and interaction mechanism between Th(IV) and O2- in the FLiNaK with different oxide added.


### 3.4. Difference between FLiBe and FLiNaK Systems: Free Fluoride Ion (F-)

#### 3.4.1. Coordination Environment of the Melts

This study was performed in FLiBe and FLiNaK molten salts, and the experimental results in the two systems showed significant difference, which might be related to the difference of free fluoride ion (F-) content in the two systems. Kergoat et al. [36, 37] put forward the concept of fluoroacidity (pF−) of a molten salt containing fluoride ions, which was defined by the free F− content in a bath as expressed by Equation (17). The pF− value could characterize the acid-base property of fluoride melt [38]. The basic form, F− donors, and the acidic form, F− acceptors, are in equilibrium in molten fluoride salts, as described in Equation (18). In this way, the more a molten salt contains free fluorides (i.e. fluorodonor), the higher its basicity is.
(17)pF−=−logaF−,(18)fluorobase⟷fluoroacid+nF−.Actually, the FLiNaK melt, which is composed of the alkali fluorides LiF, NaF, and KF (Figure13), is a F− “rich” system due to the complete dissociation of LiF, NaF, and KF at the fusion state to release free F-, as shown in Equation (19). Thus, it became a typical strong basic system [39]. However, the FLiBe melt, with a large amount of BeF42− complex formed in the system [40] (Figure 14 and Equation (20)), became a F− “lacking” system with a strong acidity [41].
(19)AF⟶A++F−A=Li,Na,K,(20)BeF2+2F−⟶BeF42−.Figure 13
XRD patterns of the FLiNaK salt.Figure 14
XRD patterns of the FLiBe salt.


#### 3.4.2. Influence of F-

As mentioned in Sections3.2 and 3.3, in the FLiBe system, the dissolution and precipitation equilibrium of ThO2 were reversible reactions with involving the simple Th(IV) and O2- ions (Equations (5) and (7)). However, in the FLiNaK system, the dissolution and precipitation equilibrium of ThO2 was more complex (Equations (12)–(14)): a new insoluble substance ThOF2 appeared, and another soluble oxyfluoride species Th2OF104- became the important intermediate between Th(IV), O2-, and ThOF2 precipitation. It could be found that fluorine ions and oxyfluoride compounds were involved in the overall chemical pathway in the FLiNaK melt, and they played a key role in the process. Due to the lack of F- in the FLiBe system, there was not enough F- to participate in the interaction between Th(IV) and O2-, and no oxyfluoride was produced. On the other hand, FLiNaK was a typical strong basic molten salt and contained abundant free F- ions, and the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2).From the above discussion, it can be concluded that the differences in Th-O interaction in FLiBe and FLiNaK melts come from the difference in free F- content in different molten fluoride salt systems. Thorium-based oxyfluoride compounds will be generated when the molten salt system is rich in free F-, including insoluble ThOF2 and soluble Th2OF104-. Therefore, in order to reduce the risk of Th-O precipitation in the fuel salt of FLiBe-based MSR, the increase of free F- content (for example, increasing the ratio of LiF: BeF2) would be a feasible strategy.


## 3.1. Oxide Analysis of the Molten FLiBe and FLiNaK Salts

Before the dissolution and oxide titration experiments, the original oxide content of the FLiBe and FLiNaK salts was firstly determined, and the results are shown in Table2. Apparently, the total oxide content consisted of two parts, i.e., the oxide in oxygenated anions (including NO3−, SO42−, and PO43−) and the free oxide ions (O2-). The purpose of the oxide analysis was to obtain the practical content of the free oxide ions (O2-) in molten fluorides, which is the basis of the study of the interactions between Th(IV) and O2- in fluoride media.Table 2
Oxide analysis results of the FLiBe and FLiNaK melts used in the present study.
Salt typeTotal oxide concentration ([O]LECO)/ppmOxygenated anion concentration/ppmOxide in oxygenated anions ([O]IC)/ppmO2− concentration ([O2−])/ppmFLiBe184NO3−2734150 (9.375×10−3mol/kg)SO42−12PO43−8FLiNaK150NO3−2936114 (7.125×10−3 mol/kg)SO42−14PO43−6The total oxide concentrations ([O]LECO) of the FLiBe and FLiNaK salts used in this study were determined by the LECO instrument and were 184 ppm and 150 ppm, respectively. As shown in Table 2, the oxygenated anions in FLiBe and FLiNaK salts were mainly NO3−, SO42−, and PO43−, as determined by the IC technique. According to the mass percentage of oxygen in these three oxygenated anions, the sum of the oxide content in oxygenated anions ([O]IC) was calculated to be 34 ppm for FLiBe and 36 ppm for FLiNaK, respectively. Thus, the practical content of free oxide ions O2- ([O2-]) was obtained by subtracting the [O]IC from the [O]LECO (Equation (4)), which were 150 ppm in FLiBe and 114 ppm in FLiNaK, respectively.
(4)O2−=OLECO−OIC.


