Chemical and electrochemical studies on the solubility of UO2 in molten FLINAK with ZrF4 additive

The solubility of UO2 in molten FLINAK with ZrF4 additive was studied by using chemical and electrochemical methods. The chemical results showed that the solubility of UO2 was lower by 0.247 wt% in molten FLINAK. When more than 2.91 wt% of ZrF4 was added into the FLINAK-UO2 melts, the maximum solubility of UO2 at 1.422 wt% can be achieved. Moreover, compared with the FLINAK-UO2 melts, a new cathodic peak at approximately 0.21 V (vs. alkalis+/alkalis) was detected in FLINAK-ZrF4-UO2 melts by using cyclic voltammetry (CV) and square wave voltammetry (SWV). This cathodic peak was attributed to the higher electroactive species ZrOF2 confirmed by CV and potentiostatic electrolysis. Thus, it can be concluded that the dissolution of UO2 is caused by the reaction of UO2 + ZrF4 = UOF2 + ZrOF2. The SWVs of FLINAK-ZrF4-UO2 system further showed that the concentration of ZrOF2 species in the bath gradually increased with the addition of ZrF4, which further proves that the dissolution of UO2 is facilitated by ZrF4 additive.