The structure of liquid UO_2-x in reducing gas atmospheres

Journal Article · 22 February 2017 · Applied Physics Letters

DOI: https://doi.org/10.1063/1.4977035 · OSTI ID:1389313

Alderman, O. L. G.  ^[1]; Benmore, C. J. ^[2]; Weber, J. K. R.  ^[1]; Skinner, L. B.  ^[3]; Tamalonis, A. J. ^[4]; Sendelbach, S. ^[5]; Hebden, A. ^[6]; Williamson, M. A. ^[6]

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1. Materials Development Inc., Arlington Heights, IL (United States); Argonne National Lab. (ANL), Argonne, IL (United States). X-ray Science Division
2. Argonne National Lab. (ANL), Argonne, IL (United States). X-ray Science Division
3. Argonne National Lab. (ANL), Argonne, IL (United States). X-ray Science Division; Stony Brook Univ., NY (United States). Mineral Physics Inst.
4. Materials Development Inc., Arlington Heights, IL (United States)
5. Materials Development Inc., Arlington Heights, IL (United States)A
6. Argonne National Lab. (ANL), Argonne, IL (United States). Nuclear Engineering

High energy X-ray diffraction experiments performed on hypostoichiometric UO_2-x liquids in reducing gas mixtures of 95%Ar:5%CO and 95%Ar:5%H₂ are compared to that conducted in a pure Ar atmosphere [Skinner et al., Science 346, 984 (2014)]. The measurements are pertinent to severe accident scenarios at nuclear reactors, where core melts can encounter reducing conditions and further shed light on the oxide chemistry of the low valence states of uranium, particularly U(III), which become stable only at very high temperatures and low oxygen potentials. The radioactive samples were melted by floating small spheres of material using an aerodynamic levitator and heating with a laser beam. In the more reducing environments, a 1.7% shift to lower Q-values is observed in the position of the principal peak of the measured X-ray structure factors, compared to the more oxidizing Ar environment. This corresponds to an equivalent elongation in the U-U nearest neighbor distances and the U-U periodicity. The U-O peak (modal) bond-length, as measured from the real-space total correlation functions, is also observed to expand by 0.9–1.6% under reducing conditions, consistent with the presence of 15–27% U³⁺ cations, assuming constant U-O coordination number. The slightly larger U-U elongation, as compared to the U-O elongation, is interpreted as a slight increase in U-O-U bond angles. Difficulties concerning the determination of the hypostoichiometry, x, are discussed, along with the future directions for related research.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1389313
• Alternate ID(s): OSTI ID: 1349361
• Journal Information: Applied Physics Letters, Vol. 110, Issue 8; ISSN 0003-6951
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English