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 fuelby incorporating additives during the manufacturing process that have a strong redoxactivity. To this end, we have studied the macroscopic electrochemical properties ofthree polycrystalline uranium dioxide samples, the first containing both molybdenumand molybdenum dioxide, the second containing niobium dioxide and the lastcontaining no additives other than impurities. During experiments, samples weresubjected to reducing and subsequently oxidizing atmospheres with respect to theoxygen partial pressure at which the oxidized and reduced forms of the additives are inequilibrium. In situ monitoring of the electrochemical response of the samples revealsthat the presence of redox couples involving either niobium or molybdenumsignificantly modifies the oxygen activity of a uranium dioxide solid.Scanning Electron Microscopy (SEM), Electron Back-Scatter Diffraction (EBSD) andEnergy Dispersive X-ray analysis (EDX) prior to and following the high temperaturemeasurements, confirm the in situ data: the final microstructure purports to show thatthe electrochemical activity is controlled by the Mo/MoO2 and the NbO2/Nb2O5couples for the molybdenum and niobium containing samples respectively.