Volatilization of Technetium from Simulated Reprocessing Solutions.

Abstract

Volatilization behavior of technetium was investigated in order to evaluate the decontamination efficiency of an evaporator for reprocessing waste solutions. The gaseous species of technetium volatilized over HTcO 4 solutions had been considered as the mixture of Tc 2 O 7 and HTcO 4 . Estimation of the decontamination efficiency needs to determine whether the dominant gaseous species is Tc 2 O 7 or HTcO 4 . The continuous dis-tillation of dilute HTcO 4 solution was carried out and we obtained the purification factor (PF) defined as the concentration ratio of technetium in the concentrate and distillate. The PF was normalized by total pressure, HTcO 4 molarity and mole number in 1l solution. The normalized PF was reciprocal to the second power of HTcO 4 molarity and was proportional to the activity of water. These findings suggested the dominant gaseous species were Tc 2 O 7 and volatilization was the dehydration of HTcO4 forming gaseous Tc 2 O 7 . The vapor pressure of Tc 2 O 7 was obtained from PF and the equilibrium constant of volatilization reaction was calculated considering that the gaseous species was dominantly Tc 2 O 7 . The thermodynamic functions controlling the volatilization reaction were obtained from the dependence of equilibrium constant on temperature as, ΔH=82.3kJ.mol -1 and ΔS=205.4 J.mol -1 .K -1 , respectively.