Abstract
Thermodynamic and electrochemical properties of actinides in molten fluoride salts are one of the key data required for design and safety assessment of any molten salt reactor concept using liquid fluoride based fuel. In the case of the Molten Salt Fast Reactor (MSFR), special attention has to be paid to thorium fluoride, which is a direct fuel carrier salt constituent. This work presents experimental measurement of ThF4 activity coefficient in eutectic LiF-CaF2 melt by electrochemical techniques. The proposed approach is overcoming problems related to the lack of a thermodynamic reference electrode for high-temperature molten fluoride media. The method can be used to compare stability of fissile material and fission products in a given molten fluoride solvent by estimating their activity coefficients using the described electrochemical measurements. In addition, electrochemical methods were successfully used to evaluate and confirm purity of the used materials, with a special regard to oxygen content in the ThF4 input material synthesised during the previous work from ThO2.
Highlights
A worldwide increased interest in the molten salt reactor (MSR) technology has resulted in the inclusion of this concept amongst the six innovative reactors studied by the Generation IV initiative since 2001 [1]
The melt was characterised by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements using tungsten and gold wires as working electrodes
Since the ThF4 content in the LiF-ThF4 melt was high (78 wt.%), the results indicated that the synthesized ThF4 does not contain any important amount of oxygen
Summary
A worldwide increased interest in the molten salt reactor (MSR) technology has resulted in the inclusion of this concept amongst the six innovative reactors studied by the Generation IV initiative since 2001 [1]. In the MSFR concept, the fissile material is dissolved in a molten fluoride salt media, which circulates in the core and serves both as fuel and primary circuit coolant. The fuel salt is composed of an eutectic carrier melt LiF-ThF4 (78.0–22.0 mol.%), in which the fissile material is dissolved in the form of fluorides. The electrochemical methods were used for confirmation of the purity and acceptable oxygen content in the synthesised ThF4 material, based on the review of approaches to determining oxide concentration in molten fluoride salts published by Goh at al. Several electrochemical parameters were measured for each thorium content and the results were compared with published data [19,20,21] In these experiments, the presence of the dissolved oxygen was analysed and observed before the carrier melt purification. A LiF-ThF4 eutectic salt (78.0–22.0 mol.%) was prepared and characterised by CV and LSV techniques using a gold working electrode to detect the possible oxygen content in ThF4
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