## 3.2. Chemical Interactions between Th(IV) and O2- in the FLiBe Melt

### 3.2.1. Saturated Dissolution Experiment: FLiBe-ThO2 System

(1) Solubility of ThO2. The ThO2 pellet was directly immersed into the FLiBe melt to determine the solubility of ThO2 in this melt. The melt was sampled at different dissolution times, and the thorium content in each sample was analyzed. Since the increase of thorium concentration came from the dissolution of ThO2, the maximum value of thorium concentration could thus reflect the solubility of ThO2, and this moment corresponds to the dissolution equilibrium time.Figure2 shows the evolution of the Th(IV) concentration in the FLiBe melt with ThO2 immersed. Apparently, no thorium was present in the initial FLiBe salt. Then, the Th(IV) concentration in the melt remarkably increased as dissolution proceeds. As the dissolution time reached 6 h, the Th(IV) concentration achieved the maximum value of 0.057 mol/kg and maintained stable, which corresponded to the solubility of ThO2 in the FLiBe melt. The dissolution equilibrium time corresponded to 6 h.Figure 2
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).The variation in oxide content of the samples taken from the FLiBe-ThO2 melt was also determined, as shown in Figure 2. Apparently, the oxide content of the system also increased with dissolution time and finally reached its maximum value of 0.124 mol/kg also at 6 h. It should be noted that the actual O2- caused by the dissolution of ThO2 should be 0.115 mol/kg, which is obtained by subtracting the initial O2- content of the FLiBe (9.375×10−3 mol/kg, Table 2) from the value of 0.124 mol/kg. Figure 3 shows the concentration ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiBe salt) during the entire dissolution process of ThO2, which was maintained at around 0.5. These results confirmed that the dissolution of ThO2 will simultaneously release O2- and Th(IV) into the melt with the stoichiometric ratio of Th(IV): O2- =1: 2. Therefore, the dissolution mechanism of ThO2 in the FLiBe melt can be described by Equation (5). The specific dissolution mechanism will be discussed in detail in Section 3.4.
(5)ThO2s⟶ThIV+2O2−.Figure 3
Invariable ratio of [Th(IV)]/[O2-] = 0.5 during the dissolution of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).(2)Ksp′ of ThO2. When the dissolution equilibrium of ThO2 was achieved, the species of ThO2 compound, Th(IV), and O2- ions coexisted and equilibrated (Equation (5)). Thus, the apparent solubility product (Ksp′) of ThO2 could be obtained by multiplying the equilibrium concentration of Th(IV) and O2-, as shown in Equation (6). According to Figure 2, the equilibrium concentration of Th(IV) and O2- were experimentally determined as 0.057 mol/kg and 0.124 mol/kg, respectively. Thus, a set of Ksp′ values could be calculated after an equilibrium time of 6 h, and the results are shown in Table 3. Apparently, the obtained Ksp′ of ThO2 showed good agreement and were located in the same order of 10-4. The slight fluctuation within the range of 8.69−8.91×10−4 may be due to the testing errors caused by ICP-OES, LECO, and IC techniques. Thus, the mean value of 8.8×10−4 with an uncertainty of ±0.1×10−4 was reported as the final result.
(6)Ksp′=ThIVO2−2.Table 3
TheKsp′ of ThO2 in the FLiBe melt at 873 K obtained by dissolution method.
Dissolution time (h)[Th(IV)]eq (mol/kg)[O2-]eq (mol/kg)Ksp′(ThO2) (mol3/kg3)AverageKsp′(ThO2) (mol3/kg3)65.70×10−20.1248.81×10−48.8±0.1×10−475.73×10−20.1248.81×10−485.64×10−20.1248.70×10−4105.75×10−20.1258.91×10−4155.67×10−20.1248.69×10−4225.72×10−20.1248.75×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported averageKsp′(ThO2) is ±0.1×10−4 mol3/kg3.


### 3.2.2. Oxide Titration Experiment: FLiBe-ThF4-Li2O System

(1) Precipitation Behavior of ThO2. The interactions between oxide and thorium were investigated by artificially introducing the oxide ions O2- (in the form of Li2O) to the Th(IV)-containing FLiBe melt. When the system reached equilibrium, the supernatant of the molten salt was sampled, and variation of the equilibrium concentration of Th(IV) in the melt with oxide addition were then clarified. Table 4 shows the residual Th(IV) concentration in the FLiBe-ThF4 (0.143 mol/kg) melt after O2- additions from 0.112 to 0.286 mol/kg. Obviously, the concentration of Th(IV) in the melt gradually decreased with oxide addition, indicating the precipitation of Th(IV).Table 4
Variation of the equilibrium concentration of Th(IV) and O2- in the FLiBe-ThF4 melt with different oxide additions at 873 K.
Added [O2-] ([O2-]added) (Mol/kg)Equilibrium [O2-] ([O2-]eq) (Mol/kg)Δ[O2-] ([O2-]added-[O2-]eq) (mol/kg)Equilibrium [Th(IV)] ([Th(IV)]eq) (mol/kg)K’sp (ThO2) ([Th(IV)]eq[O2-]eq2) (mol3/kg3)StatisticalK’sp (ThO2) (mol3/kg3)0000.140—8.6±0.7×10−40.1128.54×10−22.62×10−20.1269.19×10−40.1559.22×10−26.25×10−20.1089.15×10−40.2099.84×10−20.1118.28×10−28.03×10−40.2550.1060.1496.84×10−27.61×10−40.2860.1170.1695.64×10−27.67×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported statistical result ofKsp′(ThO2) is ±0.7×10−4 mol3/kg3.To further confirm the precipitated species, the equilibrium concentration of O2- after each oxide addition was also analyzed, and this led to the calculation of the consumption of O2- (Δ[O2-]) during the precipitation reaction, which could be obtained by the difference between the added oxide and equilibrium oxide, i.e., ΔO2−=O2−added−O2−eq. Then, a linear relationship between the [Th(IV)]eq and Δ[O2-] was evidenced by Figure 4. The slope of this straight line was 0.495, which approximately corresponded to 0.5, indicating the formation of ThO2 or ThxO2xFy compound. Moreover, the FLiBe-ThF4 melt presented as colorless before oxide addition (Figure 5(a)), and a white precipitate layer was found at the bottom of the melting bath (ρ≈2 g/cm3) during the oxide titration experiment, as shown in Figure 5(b). This layer corresponds to ThO2 precipitate (ρ≈9.86 g/cm3) as further proved through XRD analysis (Figure 5(c)). Therefore, the interactions between Th(IV) and O2- in the FLiBe melt would produce ThO2 through the following reaction (Equation (7)).
(7)ThF4+2O2−⟶ThO2s+4F−.Figure 4
Linear relationship between the equilibrium [Th(IV)] and theΔ[O2-] in the FLiBe-ThF4 melt with oxide additions at 873 K.Figure 5
Photography of the FLiBe-ThF4 (0.143 mol/kg) melt before (a) and after (b) O2- addition, observed at high temperature: Upper transparent liquid corresponding to FLiBe-ThF4 melts and the bottom white layer corresponding to solid precipitate. (c) XRD patterns of the bottom white precipitate after distillation treatment to remove the residual molten salt substrate.
(a)(b)(c)(2)Ksp′ of ThO2. After oxide addition, ThO2 precipitate was formed, and its precipitation-dissolution equilibrium would be established in the system. Thus, a set of Ksp′(ThO2) data can be calculated in accordance with Equation (6), and the results are shown in Table 4. The obtained Ksp′(ThO2) exhibited a good consistency and located within 7.61−9.19×10−4 mol3/kg3 except for some slight fluctuations, which could be due to the testing errors of the used analytical methods. A linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 was plotted in Figure 6, and the slope of this straight line corresponded to the statistical result of Ksp′(ThO2) because Eq. (6) can also be written as
(8)ThIV=Ksp′·1O2−2.Figure 6
Linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 in the FLiBe-ThF4 melt with oxide additions at 873 K.Thus, the resultingKsp′(ThO2) was finally determined to be 8.6±0.7×10−4 mol3/kg3. As an equilibrium constant, the value of solubility product theoretically only depends on temperature. The present result by oxide titration method was highly consistent with that obtained by the dissolution method in the FLiBe-ThO2 system at 873 K (8.8±0.1×10−4 mol3/kg3), as stated in Section (2) Ksp′ of ThO2 of Section 3.2.1.


