High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔHf0 (kJ/mol), for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (-3656.0 ± 5.6), Ca1.46U0.234+U0.466+T 1.85O7.00 (-3610.6 ± 4.1) and Gd2Ti2O7 (-3822.5 ± 4.9). Enthalpy of drop solution data, ΔHds, were used to calculate enthalpies of formation with respect to an oxide phase assemblage, ΔHf-ox0: CaO + MO2 + 2TiO2 = CaMTi2O7 or Gd2O3 + 2TiO2 = Gd2Ti2O7, and an oxide/perovskite phase assemblage, ΔHf-pv+ox0: CaTiO3 + MO2 + TiO2 = CaMTi2O7, where M = Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔHf-ox0 (kJ/mol) (Gd2Ti2O7) = -113.4 ± 2.8; ΔHf-ox0(Ca1.46U0.234+ U0.466+Ti1.85O7.00 = -123.1 ± 3.4; ΔHf-ox0(Ca0.93Ce1.00Ti2.0 35O7.00 = - 54.1 ± 5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔHf-pv+ox0 = -5.1 ± 4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔHf-pv+ox0 = +21.0 ± 5.5 kJ/mol). A significant metastability field was defined with respect to an oxide/perovskite phase assemblage. However, the proposed waste form baseline composition lies in the stable regions of the phase diagrams.
ASJC Scopus subject areas
- Nuclear and High Energy Physics
- Materials Science(all)
- Nuclear Energy and Engineering