Atomistic scale calculations are being used to determine the underlying chemical controls on uranium phase formation and alteration. The national and international nuclear forensic community aims at relating microstructural information with materials process history. However, the breadth of experiments and analysis necessary to develop a comprehensive list would be overwhelming. The fundamental chemistry governing phase formation and alteration can be probed at an atomistic scale using quantum-mechanical calculations. Observation and quantification of energetics and changes to electron density enable a predictive understanding of the fundamental chemistry controlling the presentation of actinide oxide phases. For example, the relationship of the surface energies of a uranium alteration phase can indicate the crystal form of the bulk phase. In this work, the (100), (110), (001), and (111) surfaces of coffinite (USiO4) were computed showing that, like zircon (ZrSiO4) the (100) face has the lowest surface energy. While zircon can form large euhedral crystals, coffinite tends to form more anhedral crystals. The range of the evaluated surface energies of coffinite (ranging from 0.67 J/m2 to 1.29 J/m2) is an indication that large euhedral crystals are unlikely to form, as compared with the range in surface energies for the same surfaces of zircon (0.88 J/m2 to 3.68 J/m2). Another example of atomistic scale investigations for nuclear forensics applications is the incorporation of a dopant into a uranium oxide system. Divalent cations may be present in the formation of uranium dioxide as an impurity in a processing facility (e.g., Ca remnant from U-metal and oxide processing), as a fission product remaining in a used fuel (e.g., Sr in used UO2 fuel), or as a daughter product in natural uranium (e.g., Pb in natural UO2). The incorporation mechanism of divalent cations into UO2 is not confirmed, but may occur via oxidation of U4+ to U6+. Here, density functional theory calculations were used to quantify the incorporation mechanism and confirm the association of uranium oxidation with divalent cation incorporation.

Atomistic insight into phase formation and alteration of uranium phases

254th National Meeting (2017)

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