Suppressing Natural Convection for Self-diffusion Measurement in Liquid Pb Using Shear Cell Technique by Stable Density Layering of Isotopic Concentration

This study aims to demonstrate a condition for suppressing natural convection for self-diffusion measurement in liquid metal by stable density layering of isotopic concentration. The self-diffusion coefficients (D_Pb) of ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb were measured using the shear cell technique at 773 K (500 °C). Simultaneously, the impurity diffusion coefficient (D_Ag) of Ag, which is a marker to compare D_Ag with the reference data of suppressing natural convection, was measured. From the concentration profiles of Pb isotopes, density distributions were calculated using an approximation of the linear relationship between the density and isotopic concentration; in addition, density gradients, which were defined as the inner product between gravity vector and density gradient calculated from the density distribution, were obtained. The results suggest that D_Ag was in good agreement with the relative error in the reference data (within 0.47 pct), which can be attributed to the suppression of natural convection by predicted stable conditions. In contrast, natural convection was confirmed as D_Ag was three times larger than the reference data in predicted unstable conditions. From the density distribution obtained after the diffusion process, when the density gradient with the largest absolute value was greater than zero, the suppression of natural convection was confirmed in the self-diffusion measurements.