Exploring YSZ/ZrO2 - PuO2 systems: Candidates for inert matrix fuel

The investigations in phase behavior of plutonia-based system are generally carried out by employing surrogates for the ease of handling. However, the curiosity to understand the phase relations by employing PuO2 directly instead of surrogates led to the present work on phase relations in yttria stabilized zirconia (YSZ)-PuO2 and ZrO2-PuO2 systems. These systems have relevance to inert matrix fuel concept. Phase relations have been discussed in both the systems under different conditions. The nominal compositions in YSZ-PuO2 system under vacuum exhibit fluorite structure for low PuO2 content (up to 20 mol %) while the pyrochlore structure was observed for the samples with 30-50 mol% PuO2 content. Annealing under oxidizing conditions did not alter the F-type phases but decreased the ordering in pyrochlore phases. The samples with composition Zr0.8Pu0.2O2 and Zr0.5Pu0.5O2 in ZrO2-PuO2 systems under reduced condition form F-type phase and pyrochlore, respectively. The F-type phase, upon oxidation, transformed to tetragonal phase while the pyrochlore phase retains its identity with concomitant decrease in intensity of superstructure reflections indicating the gradual pyrochlore to fluorite phase transition. The sintering studies on YSZ-PuO2 samples show that no appreciable shrinkage were observed on the samples beyond 1773 K. Linear thermal expansion on the representative nominal compositions was measured in a vertical dilatometer over the temperature range from RT to 1573 K in argon atmosphere. Though the coefficients of linear thermal expansion of 0.6 YSZ-0.4 PuO2 (40 mol% PuO2 containing YSZ) is less compared to YSZ (x = 0.0) at lower temperature, the trend reverses beyond 1473 K.