A miniature solid state galvanic cell was used to measure the oxygen potential Delta (G) over bar(O-2) of reactor irradiated UO2 fuel at different burnups in the range of 28 to greater than or equal to 150 GWd d/t M. This very high burnup was achieved in the rim region of a fuel with a cross section average burnup of 75 GWd d/t M. The fuels had different enrichments and therefore different contributions of fission of U-235 and Pu-239. The temperature range covered was 900 to 1350 K. None of the fuels showed a significant oxidation. Rather, if allowance is made for the dissolved rare earth fission products and the Pu formed during irradiation, some of the fuels were very slightly substoichiometric and the highest possible degree of oxidation corresponded to UO2.001. In general, the Delta (G) over bar(O-2) at 750 degrees C was about -400 kJ/mol, corresponding to the Delta (G) over bar(O-2) of the reaction Mo + O-2 --> MoO2. The implication of these results which are in contrast to commonly assumed ideas that UO2 fuel oxidizes due to burnup, are discussed and the importance of the fission product Mo and of the zircaloy clad as oxygen buffers is outlined.
