  In PWR, the Zircaloy based clad is the first safety barrier of the fuel rod,
it must prevent the dispersion of the radioactive elements, which are formed by
fission inside the UO2 pellets filling the clad. We focus here on internal
corrosion that occurs when the clad is in tight contact with the UO2 pellet. In
this situation, with temperature of 400^{\circ}C on the internal surface of the
clad, a layer of oxidised Zircaloy is formed with a thickness ranging from 5 to
15 $\mu$m. In this paper, we will underline the specific behaviour of this
internal corrosion layer compared to wet corrosion of Zircaloy. Simulations
will underline the differences of stress field and their influences on
corresponding dissolved oxygen profiles. The reasons for these differences will
be discussed as function of the mechanical state at inner surface of the clad
which is highly compressed. Differences between mechanical conditions generated
by an inner or outer corrosion of the clad are studied and their influences on
the diffusion phenomena are highlighted.
