Specific heat of UO2, ThO2, PuO2 and the mixed oxides (ThxU1 –x)O2 and (Pu0.2U0.8)O1.97 by enthalpy data analysis

Abstract

A recent reanalysis of enthalpy data for UO₂ using a local smoothing technique has revealed a weak sigmoidal structure in the data and a corresponding specific heat peak, indicating the existence of a Bredig transition occurring at ca. 2610 K (ca. 0.84T_m). The same smoother applied to ThO₂, PuO₂ and the mixed oxides (Th_xU_{1 –x})O₂ with x= 0.7, 0.85 and 0.92 and (Pu_0.2U_0.8)O_1.97 reveals a similar structure in the enthalpy data for the oxide (Th_0.7U_0.3)O₂ alone. Although the smoother does not detect enthalpy sigmoids (and thus specific heat peaks) in the data for the other oxides, this does not necessarily imply that Bredig transitions are not occurring; simulation studies show that it could be caused by insufficiently dense data in the vicinity of 0.8 of their absolute melting temperatures.