Transuranium Element Incorporation into the β-U3O8 Uranyl Sheet

ABSTRACTSpent nuclear fuel (SNF) is unstable under oxidizing conditions. Although recent studies have determined the paragenetic sequence for uranium phases that result from the corrosion of SNF, there are only limited data on the potential of alteration phases for the incorporation of transuranium elements. The crystal chemical characteristics of transuranic elements (TUE) are to a certain extent similar to uranium; thus TUE incorporation into the sheets of uranyl oxide hydrate structures can be assessed by examination of the structural details of the β-U3O8 sheet type.The sheets of uranyl polyhedra observed in the crystal structure of β-U3O8 also occur in the mineral billietite (Ba[(UO2)3O2(OH)3]2(H2O)4), where they alternate with α-U3O8 type sheets. Preliminary crystal structure determinations for the minerals ianthinite, ([U24+(HO2)4O6(HO)4(H2O)4](H2O)5), and “wyartite II” (mineral name not approved by IMA committee on mineral names), {CaCo3}[U4+(UO2)2O3(OH)2](H2O)4, indicate that these phases also contain β-U3O8 type sheets. The β-U3O8sheet anion topology contains triangular, rhombic, and pentagonal sites in the proportions 2: 1:2. In all structures containing β-U3O8 type sheets, the triangular sites are vacant. The pentagonal sites are filled with U6+O2 forming pentagonal bipyramids. The rhombic dipyramids filling the rhombic sites contain U6+O2 in billietite, U4+O2 in β-U3O8U4+(H2O)2 in ianthinite, and U4+O3 in “wyartite-II” (in which one apical anion is replaced by two O atoms forming a shared edge with a carbonate triangle of the interlayer). Interlayer species include: H2O (billietite, “wyartite II”, and ianthinite), Ba2+ (billietite) Ca2+ (”wyartite II”), and CO3−2 (”wyartite II”); there is no interlayer in β-U3O8. The similarity of known TUE coordination polyhedra with those of U suggests that the β-U3O8 sheet will accommodate TUE substitution coupled with variations in apical anion configuration and interlayer population providing the required charge balance.