X-ray diffraction and Raman spectroscopic studies on ThO2−UO2 solid solutions

In view of the vast resources of thorium in India, thorium-based fuel cycle has been envisaged as one of the most promising options to meet sustainable energy requirements. In this direction, ThO ₂− ²³³UO ₂ and ThO ₂−UO ₂ (low-enriched) mixed oxide fuels with < 30% UO ₂ have been proposed for the Advanced Heavy Water Reactor. In the present work, phase relations in the Th _1-xU _xO _{2 + δ} (x ≤ 0.3) system under reducing and oxidizing conditions have been investigated by means of X-ray diffraction and Raman spectroscopic studies. Diffraction studies confirmed the formation of single-phase fluorite-type solid solutions; however, the samples synthesized in air exhibited slightly lower lattice parameters than those of their reduced counterparts, attributed to oxidation of U ⁴⁺. Raman spectroscopic studies indicate the formation of various defects upon uranium substitution in parent ThO ₂ , depending on synthesis conditions. It also highlights the existence of more disorder in the air-synthesized samples as compared to their reduced counterparts, plausibly due to the presence of a variety of oxidation states of uranium and associated oxygen anions interstitials. Thermal annealing of the solid solutions under air at T ≥ 1873 K results in heavy uranium evaporation loss, which has been established by XRD and XRF studies.