Defect Structure and the Related Properties of UO2 and Doped UO2

Abstract

The oxygen potentials and the electrical conductivities of UO2 doped with various cations (La, Gd, Pu, Th, Nb, Cr, Ti) measured by the authors were reviewed in comparison with the previous results on UO2 doped with other cations (Mg, Y, Gd, Th, Zr) by other investigators. The defect structures of UO2 doped with these various cations were discussed based on these data. Diffusion coefficients of uranium in UO2 doped with various cations (La, Y, Ti, Nb) previously measured by some researchers together with our recent results on uranium diffusion in UO2 doped with Ti ion were also reviewed in relation to the defect structures. It was generally concluded that (1) the oxygen potential and the electrical conductivity of UO2+x are increased (or decreased) by doping with lower (or higher) valent cations, and (2) the diffusion coefficients of uranium in UO2+x are decreased (or increased) by doping with lower (or higher) valent cations. These facts are explained from the valence control rule, assuming that these metals are present as the substitutionals for uranium ions. However our recent results on the oxygen potential, the electrical conductivity and the diffusion coefficient of uranium in UO2+x doped with Ti ion can not be explained by the general rule mentioned above, suggesting the presence of titanium interstitials instead of titanium substitutionals for uranium ions. The effects of dopants with various valences (Nb, Cr, Ti, Gd, Y) on the creep rate, the fission gas release and the heat capacity were also discussed.