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, ThO2- (UO2)-U-233 and ThO2- UO2 (low-enriched) mixed oxide fuels with < 30% UO2 have been proposed for the Advanced Heavy Water Reactor. In the present work, phase relations in the Th(1-x)UxO(2+delta) (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 U4+. Raman spectroscopic studies indicate the formation of various defects upon uranium substitution in parent ThO2, 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.