EXAFS analysis of uranium(IV) and thorium(IV) complexes in concentrated CaCl2 solutions

A. Uehara 1 , T. Fujii 2 , H. Matsuura 3 , N. Sato 4 , T. Nagai 5 , K. Minato 6 , H. Yamana 7 , and Y. Okamoto 8
  • 1  Kyoto University, Research Reactor Institute, Osaka 590-0494, Japan
  • 2  Kyoto University, Research Reactor Institute, Sennan Osaka 590-0494, Japan
  • 3  Japan
  • 4  Japan Atomic Energy Agency, Advanced Science Research Centre, Tokai, Ibaraki 319-1195, Japan
  • 5  Japan Atomic Energy Agency, Nuclear Fuel Cycle Engineering Laboratories, Ibaraki 319-1194, Japan
  • 6  Japan
  • 7  Japan
  • 8  Japan

Abstract

The coordination of U4+ and Th4+ in concentrated CaCl2 solutions is studied by U and Th LIII edge extended X-ray absorption fine structure (EXAFS) spectroscopy. With the decrease of concentration of CaCl2 from 6.9 to 4 M, the Cl ion coordination number NCl in the U4+ coordination sphere decreases from 3.4 to 1.3, while the hydration number NO increases from 4.5 to 6.7. The combined coordination number NO+NCl of U4+ in concentrated Cl solution (8.0), is lower than that in 1.5 M perchloric acid aqueous solution (9.0). For Th4+, the decrease of concentration from 6.9 to 4 M CaCl2, the coordination number NCl in the Th4+ coordination sphere decreased slightly from 1.9 to 1.5, while coordination number NO increased from 7.6 to 8.8. The NO+NCl of Th4+ in concentrated Cl solution (9.0), is similar to that in 1.5 M perchloric acid. The bond distance of U–Cl (2.67 Å), is shorter than that of Th–Cl (2.76 Å), because of low coordination number NO to U4+. By adding HCl into the system, the U4+ and Th4+ coordination sphere is unchanged. The coordination structures of U4+ and Th4+ in concentrated LiCl is also discussed.

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