Modeling of Lanthanide Transport in Metallic Fuels: Recent Progresses

Experimental and modeling efforts are underway to determine the solubility and diffusivity of certain lanthanides in liquid metals to assess the feasibility of a liquid-like lanthanide transport (1) mechanism in metallic fuels. Using ab-initio Molecular Dynamics (AIMD), the solubility of cerium in liquid sodium at 1000 K was calculated to be less than 0.8 at.%. The diffusion coefficient of cerium in liquid sodium was calculated to be 5.6 10^-5 cm²/s [2]. We extended the MD work to include two temperatures, 723 K and 1000 K, for the diffusion of cerium (Ce), praseodymium (Pr), and neodymium (Nd) in both sodium (Na) and cesium (Cs) [3]. The lanthanide diffusivities were found to be in the same order of magnitude observed in liquid diffusion (10^-5 cm²/s) and the temperature dependence of the diffusivity was developed according to the Arrhenius equation. Experiments have been performed to measure the solubility of lanthanides in liquid sodium. While using ICPMS is used to measure the concentration of a particular lanthanide in a sodium sample, the solubility at that testing temperature is calculated. The experimental results indicated that the solubility of cerium, praseodymium, and neodymium in liquid sodium varied from 1×10^-5 to 3×10^-5 at.% in the temperature range of 723 to 823 K. The time dependence of dissolution was also obtained from experiments conducted at different equilibration times. The results showed that the solubility limit was reached within 30 minutes, indicating the dissolution rate is likely high relative to diffusion, thus suggesting lanthanide transport in liquid metals would be diffusion limited. We developed a conceptual pore model to better describe the lanthanide transport behavior through the fuel. Our results suggests that isolated pores will most likely act as a lanthanide sink and that the transport mechanism of lanthanides from fuel to cladding must occur through interconnected pores and cracks which are filled with sodium. (author)