  Atomic scale computer simulations using density functional theory were used
to investigate the behaviour of tin in the tetragonal phase oxide layer on
Zr-based alloys. The $Sn_{Zr}^{\times}$ site defect was shown to be dominant
across most oxygen partial pressures, with $Sn_{Zr}^{"}$ charge compensated by
fully charged oxygen vacancies occurring at partial pressures below $10^{-31}$
atm. Insertion of additional positive charge into the system was shown to
significantly increase the critical partial pressure at which $Sn_{Zr}^{"}$ is
stable. Recently developed low-Sn nuclear fuel cladding alloys have
demonstrated an improved corrosion resistance and a delayed transition compared
to Sn-containing alloys, such as Zircaloy-4. The interaction between the
positive charge and the tin defect is discussed in the context of alloying
additions, such as niobium and their influence on corrosion of cladding alloys.
