Quantum-mechanical evaluation of Np-incorporation into studtite
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Lindsay C. Shuller
,
Rodney C. Ewing
Abstract
Radionuclide incorporation into the alteration products of corroded UO2 in used nuclear fuel may control the release and mobility of key radionuclides, such as the very long-lived minor actinide, 237Np (τ1/2 = 2.1 Ma). Studtite, [UO2(O2)(H2O)2](H2O)2, may form in the presence of peroxide produced by radiolysis of water in contact with the spent fuel. Experiments have indicated that the studtite structure can incorporate Np; however, due to the low concentrations in the solid, the incorporation mechanism could not be determined. In this study, density functional theory is used to calculate an optimized structure, determine the electronic density of states, and calculate the energetics of the incorporation of Np6+ vs. Np5+ + H+ into the studtite structure. The definition of the source/sink phase (reference phase) for the cations involved in the incorporation process greatly affects the final incorporation energy. The incorporation energy of Np into studtite based on the 4+ oxide reference phases (e.g., source/sink = NpO2/UO2) results in lower incorporation energies (-0.07 and 0.63 eV for Np6+ and Np5+ incorporation, respectively) than the incorporation energy calculated using higher-oxide reference phases (e.g., Np2O5/UO3), where the incorporation energies for Np6+ and Np5+ into studtite are 0.42 and 1.12 eV, respectively. In addition, Np6+-incorporation into studtite is energetically more favorable than Np5+- incorporation as assessed from the lower incorporation energy. Estimates of the solid-solution behavior from a combination of quantum-mechanical calculations and Monte-Carlo simulations indicate that the Np6+- and U6+-studtite solid solution is completely miscible at room temperature with respect to a hypothetical Np6+-studtite structure. The Np-studtite structure was calculated to be stable with respect to the corresponding oxides, but its formation may be kinetically hindered. Knowledge of the electronic structure provides insight into Np-bonding in the studtite structure. The Np 5f orbitals are within the band gap of studtite, which results in the narrow band gap of Np-incorporated studtite (1.09 eV), as compared with the band gap of studtite alone (2.29 eV).
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- Uranyl phosphate sheet reconstruction during dehydration of metatorbernite [Cu(UO2)2(PO4)2·8H2O]
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- Electronic structure and local environment of substitutional V3+ in grossular garnet Ca3Al2(SiO4)3: K-edge X-ray absorption spectroscopy and first-principles modeling
- Elbrusite-(Zr)—A new uranian garnet from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia
- What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks
- Radiation damage and uranium concentration in zircon as assessed by Raman spectroscopy and neutron irradiation
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- Protracted oceanic subduction prior to continental subduction: New Lu-Hf and Sm-Nd geochronology of oceanic-type high-pressure eclogite in the western Dabie orogen
- Time-resolved in situ studies of apatite formation in aqueous solutions
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