U and Pu incorporation in zircon and their effect upon helium diffusion in the host lattice

Zircon mineral (ZrSiO4) is a potential host matrix for the encapsulation of highly radioactive materials, such as dismantled weapons and nuclear waste from power stations (Ewing, 2001). We report a detailed computer modelling study, based on interatomic potentials, to address two fundamental issues concerning radiation processes in the zircon mineral: first, the physics and chemistry of U and Pu inclusion in the crystalline matrix of zircon and, second, the effect of U/Pu dopants on the incorporation and diffusion behaviour of helium in zircon. Our results show that U and Pu can incorporate isomorphously in zirconium sites (Zr). The solution energies of U and Pu are 0.18 eV and 0.03 eV respectively, which indicates that Pu is more readily incorporated as a dopant than U. However, the positive solution energies show that the inclusion of the two species in the crystalline matrix would occur only under high temperature conditions. Interstitial incorporation of helium in zircon is an energetically unfavourable process, which causes strong deformation in the host lattice due to movement of atoms in the channel walls to accommodate He. Moreover, the presence of Pu and U dopants in the zircon lattice has a direct effect upon the energetics of helium diffusion between interstitial sites. For instance, activation energies calculated for He diffusion are dependent upon the distribution of U and Pu in the lattice, and can differ by as much as 0.2 eV from those calculated for the perfect zircon.