Interaction of Rare Earth Elements with Cadmium: Systems Ce-CD, Pr-Cd, Nd-Cd

It is proposed to study the interaction of the three rare earth (RE) elements cerium, praseodymium, and neodymium with cadmium to learn more about the corresponding reaction products and their thermal as well as thermochemical stability. One part of the project consists of the investigation of the phase equilibria in the three systems Ce-Cd, Pr- Cd, and Nd-Cd using powder X-ray diffraction (XRD), electron microprobe analysis (EPMA), and differential thermal analysis (DTA). From the experimental results it will be possible to derive the corresponding phase diagrams for the three systems. Due to the high reactivity of the RE elements it will be necessary use crucibles made of tantalum which are sealed under vacuum, and to do all sample preparation and sample handling in an inert gas glove box. A second part of the project addresses the thermochemical stability of Ce-Cd, Pr-Cd, and Nd-Cd alloys. A drop calorimetric method will be used to determine integral enthalpies of mixing of liquid binary alloys in the Cd-rich part of the systems. The concentration range will probably be restricted to about 10 to 15 at% of the corresponding RE element due to their limited solubility in liquid Cd at temperatures below 700C whereas at higher temperatures (where the solubility increases rapidly) the volatility of Cd will become a problem. Integral enthalpies of formation of solid intermetallic compounds can be obtained by two different drop calorimetric methods: by solution calorimetry in liquid Cd and by direct reaction calorimetry in sealed tantalum crucibles. The latter experiments will be performed in cooperation with colleagues of the University of Genoa in Italy and will allow a cross check of the results from solution calorimetry. Furthermore, it is proposed to determine partial thermodynamic properties of Cd by two different experimental methods. An isopiestic vapor pressure method will be employed to measure Cd partial pressures over Ce-Cd, Pr- Cd, and Nd-Cd alloys from which partial Gibbs energies for Cd can be deduced together with their temperature dependence. These measurements are to be supported by emf (electromotive force) measurements in cooperation with scientist from the Indira Gandhi Centre for Atomic Research in Kalpakkam, India. The combination of the two methods should allow a full description of the partial thermodynamic properties of Cd for all three systems, including the possibility to calculate integral thermodynamic properties by Gibbs-Duhem integration. Finally, all phase diagram information and thermodynamic data will be used as input into a CALPHAD (CALculation of PHAse Diagrams) type optimization of the three systems, again in cooperation with the two partners from Italy and from India. The final result will be a consistent thermodynamic description of the alloy systems Ce-Cd, Pr-Cd, and Nd-Cd in parametric form which allows the calculation of the corresponding optimized phase diagrams.

Although Austria has opted against the use of nuclear energy many countries in the world rely, or have to rely, on this type of energy production. Some of them are running strong programs in an attempt to become independent from foreign energy sources by developing an effective nuclear program. However, disposal of nuclear waste that contains a number of light rare earth (RE) elements (from cerium up to gadolinium) is one of the key issues for the future use of nuclear energy; thus innovative safe reactor concepts that minimize waste output and effective reprocessing processes are currently being developed. For high burn-up fuels, traditional aqueous reprocessing may not be a preferred method for various reasons, therefore, a number of different so-called pyrochemical separation techniques are currently under development.One central process in most of these techniques is an electrorefining step where the RE elements first dissolve in the liquid salt electrolyte and are then extracted by a liquid metal like cadmium (Cd). To understand and optimize this process it is necessary to know their solubility in liquid Cd and the thermodynamic stability of the RE-Cd alloys that will be formed. In addition, one has to know the type and stability of solid RE-Cd compounds that may precipitate out of the solution and for this an accurate knowledge of the phase diagram is necessary.During the current research project, the thermodynamic stability of solid RE-Cd alloys was determined using a particular vapor pressure method, the so-called isopiestic method. This was done for the alloy systems with the three RE elements cerium (Ce), praseodymium (Pr), and neodymium (Nd), and in logical extension of the program also for gadolinium (Gd). From the results of these experiments, the Gibbs energy of formation (a measure of the thermodynamic stability) was derived for alloys in the composition range between about 50 and 100 at% Cd. Furthermore, for some Pr-Cd compounds the heat of formation (another measure of their stability) was measured by a calorimetric method.At the same time, all four phase diagrams were studied in detail using X-ray powder diffraction, optical and electron microscopy, thermal analyses, as well as in one case also dilatometry. For the two alloy systems Pr-Cd and Nd-Cd only limited information had been available before, and the phase equilibria were elucidated in full detail. In the case of the other two systems, Ce-Cd and Gd-Cd, the available phase diagrams were re-investigated and are now available in a much more detailed version. The Pr-Cd system was optimized by the so-called CALPHAD (CALculation of PHAse Diagrams) method, based on the experimental results obtained in the present study. With this, thermodynamic properties are now available in parametric form that allows, with proper commercial software, the calculation of all thermodynamic data as well as of the full phase diagram.With the outcome of this study, it will be possible to model and optimize the electrorefining step in the pyrochemical reprocessing of burnt-up nuclear fuels.