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

摘要

Standard enthalpies of formation, ΔH°_f (kJ/mol) at 298 K, were derived for three pyrochlore phases: Ca_(0.93)Ce_(1.00)Ti_(2.035)O_(7.00)(-3656.0±5.6), Ca_(1.46)U~(4+)_(0.23)U~(6+)_(0.46)Ti_(1.85)O_(7.00) (-3610.6±4.1), Gd_2Ti_2O_7 (-3822.5±4.9) and two zirconolite phases: CaZr_(1.03)Ti_(1.97)O_7 (-3719.4±3.9) and CaHf1.02Ti1.9uO7 (-3720.5±3.9). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°_(f-ox) (kJ/mol) at 298 K: CaO + MO_2 + 2TiO_2 = CaMTi_2O_7 or Gd_2O_3 + 2TiO_2 = Gd_2Ti_2O_7, and an oxide/perovskite phase assemblage: ΔH°_(f-pv+ox) (kJ/mol) at 298 K: CaTiO_3 + MO_2 + TiO_2 = CaMTi_2O_7, 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): Ca_(0.93)Ce_(1.00)Ti_(2.035)O_(7.00) (-54.1±5.2), Ca_(1.46)U~(4+)_(0.23)U~(6+)_(0.46)Ti_(1.85)O_(7.00) (-123.1±3.4), Gd_2Ti_2O_7 (-113.4±2.8), CaZr_(1.03)Ti_(1.97)O_7 (-89.6±2.8) and CaHf_(1.02)Ti_(1.98)O_7 (-74.8±3.1). With respect to a perovskite plus oxide phase assemblage the hafnium zirconolite was marginally metastable in enthalpy (ΔH°_(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) = Ca_(0.93)Ce_(1.00)Ti_(2.035)O_(7.00) (+21.0±5.5). The U-pyrochlore sample was marginally stable in enthalpy relative to a perovskite plus oxide assemblage: ΔH°_(f-pv+ox) = Ca_(1.46)U~(4+)_(0.23)U~(6+)_(0.46)Ti_(1.85)O_(7.00) (-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.