KNa3(UO2)2(Si4O10)2(H2O)4, a new compound formed during vapor hydration of an actinide-bearing borosilicate waste glass

Vapor hydration experiments on a U-doped borosilicate waste glass at 200 degrees C produced a novel uranium silicate. Single crystal X-ray structure analysis of this phase indicate the ideal formula KNa3(UO2)(2)(Si4O10)(2)(H2O)(4), although the compound shows some compositional variability. It is monoclinic, space group C2, Z = 2, a = 1.2782(1), b = 1.3654(1), c = 0.82677(8) nm, beta = 119.240(1)degrees. The structure was solved by direct methods and refined to an agreement index (R) of 3.6% for 2239 unique observed (\F-0\ greater than or equal to 4 sigma(F)) reflections and a goodness-of-fit of 1.05. The structure contains vertex-sharing silicate tetrahedra arranged in four and eight-membered rings that are linked to give sheets parallel to (0 0 1). The sheets are cross-linked by vertex-sharing with UrO(4) square bipyramids [Ur = (UO2)(2+) uranyl ion], forming a framework of polyhedra of higher bond-valence. The title phase is the major sink for U during glass corrosion at 200 degrees C after approximately 60 days in a saturated vapor environment. Consideration of the structural sites reveals the potential of this compound to incorporate radionuclides from a variety of nuclear-waste glasses over a wide range of environmental conditions. (C) 2000 Elsevier Science B.V. All rights reserved.