Oxidation Properties of (U, Pu)O₂ Solid Solutions

Abstract

Air oxidation of (U, Pu)O2 solid solutions having Pu/(U+Pu) ratios of 0.2 and 0.25 was studied using a recording thermal balance. For Pu/(U+Pu)=0.2, the original fluorite MO2 phase oxidized first to tetragonal M3O7; further oxidation at 450°C<T<600°C gave a two‐phase mixture of M'3O8 and a cubic phase having a0≃5.43 Å. Kinetic analysis of these oxidation stages indicated that the Erofeev nucleation model explains the data reasonably well, with ΔH≃40 kcal/mol for the formation of M3O7 and ΔH≃40 kcal/mol for the oxidation of M3O7 to M'3O8+ the cubic phase. Oxidation above M3O7 (O/M=2.35) resulted in mechanical disintegration of the samples because of the presence of the M'3O8 phase. For Pu/(U+Pu) = 0.25, both isothermal and programed air oxidation of homogeneous samples up to 800°C led to a compound with a maximum O/M≃2.35, i.e. M4O9.4, an fcc phase having a0=5.428 Å; these samples were not converted to powder by the oxidation.

Raman and X‐ray Studies of Uranium–Lanthanum‐Mixed Oxides Before and After Air Oxidation

“…It was also reported that the air oxidation product of Gd‐doped UO 2 has the composition (U,Gd)O 2.4 , which is even more oxygen‐rich than U 3 O 7 (UO 2.33 ) . Tennery and Godfrey obtained O/M = 2.35 for air‐oxidized (U,Pu)O 2 samples (Pu/(U + Pu) = 0.25). Therefore, it cannot be excluded, even in the current research, that after air oxidation of the La‐doped UO 2 samples, the resulting M 4 O 9 phase could have excess oxygen beyond the nominal stoichiometry of MO 2.25 .…”

High Temperature X-ray Diffraction Study of the Oxidation Products and Kinetics of Uranium–Plutonium Mixed Oxides

“…25 The authors conducted several thermogravimetry experiments on differently fabricated MOX samples with Pu/(U + Pu) of 0.2 and 0.25. One part of the experimental campaign comprised a programmed heating with a rate of 5°C/min, which is comparable to our overall heating rate throughout the heat treatments.…”

Thermal studies on fluorite type ZryU1−yO2 solid solutions

“…These results show that oxidation of Zr y U 1Ày O 2 (y = 0.3 and 0.35) in air leads to the formation of ZrU 2 O 7 via fluorite phase Zr y U 1Ày O 2+x and the value of x depends on time and temperature of heating which alters the oxygen content thereby changing the lattice parameter. It has been reported from XRD data that oxidation of pure UO 2 powder proceeds via U 3 O 7 (tetragonal phase), whereas when UO 2 was doped with 25 mol% PuO 2 , face-centered cubic phase, M 4 O 9 (M = U + Pu) was formed, even when the material was heated up to 1073 K [24].…”