  Epitaxial thin films have been utilised to investigate the radiolytic
dissolution of uranium dioxide interfaces. Thin films of UO$_2$ deposited on
single crystal yttria stabilised zirconia substrates have been exposed to water
in the presence of a high flux, monochromatic, synchrotron x-ray source. In
particular, this technique was applied to induce dissolution of three UO$_2$
thin films, grown along the principle UO$_2$ crystallographic orientations:
(001), (110) and (111). Dissolution of each film was induced for 9 accumulative
corrosion periods, totalling 270s, after which XRR spectra were recorded to
observe the change in morphology of the films as a function of exposure time.
While the (001) and (110) oriented films were found to corrode almost linearly
and at comparable rates, the (111) film was found to be significantly more
corrosion resistant, with no loss of UO$_2$ material being observed after the
initial 90s corrosion period. These results distinctly show the effect of
crystallographic orientation on the rate of x-ray induced UO$_2$ dissolution.
This result may have important consequences for theoretical dissolution models,
as it is evident that orientation dependence must be taken into consideration
to obtain accurate predictions of the dissolution behaviour of UO$_2$.
