Mathematical Model of Dry Reprocessing Electrorefining of Molten Salt

Abstract

Molten salt electrorefining is the core process unit of spent fuel dry reprocessing, and the chemical and electrochemical changes of high-temperature molten salt electrorefining process can be explored through mathematical model research, which can provide a reference basis for electrorefining process optimization and equipment design. In this paper, based on electrochemical thermodynamics and material transport formulas, a mathematical model of the electrorefining process of molten salt in spent fuel was established, and the changes of electrode potential, partial current and material distribution of key elements in spent fuel with time were calculated. The central difference method was used to discretize the material distribution change equation and obtain the oscillation result. The backward difference method of discrete material distribution change equation was used to verify the accuracy of the established mathematical model through literature experimental data. The results show that the relative error between simulated uranium product deposited at the cathode and the experimental data is 2.80%, which shows that the mathematical model has good fit. During electrorefining, there are two mutations in the anode potential, indicating complete dissolution of Pu and complete dissolution of U separately. At the same time as the electrochemical dissolution of Pu, a displacement reaction occurs with UCl3 in the molten salt, so that the anode division current of Pu is larger than the input current. The order of dissolution of the three elements at the anode is Pu>U>Zr. U elements are deposited in large quantities at the cathode, and the deposition rate slows down after Zr dissolved at the anode. The concentration of elements in molten salt varies with the electrorefining process. In the simulated time range, the concentration of U element decreases to a certain extent at first, and the concentration of Pu element increases with time, and then the concentration of the two elements remains almost unchanged. The molar concentration of Zr elements in molten salt remains traces. At the same time, the influence of current intensity on the spent fuel electrorefining process is simulated and calculated. The results show that with the increase of current intensity, the anode potential shows an overall increasing trend, and the cathode potential shows an overall decreasing trend. The rate of anodic dissolution and cathodic deposition of spent fuel accelerates, and the partial current of each element increases. The change rate of element concentration in molten salt is accelerated. In actual working conditions, the current can increase within the permissible range to improve cell efficiency.

Keywords

• spent fuel
• dry reprocessing
• molten salt electrorefining
• mathematical model
• varieties in mass distribution