Hydrogen concentration dependence of tritium tracer diffusion coefficient in alpha phase of niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients D-t of tritium in alpha phase of hydrogenated and deuterized niobium (alpha-NbHxTy and alpha-NbDxTy (x<0.8, y<<x)) have been measured at 473 K, 493 K and 553 K. The data on Dt show typical hydrogen concentration dependence: D-t of tritium for both alpha-NbHxTy and alpha-NbDxTy decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of D-t of tritium differs from that of D-t of protium in alpha-NbHx or of deuterium in alpha-NbDx. On the other hand, no appreciable differences in the concentration dependence of D-t of tritium between alpha-NbHxTy and alpha-NbDxTy are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that D-t of tritium for a tritiated mobium (alpha-NbTx) is not very different from that for alpha-NbHxTy and alpha-NbDxTy. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained D-t of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.