Rare Earth Silicide Formation on Si Electrode in LiCl-KCl Melt to Establish a Novel Used Salt Treatment Process
Authors: Murakami Tsuyoshi , Sakamura Yoshiharu , Uozumi Koichi , Iizuka Masatoshi 
Published: December 2020

Pyroprocessing of spent nuclear fuels which uses chemical and electrochemical reactions in molten salts has been developed as one of the promising technologies for advanced nuclear fuel cycle. A main step of the pyroprocessing is electrorefining process where a recovery of actinides from spent nuclear fuels is accomplished in an electrolyte, LiCl-KCl melt. After the electrorefining process, fission products (rare earths (REs), alkalis and alkaline earths) chlorides accumulate in the melt, the used LiCl-KCl melt.
The accumulated fission products must be removed from the used melt to be stabilized in a waste form suitable for a geological disposal. For this purpose, a zeolite is utilized in the conventional used salt treatment process: The zeolite is contacted with the used melt to absorb and occlude the fission products in its structure and then the zeolite is converted to a stable chlorine-containing waste form, a glass-bonded sodalite. However, the achievable concentration of fission products in the glass-bonded sodalite is limited to lower values (1.6 ~ 4.3 wt%) compared with that in the conventional oxide-base glass waste form (6.4 ~ 15.2 wt%).
In order to pursue a reduction of an environmental burden, this paper proposes a novel used salt treatment process utilizing the glass to decrease the volume of the waste form. REs, dominant among fission products, are electrochemically recovered from the used melt in a form of their silicides (REn+ + xSi + ne- → RESix). Then, the silicides are oxidized to be dissolved in the glass matrix. Si is selected as the alloying material, meaning that REs are not required to be separated from the Si before stabilizing in the glass waste form which is composed mainly of Si. After the electrochemical recovery of REs, the remaining fission products, alkalis and alkaline earths, are removed from the used melt in a similar manner to the conventional one; absorption and occlusion in the zeolite to be converted to the glass-bonded sodalite.
To investigate a basic behavior of a series of an individual RE silicide (La-Si, Ce-Si, Pr-Si, Nd-Si, Sm-Si, Eu-Si and Gd-Si) formation, cyclic voltammetry and potentiostatic electrolysis were performed using a Si wafer (n-type, <110>, 0.02 Ωcm) as the working electrode in LiCl-KCl melt containing the corresponding RE chloride at 723 K. It was confirmed that all of the above REs were recovered from the melt as their silicides. Silicides with various morphologies were found to be formed on the Si electrode; a pillar shaped silicide growing perpendicularly to the Si substrate for Sm, a dense and thin silicide layer covering the Si substrate for Eu and a porous silicide layer between a dense silicide layer and the remaining Si substrate for the rest of the REs. The obtained results confirmed a feasibility of the key step composing of the proposed used salt treatment process; REs recovery as silicides from the used LiCl-KCl melt. In addition, a preliminary test of the proposed used salt treatment process, Ce recovery as silicide from LiCl-KCl-CeCl3 melt and the oxidation of the Ce silicide to be dissolved in a glass matrix, was demonstrated and an optimization of the processing condition is under way.
Acknowledgement Parts of the presented results were obtained in the project “Development of highly flexible technology for recovery and transmutation of minor actinide” entrusted to CRIEPI by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).