Oxidation studies of (U1−yGdy)O2.00; (y ≤ 0.2) under different oxygen partial pressures by thermogravimetry and in situ X-ray diffraction

Oxidation behaviour of (U 1−y Gd y )O 2.00 ; ( y = 0.05, 0.10, 0.15 and 0.20) was examined under controlled oxygen partial pressures ranging from 101.3 to 0.2 kPa. Stoichiometric gadolinium (Gd 3+ )-doped UO 2 was synthesized by a gel combustion method followed by reduction at 1673 K in Ar–8%H 2 atmosphere. Thermogravimetry (TG) measurements of (U 1−y Gd y )O 2.00 were recorded under different oxygen partial pressures, and end products were analysed using powder X-ray diffraction (XRD) technique. Rietveld analysis of the XRD data of the oxidized products shows the formation of mixture of orthorhombic M 3 O 8 and FCC MO 2+x phases. Based on TG and XRD data, quantitative information on the distribution of uranium and gadolinium between all product phases is evaluated. Lowering the oxygen partial pressure and increasing Gd concentration in (U,Gd)O 2.00 resist the formation of M 3 O 8 and hence stabilize the FCC phase. The oxidation study under varying oxygen partial pressures implies that minimum oxygen potential of approximately − 72 kJ mol −1 is required to retain the FCC phase for oxidizing conditions up to 1273 K. High-temperature X-ray diffraction (HT-XRD) measurements in vacuum and oxygen atmospheres are used to track the oxidation reaction. HT-XRD and TG studies in vacuum and oxygen atmospheres were coupled together to correlate the change in lattice parameter (LP) of FCC phase, with a change in O/M ratio.