## 3.2.1. Saturated Dissolution Experiment: FLiBe-ThO2 System

(1) Solubility of ThO2. The ThO2 pellet was directly immersed into the FLiBe melt to determine the solubility of ThO2 in this melt. The melt was sampled at different dissolution times, and the thorium content in each sample was analyzed. Since the increase of thorium concentration came from the dissolution of ThO2, the maximum value of thorium concentration could thus reflect the solubility of ThO2, and this moment corresponds to the dissolution equilibrium time.Figure2 shows the evolution of the Th(IV) concentration in the FLiBe melt with ThO2 immersed. Apparently, no thorium was present in the initial FLiBe salt. Then, the Th(IV) concentration in the melt remarkably increased as dissolution proceeds. As the dissolution time reached 6 h, the Th(IV) concentration achieved the maximum value of 0.057 mol/kg and maintained stable, which corresponded to the solubility of ThO2 in the FLiBe melt. The dissolution equilibrium time corresponded to 6 h.Figure 2
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).The variation in oxide content of the samples taken from the FLiBe-ThO2 melt was also determined, as shown in Figure 2. Apparently, the oxide content of the system also increased with dissolution time and finally reached its maximum value of 0.124 mol/kg also at 6 h. It should be noted that the actual O2- caused by the dissolution of ThO2 should be 0.115 mol/kg, which is obtained by subtracting the initial O2- content of the FLiBe (9.375×10−3 mol/kg, Table 2) from the value of 0.124 mol/kg. Figure 3 shows the concentration ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiBe salt) during the entire dissolution process of ThO2, which was maintained at around 0.5. These results confirmed that the dissolution of ThO2 will simultaneously release O2- and Th(IV) into the melt with the stoichiometric ratio of Th(IV): O2- =1: 2. Therefore, the dissolution mechanism of ThO2 in the FLiBe melt can be described by Equation (5). The specific dissolution mechanism will be discussed in detail in Section 3.4.
(5)ThO2s⟶ThIV+2O2−.Figure 3
Invariable ratio of [Th(IV)]/[O2-] = 0.5 during the dissolution of ThO2 in the FLiBe melt at 873 K (the initial O2- concentration in the FLiBe salt has been deducted).(2)Ksp′ of ThO2. When the dissolution equilibrium of ThO2 was achieved, the species of ThO2 compound, Th(IV), and O2- ions coexisted and equilibrated (Equation (5)). Thus, the apparent solubility product (Ksp′) of ThO2 could be obtained by multiplying the equilibrium concentration of Th(IV) and O2-, as shown in Equation (6). According to Figure 2, the equilibrium concentration of Th(IV) and O2- were experimentally determined as 0.057 mol/kg and 0.124 mol/kg, respectively. Thus, a set of Ksp′ values could be calculated after an equilibrium time of 6 h, and the results are shown in Table 3. Apparently, the obtained Ksp′ of ThO2 showed good agreement and were located in the same order of 10-4. The slight fluctuation within the range of 8.69−8.91×10−4 may be due to the testing errors caused by ICP-OES, LECO, and IC techniques. Thus, the mean value of 8.8×10−4 with an uncertainty of ±0.1×10−4 was reported as the final result.
(6)Ksp′=ThIVO2−2.Table 3
TheKsp′ of ThO2 in the FLiBe melt at 873 K obtained by dissolution method.
Dissolution time (h)[Th(IV)]eq (mol/kg)[O2-]eq (mol/kg)Ksp′(ThO2) (mol3/kg3)AverageKsp′(ThO2) (mol3/kg3)65.70×10−20.1248.81×10−48.8±0.1×10−475.73×10−20.1248.81×10−485.64×10−20.1248.70×10−4105.75×10−20.1258.91×10−4155.67×10−20.1248.69×10−4225.72×10−20.1248.75×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported averageKsp′(ThO2) is ±0.1×10−4 mol3/kg3.


