title: Energetics of metastudtite and implications for nuclear waste alteration.
abstract: Metastudtite, (UO2)O2(H2O)2, is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, -1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220 °C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400 °C to 530 °C; oxygen loss from amorphous UO3 to crystallize orthorhombic α-UO2.9; and reduction to crystalline U3O8. This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water.
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit and Department of Chemistry, University of California, Davis, CA 95616; and.
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Guo
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit and.
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Ushakov
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Labs
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
authors.forename: Hildegard
authors.initials: H
authors.lastname: Curtius
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Bosbach
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit and anavrotsky@ucdavis.edu.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
keywords: enthalpy of formation; metastudtite; nuclear fuel alteration; stability; uranium