EPMA observations of internal and external corrosion of fuel pin cladding in PFR

The Electron Probe Microanalysis (EPMA) examination of two PFR sibling pins irradiated in the same sub-assembly to 10.7% heavy atom burn-up has shown that the external clad corrosion is confined to limited sub-surface diffusion of Ni and Cr into the sodium coolant. The internal clad corrosion mode is variable and this variability in Fuel Clad Chemical Interaction (FCCI) can be attributed to differences in the clad/fuel gap width. The major active species in the internal oxidative corrosion were Cs and Mo. It is suggested that Te may play an increased role in the corrosion mechanisms when a clad/fuel gap exists. (author)