THERMODYNAMICS AND EXTRACTION MODELING OF TRIVALENT LANTHANIDES IN THE NUCLEAR FUEL CYCLE

摘要

Development of predictive modeling capabilities for solvent extraction processes requires a fundamental quantitative understanding of non-ideality effects and speciation in relevant aqueous and organic solutions. In spent nuclear fuel reprocessing trivalent lanthanides are present at significant concentration levels among the nuclear fission products, and the separation of the trivalent actinides is desirable but difficult step in closing the nuclear fuel cycle. Development of predictive modeling capabilities for the advanced fuel cycle separation processes requires gaining a fundamental quantitative understanding of non-ideality effects and speciation in relevant aqueous and organic solutions. Advancement of the aqueous and non-aqueous solution models can improve our knowledge and prediction of the trivalent metal ion extraction mechanism. Osmotic coefficients of the aqueous electrolyte solutions can be accurately measured by classical isopiestic technique. The drawback of the isopiestic method is that it is time consuming and is not suitable for collecting large volumes of experimental data. An alternate, faster technique is being tested for the reliable measurement of the water activity in the aqueous electrolyte solutions. Binary solutions of lanthanide nitrate salts were measured over a wide concentration range using a vapor pressure osmometer and water activity meter and compared with the literature values for 3:1 electrolyte systems. The obtained Pitzer thermodynamic parameters were used for the modeling and prediction of the extraction process employing the organic extractant bis(2-ethylhexyl) phosphoric acid under varying conditions. This report discusses our current research efforts in this area.