Buffering the oxygen activity of uranium dioxide fuels using niobium and molybdenum as redox additives
Résumé
We demonstrate that it is possible to control the oxygen activity of a uranium oxide fuel
by incorporating additives during the manufacturing process that have a strong redox
activity. To this end, we have studied the macroscopic electrochemical properties of
three polycrystalline uranium dioxide samples, the first containing both molybdenum
and molybdenum dioxide, the second containing niobium dioxide and the last
containing no additives other than impurities. During experiments, samples were
subjected to reducing and subsequently oxidizing atmospheres with respect to the
oxygen partial pressure at which the oxidized and reduced forms of the additives are in
equilibrium. In situ monitoring of the electrochemical response of the samples reveals
that the presence of redox couples involving either niobium or molybdenum
significantly modifies the oxygen activity of a uranium dioxide solid.
Scanning Electron Microscopy (SEM), Electron Back-Scatter Diffraction (EBSD) and
Energy Dispersive X-ray analysis (EDX) prior to and following the high temperature
measurements, confirm the in situ data: the final microstructure purports to show that
the electrochemical activity is controlled by the Mo/MoO2 and the NbO2/Nb2O5
couples for the molybdenum and niobium containing samples respectively.
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