Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi2O6, (M=actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, ΔH0f at 298 K, for three brannerite compositions (kJ/mol): CeTi2O6 (−2948.8 ± 4.3), U0.97Ti2.03O6 (−2977.9 ± 3.5) and ThTi2O6 (−3096.5 ± 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na2O · 4MoO3 solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (ΔH0f-ox at 298 K): MO2 + 2TiO2=MTi2O6. Only UTi2O6 is energetically stable with respect to an oxide assemblage: U0.97Ti2.03O6 (ΔH0f-ox=−7.7±2.8 kJ/mol). Both CeTi2O6 and ThTi2O6 are higher in enthalpy with respect to their oxide assemblages with (ΔH0f-ox=+29.4±3.6 kJ/mol) and (ΔH0f-ox=+19.4±1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature.