Crystal Chemistry of Uranyl Sulfates and Oxalates and Perrhenate Incorporation into Uranyl Phases

The understanding of uranium chemistry is of increasing importance in the search for clean, renewable energy. Nuclear power can produce energy with no greenhouse gas emissions and far fewer raw materials. One ton of uranium produces power equivalent to 16,000 tons of coal or 80,000 barrels of oil (Nuclear Energy Institute, 2009). However, controversy surrounds nuclear power due to the disposal of spent nuclear fuel. There is currently no long-term storage facility in the United States. Yucca Mountain, Nevada has been proposed as a geological repository, but more information on the chemistry of spent fuel components, such as uranium and technetium is needed to ensure safety and secure a license for operation. This research expands the knowledge of uranium crystal chemistry and the ability of uranyl phases to incorporate components of spent nuclear fuel.

A novel new uranyl sulfate and three new uranyl oxalate compounds are presented, along with a study of the incorporation of perrhenate (ReO4-), as a analog for pertechnetate (TcO4 ), into uranyl phases. The uranyl sulfate K2[(UO2)(SO4)2H2O]H2O crystallized in the Cmca space group and is composed of uranyl pentagonal bipyramids linked into infinite chains with sulfate tetrahedra. The uranyl oxalate Cs2(UO2)2(C2O4)3 crystallized in the P21 space group and is composed of infinite sheets of uranyl pentagonal bipyramids connected via oxalate ions. The uranyl oxalate hydroxide Cs(UO2)2(C2O4)(OH)3 crystallized in the P21/m space group and is composed of infinite sheets of uranyl pentagonal bipyramids connected via oxalate ions and edge sharing polyhedra. The uranyl oxalate K6(UO2)2(C2O4)4O2 crystallized in the space group P21/c. It is composed of isolated clusters of two uranyl hexagonal bipyramids connected via edge sharing and contains a total of four coordinated oxalate ions. All crystals were analyzed with single crystal x-ray diffraction. The structures were solved with SHELL XL software.

Technetium-99 is an important dose contributor in a geological repository (Burns et al., 1997a; Chen et al., 2000). It is a potentially mobile component of spent nuclear fuel with a relatively long half-life. The uranyl mineral analogues, uranophane, Ca[(UO2)(SiO3OH)]2Ì¢‰âÂå¢5H2O, sodium boltwoodite, Na(UO2)(SiO3OH)Ì¢‰âÂå¢1.5H2O, and soddyite, (UO2)2(SiO4)Ì¢‰âÂå¢2H2O, are known to form from spent nuclear fuel and are expected to form in a geologic repository (Finch and Ewing, 1992; Wronkiewicz et al., 1996). The extent to which Tc7+, found as pertechnetate (TcO4-), is incorporated into these uranyl phases will impact its mobility in the repository (Burns et al., 1997a; Chen et al., 2000).

In this research, the above uranyl phases were hydrothermally synthesized in the presence of perrhenate (ReO4-), as a crystal chemical analog for pertechnetate (TcO4 ). The identity of the uranyl phases were verified with powder x-ray diffraction. The presence of rhenium was analyzed using inductively coupled plasma Ì¢‰âÂ' optical emission spectroscopy (ICP-OES). No rhenium was found in the hydrothermally synthesized samples, indicating that perrhenate was not incorporated. Due to the similarities in size, tetrahedral shape, and bond valence arguments, we postulate that pertechnetate will not incorporate into uranyl mineral analogues found in spent nuclear fuel.