## 3.2.2. Oxide Titration Experiment: FLiBe-ThF4-Li2O System

(1) Precipitation Behavior of ThO2. The interactions between oxide and thorium were investigated by artificially introducing the oxide ions O2- (in the form of Li2O) to the Th(IV)-containing FLiBe melt. When the system reached equilibrium, the supernatant of the molten salt was sampled, and variation of the equilibrium concentration of Th(IV) in the melt with oxide addition were then clarified. Table 4 shows the residual Th(IV) concentration in the FLiBe-ThF4 (0.143 mol/kg) melt after O2- additions from 0.112 to 0.286 mol/kg. Obviously, the concentration of Th(IV) in the melt gradually decreased with oxide addition, indicating the precipitation of Th(IV).Table 4
Variation of the equilibrium concentration of Th(IV) and O2- in the FLiBe-ThF4 melt with different oxide additions at 873 K.
Added [O2-] ([O2-]added) (Mol/kg)Equilibrium [O2-] ([O2-]eq) (Mol/kg)Δ[O2-] ([O2-]added-[O2-]eq) (mol/kg)Equilibrium [Th(IV)] ([Th(IV)]eq) (mol/kg)K’sp (ThO2) ([Th(IV)]eq[O2-]eq2) (mol3/kg3)StatisticalK’sp (ThO2) (mol3/kg3)0000.140—8.6±0.7×10−40.1128.54×10−22.62×10−20.1269.19×10−40.1559.22×10−26.25×10−20.1089.15×10−40.2099.84×10−20.1118.28×10−28.03×10−40.2550.1060.1496.84×10−27.61×10−40.2860.1170.1695.64×10−27.67×10−4Note: The expanded uncertainties (0.95 level of confidence) of the reported statistical result ofKsp′(ThO2) is ±0.7×10−4 mol3/kg3.To further confirm the precipitated species, the equilibrium concentration of O2- after each oxide addition was also analyzed, and this led to the calculation of the consumption of O2- (Δ[O2-]) during the precipitation reaction, which could be obtained by the difference between the added oxide and equilibrium oxide, i.e., ΔO2−=O2−added−O2−eq. Then, a linear relationship between the [Th(IV)]eq and Δ[O2-] was evidenced by Figure 4. The slope of this straight line was 0.495, which approximately corresponded to 0.5, indicating the formation of ThO2 or ThxO2xFy compound. Moreover, the FLiBe-ThF4 melt presented as colorless before oxide addition (Figure 5(a)), and a white precipitate layer was found at the bottom of the melting bath (ρ≈2 g/cm3) during the oxide titration experiment, as shown in Figure 5(b). This layer corresponds to ThO2 precipitate (ρ≈9.86 g/cm3) as further proved through XRD analysis (Figure 5(c)). Therefore, the interactions between Th(IV) and O2- in the FLiBe melt would produce ThO2 through the following reaction (Equation (7)).
(7)ThF4+2O2−⟶ThO2s+4F−.Figure 4
Linear relationship between the equilibrium [Th(IV)] and theΔ[O2-] in the FLiBe-ThF4 melt with oxide additions at 873 K.Figure 5
Photography of the FLiBe-ThF4 (0.143 mol/kg) melt before (a) and after (b) O2- addition, observed at high temperature: Upper transparent liquid corresponding to FLiBe-ThF4 melts and the bottom white layer corresponding to solid precipitate. (c) XRD patterns of the bottom white precipitate after distillation treatment to remove the residual molten salt substrate.
(a)(b)(c)(2)Ksp′ of ThO2. After oxide addition, ThO2 precipitate was formed, and its precipitation-dissolution equilibrium would be established in the system. Thus, a set of Ksp′(ThO2) data can be calculated in accordance with Equation (6), and the results are shown in Table 4. The obtained Ksp′(ThO2) exhibited a good consistency and located within 7.61−9.19×10−4 mol3/kg3 except for some slight fluctuations, which could be due to the testing errors of the used analytical methods. A linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 was plotted in Figure 6, and the slope of this straight line corresponded to the statistical result of Ksp′(ThO2) because Eq. (6) can also be written as
(8)ThIV=Ksp′·1O2−2.Figure 6
Linear relationship between the equilibrium [Th(IV)] and 1/[O2-]2 in the FLiBe-ThF4 melt with oxide additions at 873 K.Thus, the resultingKsp′(ThO2) was finally determined to be 8.6±0.7×10−4 mol3/kg3. As an equilibrium constant, the value of solubility product theoretically only depends on temperature. The present result by oxide titration method was highly consistent with that obtained by the dissolution method in the FLiBe-ThO2 system at 873 K (8.8±0.1×10−4 mol3/kg3), as stated in Section (2) Ksp′ of ThO2 of Section 3.2.1.


## 3.3. Chemical Interactions between Th(IV) and O2- in the FLiNaK Melt

### 3.3.1. Solubility of ThO2 in the FLiNaK Melt

Figure7 shows the variation of Th(IV) and O2- concentrations during the dissolution of ThO2 in the FLiNaK melt at 873 K. The results showed that the concentrations of Th(IV) and O2- remarkably increased with dissolution time and were finally maintained at the maximum value of 0.048 mol/kg and 0.121 mol/kg (with initialO2−subtractionof7.125×10−3 mol/kg in FLiNaK, Table 2), respectively, and an equilibration time of 8 h was obtained. The dissolution mechanism of ThO2 in FLiNaK was probably due to the following reactions (Equations. (9)–(12)):
(9)ThO2s⟶Th4++2O2−ThIVeqO2−eq=1:2,(10)ThO2s+2F−⟶ThOF2s+O2−ThIVeqO2−eq=0,(11)2ThO2s+10F−⟶Th2OF104−+3O2−ThIVeqO2−eq=1:2,(12)3ThO2s+12F−⟶ThOF2s+Th2OF104−+4O2−ThIVeqO2−eq=2:5.Figure 7
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).FLiNaK is a typical strong basic molten salt and contains abundant free F- ions, which will participate in the dissolution process of ThO2 to form oxyfluoride species, such as ThOF2(s) and Th2OF104-, as suggested by Wang et al. [29]. Therefore, as a simple dissolution process without yielding oxyfluoride species, Equation (9) was obviously not the mechanism. To further confirm the dissolution mechanism, the ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiNaK salt) during ThO2 dissolution was determined, as shown in Figure 8. The ratio of [Th(IV)]/[O2-] maintained at around 0.4, indicating the stoichiometric relation of Th(IV) and O2- ions caused by ThO2 dissolution is 2:5, which is in good agreement with the situation of Equation (12). Therefore, the dissolution pathway of ThO2 in the FLiNaK melt could be best described by Equation (12), with the ratio of 2:5 of the dissolved Th(IV) to O2-. The specific dissolution mechanism will be discussed in detail in Section 3.4.Figure 8
Invariable ratio of [Th(IV)]/[O2-] =0.4 during the dissolution of ThO2 in the FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).


