Thermochemical Investigations of Zirconolite, Pyrochlore and Brannerite: Candidate Materials for the Immobilization of Plutonium

Standard enthalpies of formation, ΔH°f (kJ/mol) at 298 K, were derived for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1), Gd2Ti2O7 (−3822.5±4.9) and two zirconolite phases: CaZr1.03Ti1.97O7 (−3719.4±3.9) and CaHf1.02Ti1.98O7 (−3720.5±3.9). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: CaO + MO2 + 2TiO2 = CaMTi2O7 or Gd2O3 + 2TiO2 = Gd2Ti2O7, and an oxide/perovskite phase assemblage: ΔH°f-pv+ox (kJ/mol) at 298 K: CaTiO3 + MO2 + TiO2 = CaMTi2O7, M = Ce, U, Hf, Zr were also calculated. All of the pyrochlore and zirconolite materials studied were stable in enthalpy with respect to their oxides. ΔH°f-ox: Ca0.93Ce1.00Ti2.035O7.00 (−54.1±5.2), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−123.1±3.4), Gd2Ti2O7 (−113.4±2.8), CaZr1.03Ti1.97O7 (−89.6±2.8) and CaHf1.02Ti1.98O7 (−74.8±3.1). With respect to a perovskite plus oxide phase assemblage the hafnium zirconolite was marginally metastable in enthalpy (ΔH°f-pv+ox = +6.0±3.5 kJ/mol) while zirconolite was marginally stable in enthalpy (ΔH°f-pv+ox = −8.8±3.3 kJ/mol). The Ce-pyrochlore was not thermodynamically stable as measured by the enthalpy change relative to perovskite plus oxides: ΔH°f-pv+ox = Ca0.93Ce1.00Ti2.035O7.00 (+21.0±5.5). The U-pyrochlore sample was marginally stable in enthalpy relative to a perovskite plus oxide assemblage: ΔH°f-pv+ox = Ca1.46U4+0.23U6+0.46+Ti1.85O7.00 (−5.1±4.0). A significant field where the proposed waste form for excess weapons plutonium is not thermodynamically stable both at room temperature and at high temperature was defined. ΔH°f (kJ/mol) at 298 K were derived for three brannerite phases: CeTi2O6 (−2948.8±4.3), U0.97Ti2.03O6 (−2977.9±3.5) and ThTi2O6 (−3096.5±4.3). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: MO2 + 2TiO2 = MTi2O6, M = Ce, U, Th were also calculated. ΔH°f-ox: CeTi2O6 (+29.4±3.6), U0.97Ti2.03O6 (−7.7±2.8) and ThTi2O6 (+19.4±1.6). Thus, Ce and Th-brannerite are entropy stabilized and thermodynamically stable only at high temperature. These thermochemical data are correlated to the observed trends in the radiation “susceptibility” of these phases. Thermodynamic data relevant to waste form processing and optimization are also discussed.