Reflux Dissolution of Thorium Dioxide in Nitric Acid

This study aims to investigate the reflux dissolution process of thorium dioxide in nitric acid, seeking an alternative to the current fluoride ion-dependent dissolution methods to prevent corrosion of stainless steel dissolvers and subsequent processing issues. By employing a reflux dissolution apparatus and utilizing nitric acid as the solvent, the effects of initial nitric acid concentration, temperature, and initial thorium ion concentration on the dissolution efficiency of thorium dioxide were systematically studied. The experimental results reveal that the apparent activation energy for the dissolution process of thorium dioxide in nitric acid is 41.47 kJ/mol. An increase in initial nitric acid concentration from 9 mol/L to 12 mol/L corresponded with an increased dissolution rate of thorium dioxide, exhibiting an apparent order of about 3. However, a decline in the dissolution rate was observed when the initial nitric acid concentration surpassed 12 mol/L. Additionally, the initial thorium ion concentration was found to have a minimal impact on the dissolution rate. Further kinetic studies employing the shrinking core model indicate that the dissolution process of thorium dioxide is predominantly controlled by interfacial chemical reactions. Data fitting yield an apparent reaction rate constant k of (0.132±0.007) min⁻¹. These findings provide significant theoretical support for optimizing the dissolution process in thorium-based fuel cycles and improving subsequent processing techniques. By systematically examining the impact of various factors on the dissolution of thorium dioxide, this research offers new insights and solutions for the dissolution process within thorium-based fuel cycles. Effective dissolution of thorium dioxide can be achieved without the use of fluoride ions by optimizing nitric acid concentration and controlling dissolution temperature, which is of considerable importance for the subsequent processing and application of thorium-based nuclear fuels.