Phase evolution in [Nd1-xUx]2Zr2O7+δ system in oxidizing and reducing conditions: A nuclear waste form

Nd2Zr2O7 pyrochlore has been proposed as a promising candidate for the immobilization of actinides and inert matrix fuel in the nuclear industry. In view of this, uranium substituted Nd2Zr2O7 samples with general formula [Nd1-xUx]2Zr2O7+δ (0.0 ≤ x ≤ 0.5) have been synthesized under reducing and oxidizing conditions by a solid-state route and thoroughly characterized by X-ray diffraction, Raman and IR spectroscopy to determine uranium solubility in Nd2Zr2O7. All the characterization studies confirm the formation of pyrochlore/fluorite-type phases in reduced samples at x ≤ 0.5 indicating 50 at.% uranium solubility. However, Rietveld analysis in conjunction with Raman spectroscopic studies indicates that the concentration of F-type domains increases with increasing uranium substitution in the largely pyrochlore-type phase (x ≤ 0.3). The solubility of uranium decreases when the samples are synthesized in an oxidizing atmosphere as phase separation is observed at x = 0.5. Rietveld analysis also indicates the increase of F-type phase fractions in the oxidizing atmosphere compared to that of the reduced counterpart. This increase of the F-type phase has been attributed to the formation of smaller-sized U5+/U6+ (as compared to U4+) and the concomitant incorporation of additional oxygen anions in the lattice in oxidizing conditions. The thermogravimetry method has been used to study the oxidation of reduced samples in a flowing oxygen atmosphere. The weight gain corresponds to the oxidation of U4+ to higher oxidation states and the oxygen to uranium ratio (O/U) of the oxidized samples is close to 2.67 similar to U3O8.