Simulated advanced gas-cooled reactor spent nuclear fuels: Determination of the O/U ratio - an XRD, XPS and Raman study

This paper reports a detailed chemical and materials characterization study of novel Simulated Spent Nuclear Fuels (SIMFuel) which replicate the chemical and microstructural state of Spent Nuclear Fuel (SNF) discharged from a UK Advanced Gas-cooled Reactor (AGR). Recent advances in the analysis of pure UO 2 samples by XPS, micro-Raman spectroscopy and XRD, have been assessed for deployment in the characterisation of SIMFuels generally and these AGR SIMFuels in particular. All three analytical methods reveal the extent that the UO 2 bulk matrix is defected in the SIMFuels by the presence of lanthanide dopants. XPS, by inspection of the U4f peaks and their satellites, indicates the presence of U(V) in the UO 2 matrix as a means to charge compensate for the incorporation of Ln(III) states and presence of a slight hyperstoichiometry in the UO 2 solid solution. This was corroborated by detailed Raman analysis of the UO 2 matrix of the SIMFuels – wherein the (Ba,Sr)ZrO 3 and metallic particle phases, that simulate the precipitated grey phases and ε-particles formed in real spent fuel, were avoided – which indicated the presence of both interstitial oxygens and oxygen vacancies, the latter as a parallel means of charge compensation for the presence of Ln(III). Further confirmation was provided by XRD measurements through observation of a lattice parameter contraction arising from the presence of the smaller U(V) ion in the U(IV)O 2 matrix. XPS and Raman allow for the reporting of O/U ratios in the SIMFuels. These XPS and Raman-derived O/U ratios are in good agreement, indicating: (i) that the SIMFuels are near stoichiometric/slightly hyperstoichiometric, concurring with previous analogous studies on real and simulated light water reactor fuels; and (ii) that either technique may find application in the analysis of real fuel analysis, both pre- and post-irradiation.