X-ray absorption in plutonium uranium mixed oxide fuel: Thorium characterization

• Impurity thorium forms a solid solution with UO2 in pristine and irradiated MOX.
• UO2 structure is distorted near the thorium atoms due to the different ionic.
• EXAFS results are supported by FEFF modeling.

Plutonium uranium mixed oxide (MOX) fuels are currently used in nuclear light water reactors. Thorium can be found in pristine as well as irradiated MOX samples. In normal conditions this element is only present in one oxidation state (IV), therefore its characterization can be used as a solid base for later studies of other actinides with the same oxidation state.In this work, the atomic structure and nearest-neighbor environment of thorium within pristine and irradiated MOX samples were analyzed by micro-X-ray absorption fine structure (μ-XAFS) spectroscopy measured in fluorescence mode. To serve as a reference, spectra of pure thoria (ThO2) powder samples were recorded. It was found that thorium is only present in its most common state (tetravalent) in both pristine and irradiated fuels. The structure of the thorium dioxide and MOX samples was modeled and their respective spectra were calculated. This modeling is consistent with the experimental conclusions.