Buffering the oxygen activity of uranium dioxide fuels using niobium and molybdenum as redox additives

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.