Determination of mass transfer coefficients of Tc-99(VII) in forward and backward transport in PUREX-relevant systems by using analytical ultracentrifugation

The lack of published data on the extraction kinetics of Tc(VII) in the sys tem U(VI)/HNO3//TBP/dodecane prompted us to investigate its transport behav iour in detail. The aim was to improve the U(VI)/Tc(VII) separation in the PUREX process. We choose the method of analytical ultracentrifugation to de termine the concentration gradient profile, the initial flux density and th e integral mass transport for both sides of the unstirred liquid-liquid int erface as well as the overall mass transfer coefficients as a function of T c(VII)-, U(VI)-concentrations and temperature from time-dependent changes o f the refractive index gradients. The results were supplemented by comparin g the experimental with calculated concentration profiles, obtained by mode ling a diffusion-limited extraction process. In some of the investigated sy stems, the transport at the interface is reaction-limited after phase conta ct. The determined values of (J) over right arrow (max) I, and <(<alpha>)ov er right arrow> are discussed in terms of the extraction conditions.