### 3.3.2. Oxide Titration Experiment: FLiNaK-ThF4-Li2O System

The chemical interactions between Th(IV) and O2- in the FLiNaK melt showed significant difference from that in the FLiBe melt. As shown in Figure 9 and Table 5, the evolution of the equilibrium concentration of Th(IV) and O2- in the FLiNaK-ThF4 melt with oxide additions could be regarded as three stages, indicating a more complicated speciation in this system.Figure 9
Evolution of the equilibrium [Th(IV)] and [O2-] in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K.Table 5
Summary of speciation and chemical interaction between Th(IV) and O2- in the FLiNaK molten salt at different r ([O2-]added/[Th(IV)]0) at 873 K.
Reaction/equilibriumSpeciation[O2-]eq/[Th(IV)]eqStage 1r=0—ThF4(ThFx4-x)00<r<0.52ThF4excessive+2F−+O2−⟶Th2OF104−Th2OF104-, ThF4(ThFx4-x)<0.5r=0.52ThF4+2F−+O2−⟶Th2OF104−Th2OF104-0.5Stage 20.5<r<1Th2OF104−excessive+O2−⟶2ThOF2s+6F−Th2OF104-, ThOF2(s)0.5r=1∗Th2OF104−+O2−⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and O2- (1 : 1)1Stage 3r>1Th2OF104−+O2−excessive⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and excessive O2->1(∗denotes the establishment of precipitation-dissolution equilibrium.).Stage 1. 0≤O2−added≤0.0721 mol/kg. 0≤O2−added/ThIV0r≤0.5.With oxide additions less than 0.0721 mol/kg, the equilibrium Th(IV) concentration in the melt remained invariable and maintained at the initial concentration of 0.1433 mol/kg ([Th(IV)]0), while the equilibrium O2- in the melt linearly increased with oxide addition with the slope of 1 (k1=0.99), as shown in Figure 9. These results indicated that all the added O2- exists in a soluble form in the melt without precipitation of Th(IV), probably forming soluble Th(IV) oxyfluoride ions. Therefore, the ratio of [O2-]eq/[Th(IV)]eq at this stage gradually increases with oxide addition, as shown in Figure 10. When the added O2- was increased to be 0.0721 mol/kg, the equilibrium [O2-] reached its maximum value of 0.0715 mol/kg, which was the upper limit of the formed oxyfluoride species dissolved in the melt. At this point, the ratio of [O2-]added/[Th(IV)]0 (defined as r) corresponded to 0.5 (Figure 9), indicating that the stoichiometry of the oxide and thorium in this formed soluble Th(IV) oxyfluoride species was 1 : 2, presumably Th2OF104-. This hypothesis was also comforted by the recent observation by Raman measurements of a stable Th2OF104- cluster structure in fluoride media [29]. Therefore, the ThF4 would combine with the added oxide to form Th2OF104- in this stage, and all the initial ThF4 in the melt were converted to the Th2OF104- at the end of this stage (r=0.5). At the same time, the ratio of [O2-]eq/[Th(IV)]eq increased to 0.5 due to the single species of Th2OF104- existing in the melt (Figure 10). Therefore, the reaction pathway of the process in this stage can be described by Equation (13). The soluble and colorless nature of the Th2OF104- species caused no obvious change in the appearance of the molten salt, as shown in Figures 11(a) and 11(b).
(13)2ThIV+10F−+O2−⟶Th2OF104−.Figure 10
Evolution of the ratio of [O2-]eq/[Th(IV)]eq in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K. (The slopes in stage 1 and stage 3 were obtained due to [O2-]eq = [O2-]added).Figure 11
Photography of the FLiNaK-ThF4 (0.1433 mol/kg) melt (a) with different oxide additions at 873 K: (b) 0.0354 mol/kg (r=0.247); (c) 0.0918 mol/kg (r=0.641); (d) 0.1436 mol/kg (r=1.002); (e) 0.1863 mol/kg (r=1.300).
(a)(b)(c)(d)(e)Stage 2: 0.0721<O2−added≤0.1436 mol/kg. 0.5<O2−added/ThIV0r≤1.When the added O2- was more than 0.0721 mol/kg (r>0.5), both the equilibrium [Th(IV)] and [O2-] in the melt began to decrease. This result suggested that a precipitation reaction occurred between the formed Th2OF104- and added O2-, resulting in a solid compound that simultaneously containing thorium and oxygen elements. With the added O2- further increased to 0.1436 mol/kg (r=1), both the [Th(IV)]eq and [O2-]eq almost decreased linearly and finally decreased to their minimum value of 0.0047 mol/kg and 0.0048 mol/kg, respectively. The slopes of the two fitted straight lines were approximately 2 (k4=1.94) and 1 (k2=0.93), respectively. This result suggested that the Th(IV) consumed by precipitation in molten salt is twice that of oxide, and the stoichiometric ratio of the reactants Th2OF104- and added O2- was 1 : 1. Besides, the r corresponded to 1 at O2−added=0.1436 mol/kg, indicating an equimolar of thorium and oxygen atoms contained in this formed precipitate, presumably ThOF2. During the experiment, a kind of white precipitate was observed at the bottom of the bath (Figure 11(c)), and this precipitate was proved to be ThOF2 by XRD (Figure 12). Related literature also reported that the ThOF2 compound can stably exist in fluoride salt media [30]. Therefore, Reaction (14) was speculated to occur during this stage, with a stable ratio of O2−eq/ThIVeq=0.5 due to the unreacted Th2OF104- dissolving in the melt (Figure 10).
(14)Th2OF104−+O2−⟶2ThOF2s+6F−.Figure 12
XRD patterns of the precipitate formed in stage 2 of the FLiNaK-ThF4 melt with oxide addition.It should be noted that the ratio [O2-]eq/[Th(IV)]eq suddenly increases from 0.5 to 1 at r=1 (Figure 10), which was caused by the dissolution equilibrium of ThOF2. At the point of r=1, almost all Th2OF104- was converted into ThOF2(s), and the ThOF2(s) precipitate at the bottom of the bath reached the maximum amount and formed a white insoluble layer (Figure 11(d)). Then, the precipitation-dissolution equilibrium of the ThOF2(s) was established, as shown in Equation (15). Thus, the residual [Th(IV)]eq and [O2-]eq remaining in the melt came from the dissolution of ThOF2(s), resulting in O2−eq/ThIVeq=1 with equimolar Th2OF104- and O2- dissolved.
(15)2ThOF2s+6F−⟷Th2OF104−+O2−.Based on the above analysis, Th2OF104- was the intermediate species in the overall chemical pathway of precipitation of ThF4 into ThOF2, as shown in Equations (13), (14), and (16). Thus, it could be concluded that the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2) due to the abundant F- in this system.
(16)ThIV+O2−+2F−↔ThOF2s.Stage 3: O2−added>0.1436 mol/kg. O2−added/ThIV0r>1When the added oxide was more than 0.1436 mol/kg (r>1), the O2- ions in the melt become excessive. The equilibrium O2- in the melt increased linearly with oxide addition and the slope was k3=1 (Figure 9). This result indicated that all the added Li2O has been dissolved to release O2- ions, resulting in extra oxide in the melt. The layer of the ThOF2 precipitate at the bottom did not change significantly (Figure 11(e)). Meanwhile, the equilibrium concentration of Th(IV) remains invariable (0.0047 mol/kg) due to the dissolution equilibrium establishment of ThOF2(s). Table 5 summarizes the speciation law and interaction mechanism between Th(IV) and O2- in the FLiNaK with different oxide added.


