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.