Phase evolution in M1-xPuxO2 (0.0 ≤ x ≤ 0.6) (M = Zr, Th) as potential inert matrix fuel system under reducing and oxidizing conditions

Incorporation of plutonia in zirconia and thoria based matrices is important in the context of their transmutation and geological disposal applications. In the present work, phase relations and lattice parameter–composition relationships have been reported in ZrO2–PuO2 and ThO2–PuO2 systems under reducing and oxidizing conditions. In this direction, samples with general formula Zr1-xPuxO2 (0.0 ≤ x ≤ 0.6) and Th1-xPuxO2 (0.0 ≤ x ≤ 0.5) have been prepared by a solid-state route at 1773 K in Ar–H2 and characterized by X-ray diffraction. A mixture of m-ZrO2 + F-type phase at x = 0.1, F-type stabilized zirconia at x = 0.2 & 0.3, pyrochlore phase at x = 0.4 & 0.5 and a mixture of pyrochlore + F-type phase at x = 0.6 have been observed in ZrO2–PuO2 series. All samples in Th1-xPuxO2 (0.0 ≤ x ≤ 0.5) series under similar experimental conditions form F-type phase. Upon oxidation at 973 K in air, F-type stabilized zirconia phase transforms to a tetragonal phase and pyrochlore phase maintains its identity in ZrO2–PuO2 system. In the ThO2–PuO2 system, the oxidizing atmosphere does not modify the F-type phases except the decrease in lattice parameter values for each Pu containing composition. Variation of lattice parameters of ThO2–PuO2 solid solutions prepared in reducing conditions shows a slight deviation from Vegard's law while oxidized samples follow the same accurately.