(44c) Modeling of Zone Freezing for Pyrochemical Process Waste Minimization

Authors: 
Williams, A. N., University of Idaho
Phongikaroon, S., Virginia Commonwealth University
Simpson, M. F., Idaho National Laboratory


Treatment of spent nuclear fuel using pyroprocessing technology results in accumulation of fission product chlorides in electrorefiner (ER) salt. This process is currently being carried out at Idaho National Laboratory (INL) to treat spent fuel from Experimental Breeder Reactor-II to prepare it for permanent disposal. Several purification techniques for this salt were proposed and studied by INL and Korea Atomic Energy Research Institute (KAERI) under an International Nuclear Energy Research Initiative (I-NERI) project. One of the most promising methods revealed in that project was zone salt freezing. In zone freezing, the ER salt is slowly cooled by advancing it up from a hot region through an adiabatic region and into a cold region. This causes the melt to cool from the top down. As the melt cools, pure LiCl-KCl will solidify first, causing the fission products to concentrate in the melt. Once the salt has completely cooled, the lower portion of the salt crystal containing the bulk of the fission products is removed, and the upper portion is recycled back into the electrorefiner. To better understand and optimize the zone freezing process, a mathematical model has been developed based on fundamental thermodynamic, heat transport, and mass transport theories. With this model, the temperature and concentration distributions can be determined as a function of time, providing valuable information on the zone freezing process. The temperature distribution can be found by using an implicit finite-difference method and basic laws of thermodynamics. The temperature profile was modeled by dividing the melt region into small elements and solving for the temperature of each element at a given time via the heat equation. To model the phase change phenomena, the thermodynamic property of enthalpy was used in correlation with the finite-difference method. In addition, a moving boundary condition technique was applied for each time step to simulate the advancement for the temperature distribution as a function of time of the melt into both the adiabatic and cold regions. The concentration distribution can be found using the temperature distribution along with mass transfer theory. The impurity distribution in zone freezing should be a function of both the diffusion flux of the impurity and the cooling rate. A correlation will be used between the diffusion equation and the segregation coefficient k. Details of this phase of the model are still being investigated. Preliminary results for the temperature distribution have been calculated and indicate that the temperature for a given element should decrease steadily with time until the phase change region was reached. Through the phase change region, the temperature is predicted to remain constant until the entire volume of the salt element solidifies, at which point the temperature once again should begin to drop. The temperature and concentration distribution calculations will be presented for the LiCl-KCl salt with impurities of strontium (Sr) and cesium (Cs) as a function of time.