## 3.3.1. Solubility of ThO2 in the FLiNaK Melt

Figure7 shows the variation of Th(IV) and O2- concentrations during the dissolution of ThO2 in the FLiNaK melt at 873 K. The results showed that the concentrations of Th(IV) and O2- remarkably increased with dissolution time and were finally maintained at the maximum value of 0.048 mol/kg and 0.121 mol/kg (with initialO2−subtractionof7.125×10−3 mol/kg in FLiNaK, Table 2), respectively, and an equilibration time of 8 h was obtained. The dissolution mechanism of ThO2 in FLiNaK was probably due to the following reactions (Equations. (9)–(12)):
(9)ThO2s⟶Th4++2O2−ThIVeqO2−eq=1:2,(10)ThO2s+2F−⟶ThOF2s+O2−ThIVeqO2−eq=0,(11)2ThO2s+10F−⟶Th2OF104−+3O2−ThIVeqO2−eq=1:2,(12)3ThO2s+12F−⟶ThOF2s+Th2OF104−+4O2−ThIVeqO2−eq=2:5.Figure 7
The evolutions of concentrations of Th(IV) and O2- with the dissolution time of ThO2 in FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).FLiNaK is a typical strong basic molten salt and contains abundant free F- ions, which will participate in the dissolution process of ThO2 to form oxyfluoride species, such as ThOF2(s) and Th2OF104-, as suggested by Wang et al. [29]. Therefore, as a simple dissolution process without yielding oxyfluoride species, Equation (9) was obviously not the mechanism. To further confirm the dissolution mechanism, the ratio of [Th(IV)]/[O2-] (after subtracting the initial O2- of the FLiNaK salt) during ThO2 dissolution was determined, as shown in Figure 8. The ratio of [Th(IV)]/[O2-] maintained at around 0.4, indicating the stoichiometric relation of Th(IV) and O2- ions caused by ThO2 dissolution is 2:5, which is in good agreement with the situation of Equation (12). Therefore, the dissolution pathway of ThO2 in the FLiNaK melt could be best described by Equation (12), with the ratio of 2:5 of the dissolved Th(IV) to O2-. The specific dissolution mechanism will be discussed in detail in Section 3.4.Figure 8
Invariable ratio of [Th(IV)]/[O2-] =0.4 during the dissolution of ThO2 in the FLiNaK melt at 873 K. (The initial O2- concentration in the FLiNaK salt has been deducted).


## 3.3.2. Oxide Titration Experiment: FLiNaK-ThF4-Li2O System

The chemical interactions between Th(IV) and O2- in the FLiNaK melt showed significant difference from that in the FLiBe melt. As shown in Figure 9 and Table 5, the evolution of the equilibrium concentration of Th(IV) and O2- in the FLiNaK-ThF4 melt with oxide additions could be regarded as three stages, indicating a more complicated speciation in this system.Figure 9
Evolution of the equilibrium [Th(IV)] and [O2-] in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K.Table 5
Summary of speciation and chemical interaction between Th(IV) and O2- in the FLiNaK molten salt at different r ([O2-]added/[Th(IV)]0) at 873 K.
Reaction/equilibriumSpeciation[O2-]eq/[Th(IV)]eqStage 1r=0—ThF4(ThFx4-x)00<r<0.52ThF4excessive+2F−+O2−⟶Th2OF104−Th2OF104-, ThF4(ThFx4-x)<0.5r=0.52ThF4+2F−+O2−⟶Th2OF104−Th2OF104-0.5Stage 20.5<r<1Th2OF104−excessive+O2−⟶2ThOF2s+6F−Th2OF104-, ThOF2(s)0.5r=1∗Th2OF104−+O2−⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and O2- (1 : 1)1Stage 3r>1Th2OF104−+O2−excessive⟷2ThOF2s+6F−ThOF2(s), with less Th2OF104- and excessive O2->1(∗denotes the establishment of precipitation-dissolution equilibrium.).Stage 1. 0≤O2−added≤0.0721 mol/kg. 0≤O2−added/ThIV0r≤0.5.With oxide additions less than 0.0721 mol/kg, the equilibrium Th(IV) concentration in the melt remained invariable and maintained at the initial concentration of 0.1433 mol/kg ([Th(IV)]0), while the equilibrium O2- in the melt linearly increased with oxide addition with the slope of 1 (k1=0.99), as shown in Figure 9. These results indicated that all the added O2- exists in a soluble form in the melt without precipitation of Th(IV), probably forming soluble Th(IV) oxyfluoride ions. Therefore, the ratio of [O2-]eq/[Th(IV)]eq at this stage gradually increases with oxide addition, as shown in Figure 10. When the added O2- was increased to be 0.0721 mol/kg, the equilibrium [O2-] reached its maximum value of 0.0715 mol/kg, which was the upper limit of the formed oxyfluoride species dissolved in the melt. At this point, the ratio of [O2-]added/[Th(IV)]0 (defined as r) corresponded to 0.5 (Figure 9), indicating that the stoichiometry of the oxide and thorium in this formed soluble Th(IV) oxyfluoride species was 1 : 2, presumably Th2OF104-. This hypothesis was also comforted by the recent observation by Raman measurements of a stable Th2OF104- cluster structure in fluoride media [29]. Therefore, the ThF4 would combine with the added oxide to form Th2OF104- in this stage, and all the initial ThF4 in the melt were converted to the Th2OF104- at the end of this stage (r=0.5). At the same time, the ratio of [O2-]eq/[Th(IV)]eq increased to 0.5 due to the single species of Th2OF104- existing in the melt (Figure 10). Therefore, the reaction pathway of the process in this stage can be described by Equation (13). The soluble and colorless nature of the Th2OF104- species caused no obvious change in the appearance of the molten salt, as shown in Figures 11(a) and 11(b).
(13)2ThIV+10F−+O2−⟶Th2OF104−.Figure 10
Evolution of the ratio of [O2-]eq/[Th(IV)]eq in the FLiNaK-ThF4 (0.1433 mol/kg) melt with oxide additions at 873 K. (The slopes in stage 1 and stage 3 were obtained due to [O2-]eq = [O2-]added).Figure 11
Photography of the FLiNaK-ThF4 (0.1433 mol/kg) melt (a) with different oxide additions at 873 K: (b) 0.0354 mol/kg (r=0.247); (c) 0.0918 mol/kg (r=0.641); (d) 0.1436 mol/kg (r=1.002); (e) 0.1863 mol/kg (r=1.300).
(a)(b)(c)(d)(e)Stage 2: 0.0721<O2−added≤0.1436 mol/kg. 0.5<O2−added/ThIV0r≤1.When the added O2- was more than 0.0721 mol/kg (r>0.5), both the equilibrium [Th(IV)] and [O2-] in the melt began to decrease. This result suggested that a precipitation reaction occurred between the formed Th2OF104- and added O2-, resulting in a solid compound that simultaneously containing thorium and oxygen elements. With the added O2- further increased to 0.1436 mol/kg (r=1), both the [Th(IV)]eq and [O2-]eq almost decreased linearly and finally decreased to their minimum value of 0.0047 mol/kg and 0.0048 mol/kg, respectively. The slopes of the two fitted straight lines were approximately 2 (k4=1.94) and 1 (k2=0.93), respectively. This result suggested that the Th(IV) consumed by precipitation in molten salt is twice that of oxide, and the stoichiometric ratio of the reactants Th2OF104- and added O2- was 1 : 1. Besides, the r corresponded to 1 at O2−added=0.1436 mol/kg, indicating an equimolar of thorium and oxygen atoms contained in this formed precipitate, presumably ThOF2. During the experiment, a kind of white precipitate was observed at the bottom of the bath (Figure 11(c)), and this precipitate was proved to be ThOF2 by XRD (Figure 12). Related literature also reported that the ThOF2 compound can stably exist in fluoride salt media [30]. Therefore, Reaction (14) was speculated to occur during this stage, with a stable ratio of O2−eq/ThIVeq=0.5 due to the unreacted Th2OF104- dissolving in the melt (Figure 10).
(14)Th2OF104−+O2−⟶2ThOF2s+6F−.Figure 12
XRD patterns of the precipitate formed in stage 2 of the FLiNaK-ThF4 melt with oxide addition.It should be noted that the ratio [O2-]eq/[Th(IV)]eq suddenly increases from 0.5 to 1 at r=1 (Figure 10), which was caused by the dissolution equilibrium of ThOF2. At the point of r=1, almost all Th2OF104- was converted into ThOF2(s), and the ThOF2(s) precipitate at the bottom of the bath reached the maximum amount and formed a white insoluble layer (Figure 11(d)). Then, the precipitation-dissolution equilibrium of the ThOF2(s) was established, as shown in Equation (15). Thus, the residual [Th(IV)]eq and [O2-]eq remaining in the melt came from the dissolution of ThOF2(s), resulting in O2−eq/ThIVeq=1 with equimolar Th2OF104- and O2- dissolved.
(15)2ThOF2s+6F−⟷Th2OF104−+O2−.Based on the above analysis, Th2OF104- was the intermediate species in the overall chemical pathway of precipitation of ThF4 into ThOF2, as shown in Equations (13), (14), and (16). Thus, it could be concluded that the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2) due to the abundant F- in this system.
(16)ThIV+O2−+2F−↔ThOF2s.Stage 3: O2−added>0.1436 mol/kg. O2−added/ThIV0r>1When the added oxide was more than 0.1436 mol/kg (r>1), the O2- ions in the melt become excessive. The equilibrium O2- in the melt increased linearly with oxide addition and the slope was k3=1 (Figure 9). This result indicated that all the added Li2O has been dissolved to release O2- ions, resulting in extra oxide in the melt. The layer of the ThOF2 precipitate at the bottom did not change significantly (Figure 11(e)). Meanwhile, the equilibrium concentration of Th(IV) remains invariable (0.0047 mol/kg) due to the dissolution equilibrium establishment of ThOF2(s). Table 5 summarizes the speciation law and interaction mechanism between Th(IV) and O2- in the FLiNaK with different oxide added.


