OXYGEN POTENTIAL IN THE RIM REGION OF HIGH BURNUP UO2 FUEL

Small specimens from the rim region (< 200 mum from the fuel surface) of a UO2 fuel rod with an average burnup of 7.6% FIMA were analysed in a miniaturized galvanic cell to determine their oxygen potential DELTAG(O2)BAR. These fuel pieces represented the porous rim structure with very small grains known to be formed near the periphery of high burnup UO2 fuel pellets. The oxygen potential of the rim material was very low, corresponding to that of unirradiated stoichiometric UO2, or to that of slightly substoichiometric UO2 containing rare earth fission products. No indication of oxidation due to fission was found, though most fission was that of Pu. Measurements on pieces from the inner, unrestructured fuel showed somewhat higher oxygen potentials corresponding to those of very slightly substoichiometric fuel if allowance is made for the incorporation of rare earths. These results are in contrast to some generally accepted ideas of burnup effects, and the possible reasons and implications are discussed.