K.B. Helean

Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE 3+ )PO 4 , (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na 2 O · 4MoO 3 ) and lead borate (2PbO · 2B 2 O 3 ) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO 4 , RE oxides, and P 2 O 5 . Enthalpies of formation for the (RE)PO 4 compounds with respect to the oxides at 298 K become more negative with increasing RE 3+ ionic radius; i.e., in going from ScPO 4 (−209.8 ± 1.0 kJ/mol), to LuPO 4 (−263.9 ± 1.9 kJ/mol), to LaPO 4 (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.

Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U4+0.23U6+0.46Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7: three materials relevant to the proposed waste form for excess weapons plutonium

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH0f (kJ/mol), 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) 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, ΔH0f−ox: CaO+MO2+2TiO2=CaMTi2O7 or Gd2O3+2TiO2=Gd2Ti2O7, and an oxide/perovskite phase assemblage, ΔH0f−pv+ox: CaTiO3+MO2+TiO2=CaMTi2O7, where M=Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH0f−ox (kJ/mol) (Gd2Ti2O7)=−113.4±2.8; ΔH0f−ox(Ca1.46U4+0.23U6+0.46Ti1.85O7.00)=−123.1±3.4; ΔH0f−ox(Ca0.93Ce1.00Ti2.035O7.00)=−54.1±5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH0f−pv+ox=−5.1±4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH0f−pv+ox=+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.

Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates

Abstract The enthalpies of formation of seven uranyl oxide hydrate phases and one uranate have been determined using high-temperature oxide melt solution calorimetry: [(UO2)4O(OH)6](H2O)5, metaschoepite; β-UO2(OH)2; CaUO4; Ca(UO2)6O4(OH)6(H2O)8, becquerelite; Ca(UO2)4O3(OH)4(H2O)2; Na(UO2)O(OH), clarkeite; Na2(UO2)6O4(OH)6(H2O)7, the sodium analogue of compreignacite, and Pb3(UO2)8O8(OH)6(H2O)2, curite. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpy, ΔHds. The standard enthalpies of formation from the elements, ΔH0f, at 298 K are -1791.0 ± 3.2, -1536.2 ± 2.8, -2002.0 ± 3.2, -11389.2 ± 13.5, -6653.1 ± 13.8, -1724.7 ± 5.1, -10936.4 ± 14.5, and -13163.2 ± 34.4 kJ/mol, respectively. These values are useful in exploring the stability of uranyl oxide hydrates in auxiliary chemical systems, such as those expected in U-contaminated environments.

Thermodynamics of uranyl minerals: Enthalpies of formation of rutherfordine, UO2CO3, andersonite, Na2CaUO2(CO3)3(H2O)5, and grimselite, K3NaUO2(CO3)3H2O

Abstract Enthalpies of formation of rutherfordine, UO2CO3, andersonite, Na2CaUO2(CO3)3(H2O)5, and grimselite, K3NaUO2(CO3)3(H2O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, ΔHr-ox, is .99.1 ± 4.2 kJ/mol for the reaction UO3 (xl, 298 K) + CO2 (g, 298 K) = UO2CO3 (xl, 298 K). The ΔHr-ox for andersonite is .710.4 ± 9.1 kJ/mol for the reaction Na2O (xl, 298 K) + CaO (xl, 298 K) + UO3 (xl, 298 K) + 3CO2 (g, 298 K) + 5H2O (l, 298 K) = Na2CaUO2(CO3)3(H2O)6 (xl, 298 K). The ΔHr-ox for grimselite is .989.3 ± 14.0 kJ/mol for the reaction 1.5 K2O (xl, 298 K) + 0.5Na2O (xl, 298 K) + UO3 (xl, 298 K) + 3CO2 (g, 298 K) + H2O (l, 298 K) = K3NaUO2(CO3)3H2O (xl, 298 K). The standard enthalpies of formation from the elements, ΔHfºf are .1716.4 ± 4.2, .5593.6 ± 9.1, and .4431.6 ± 15.3 kJ/mol for rutherfordine, andersonite, and grimselite, respectively. Energetic trends of uranyl carbonate formation from the binary oxides and ternary carbonates are dominated by the acid-base character of the binary oxides. However, even relative to mixtures of UO2CO3, K2CO3, and Na2CO3 or CaCO3, andersonite and grimselite are energetically stable by 111.7 ± 10.2 and 139.6 ± 16.1 kJ/mol, respectively, suggesting additional favorable interactions arising from hydration and/or changes in cation environments. These enthalpy values are discussed in comparison with earlier estimates

FORMATION ENERGETICS OF CERAMIC PHASES RELATED TO SURPLUS PLUTONIUM DISPOSITION

A database of fundamental thermodynamic functions at 298.15 K including entropy, S” , standard enthalpies and Gibbs free energies of formation, A~H 0 and Af G“, molar masses, and molar volumes, V“, for phases related to proposed ceramic waste materials for the disposal of surplus weapons plutonium has been assembled. Previously published as well as newly reported results of measured and predicted quantities for pyrochlore, brannerite, monazite, zircon and other related phases are presented. Where thermodynamic quantities are available for temperatures above 298.15 K, they are included or referenced. INTRODUCTION “) the U S Department of Energy has decreed that at With a landmark record of decision , . . least 17 ● 103 kg of surplus weapons plutonium will be incorporated into a ceramic waste material. Another 33 ● 103 kg is expected to be used in mixed-oxide (MOX) fuel reactors and the resulting irradiated fuel is to be considered a waste materiai. Both waste ceramics will be buried in a geological repository such as the proposed Yucca Mountain nuclear waste repository in the state of Nevada in the United States of America. Licensing of each waste material for incorporation into a waste repository will require clear and predictive scientific results on their durability and performance in a geological environment over nearly 106 years. Accurate thermodynamic quantities are key to both the development and modeling of these waste materials. In an effort to facilitate the optimization of a ceramic waste form, enable accurate modeling of its performance in a geological repository, and to provide a thermodynamic basis for production process engineering, we have sought to provide accurate thermodynamic quantities for phases related to these waste materials. Here, we report moiar masses, molar volumes, V“, heat capacities, Cl , entropies, S0, enthalpies, A~H 0, and Gibbs free energies, A ~Go , of formation for phases related to these proposed ceramic waste materials for surplus weapons plutonium. The * Author to whom correspondence should be sent.