## 3.4. Difference between FLiBe and FLiNaK Systems: Free Fluoride Ion (F-)

### 3.4.1. Coordination Environment of the Melts

This study was performed in FLiBe and FLiNaK molten salts, and the experimental results in the two systems showed significant difference, which might be related to the difference of free fluoride ion (F-) content in the two systems. Kergoat et al. [36, 37] put forward the concept of fluoroacidity (pF−) of a molten salt containing fluoride ions, which was defined by the free F− content in a bath as expressed by Equation (17). The pF− value could characterize the acid-base property of fluoride melt [38]. The basic form, F− donors, and the acidic form, F− acceptors, are in equilibrium in molten fluoride salts, as described in Equation (18). In this way, the more a molten salt contains free fluorides (i.e. fluorodonor), the higher its basicity is.
(17)pF−=−logaF−,(18)fluorobase⟷fluoroacid+nF−.Actually, the FLiNaK melt, which is composed of the alkali fluorides LiF, NaF, and KF (Figure13), is a F− “rich” system due to the complete dissociation of LiF, NaF, and KF at the fusion state to release free F-, as shown in Equation (19). Thus, it became a typical strong basic system [39]. However, the FLiBe melt, with a large amount of BeF42− complex formed in the system [40] (Figure 14 and Equation (20)), became a F− “lacking” system with a strong acidity [41].
(19)AF⟶A++F−A=Li,Na,K,(20)BeF2+2F−⟶BeF42−.Figure 13
XRD patterns of the FLiNaK salt.Figure 14
XRD patterns of the FLiBe salt.


### 3.4.2. Influence of F-

As mentioned in Sections3.2 and 3.3, in the FLiBe system, the dissolution and precipitation equilibrium of ThO2 were reversible reactions with involving the simple Th(IV) and O2- ions (Equations (5) and (7)). However, in the FLiNaK system, the dissolution and precipitation equilibrium of ThO2 was more complex (Equations (12)–(14)): a new insoluble substance ThOF2 appeared, and another soluble oxyfluoride species Th2OF104- became the important intermediate between Th(IV), O2-, and ThOF2 precipitation. It could be found that fluorine ions and oxyfluoride compounds were involved in the overall chemical pathway in the FLiNaK melt, and they played a key role in the process. Due to the lack of F- in the FLiBe system, there was not enough F- to participate in the interaction between Th(IV) and O2-, and no oxyfluoride was produced. On the other hand, FLiNaK was a typical strong basic molten salt and contained abundant free F- ions, and the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2).From the above discussion, it can be concluded that the differences in Th-O interaction in FLiBe and FLiNaK melts come from the difference in free F- content in different molten fluoride salt systems. Thorium-based oxyfluoride compounds will be generated when the molten salt system is rich in free F-, including insoluble ThOF2 and soluble Th2OF104-. Therefore, in order to reduce the risk of Th-O precipitation in the fuel salt of FLiBe-based MSR, the increase of free F- content (for example, increasing the ratio of LiF: BeF2) would be a feasible strategy.


## 3.4.1. Coordination Environment of the Melts

This study was performed in FLiBe and FLiNaK molten salts, and the experimental results in the two systems showed significant difference, which might be related to the difference of free fluoride ion (F-) content in the two systems. Kergoat et al. [36, 37] put forward the concept of fluoroacidity (pF−) of a molten salt containing fluoride ions, which was defined by the free F− content in a bath as expressed by Equation (17). The pF− value could characterize the acid-base property of fluoride melt [38]. The basic form, F− donors, and the acidic form, F− acceptors, are in equilibrium in molten fluoride salts, as described in Equation (18). In this way, the more a molten salt contains free fluorides (i.e. fluorodonor), the higher its basicity is.
(17)pF−=−logaF−,(18)fluorobase⟷fluoroacid+nF−.Actually, the FLiNaK melt, which is composed of the alkali fluorides LiF, NaF, and KF (Figure13), is a F− “rich” system due to the complete dissociation of LiF, NaF, and KF at the fusion state to release free F-, as shown in Equation (19). Thus, it became a typical strong basic system [39]. However, the FLiBe melt, with a large amount of BeF42− complex formed in the system [40] (Figure 14 and Equation (20)), became a F− “lacking” system with a strong acidity [41].
(19)AF⟶A++F−A=Li,Na,K,(20)BeF2+2F−⟶BeF42−.Figure 13
XRD patterns of the FLiNaK salt.Figure 14
XRD patterns of the FLiBe salt.


## 3.4.2. Influence of F-

As mentioned in Sections3.2 and 3.3, in the FLiBe system, the dissolution and precipitation equilibrium of ThO2 were reversible reactions with involving the simple Th(IV) and O2- ions (Equations (5) and (7)). However, in the FLiNaK system, the dissolution and precipitation equilibrium of ThO2 was more complex (Equations (12)–(14)): a new insoluble substance ThOF2 appeared, and another soluble oxyfluoride species Th2OF104- became the important intermediate between Th(IV), O2-, and ThOF2 precipitation. It could be found that fluorine ions and oxyfluoride compounds were involved in the overall chemical pathway in the FLiNaK melt, and they played a key role in the process. Due to the lack of F- in the FLiBe system, there was not enough F- to participate in the interaction between Th(IV) and O2-, and no oxyfluoride was produced. On the other hand, FLiNaK was a typical strong basic molten salt and contained abundant free F- ions, and the interactions between Th(IV) and O2- in FLiNaK would produce Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2).From the above discussion, it can be concluded that the differences in Th-O interaction in FLiBe and FLiNaK melts come from the difference in free F- content in different molten fluoride salt systems. Thorium-based oxyfluoride compounds will be generated when the molten salt system is rich in free F-, including insoluble ThOF2 and soluble Th2OF104-. Therefore, in order to reduce the risk of Th-O precipitation in the fuel salt of FLiBe-based MSR, the increase of free F- content (for example, increasing the ratio of LiF: BeF2) would be a feasible strategy.


## 4. Conclusions

The present work investigated the chemical interactions between thorium(IV) and O2- in different molten fluoride salt systems (i.e., FLiBe and FLiNaK) by a combination of various analytical techniques, including LECO, IC, ICP-OES, and XRD.Firstly, the oxide analysis method for molten fluoride salts was successfully established, and thus the practical free O2- concentration can be determined by subtracting the oxide in oxygenated anions from the total oxide content. In this manner, the original O2- content of the used FLiBe and FLiNaK salts was calculated. The purpose of oxide analysis is to obtain the actual content of free oxide ions.Then, the precipitation-dissolution behavior of Th(IV) oxide/oxyfluoride species, as well as the speciation law of these species and their relationship with oxide concentration in fluoride melts, were further studied the following:(i)
In FLiBe. The results showed that the situation in the FLiBe was relatively simple. The interactions between Th(IV) and O2- in the FLiBe melt would only produce ThO2 precipitate, and conversely, the dissolution products of ThO2 in FLiBe were the Th(IV) and O2- ions. Then, the key dissolution parameters of ThO2 in the FLiBe melt, including saturated solubility and apparent solubility product (Ksp′), were obtained. The Ksp′ was determined by the dissolution method and the oxide titration method, and the results showed a high degree of consistency(ii)
In FLiNaK. However, in the FLiNaK melt, the situation was totally different. The interactions between Th(IV) and O2-, as well as the dissolution mechanism of ThO2 in FLiNaK, were more complicated than those in FLiBe, involving Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2). The speciation process in the FLiNaK-ThF4 system with oxide additions was clarified, and the evolution law of these Th(IV)-containing species with added O2- amount was also revealed. The results showed that Th2OF104- was first formed in the system, and then the Th2OF104- was converted to the solid compound ThOF2 with further oxide additions. That means the soluble species Th2OF104- was the important intermediate between Th(IV), O2- and ThOF2 precipitationThe significant difference between these results between FLiBe and FLiNaK was attributed to the different content of free F- in these two systems. Due to the BeF42− complex in the FLiBe system, there is not enough F- to participate in the interaction between Th(IV) and O2-, and no oxyfluoride was produced. Actually, ThO2 was the only product in this system. On the other hand, FLiNaK is a typical strong basic molten salt and contains abundant free F- ions, which could participate in the interactions between Th(IV) and O2- in FLiNaK and would produce thorium oxyfluoride species (i.e., the soluble Th2OF104- and insoluble ThOF2). This indicates that the free F- can increase the solubility of Th-O species to a certain extent. Therefore, in order to reduce the risk of thorium oxide precipitation in the fuel salt of FLiBe-based MSR, the increase of free F- content (for example, increasing the ratio of LiF: BeF2) would be a feasible strategy.This work provided important reference and fundamental data for understanding the nature of thorium fuel precipitation and how to control it in the molten fluoride media, which is of significant importance for the safe operation of a molten salt reactor (MSR).



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*Source: 4447405-2023-03-11.xml*