Cs boltwoodite obtained by ion exchange from single crystals: Implications for radionuclide release in a nuclear repository

Abstract

Spent nuclear fuel is unstable under the oxidizing conditions present in the proposed repository at Yucca Mountain, and the dominant products of alteration are expected to be uranyl oxide hydrates and uranyl silicates. Incorporation of radionuclides into the structures of the uranyl phases may impact upon the mobility of radionuclides under repository conditions. Boltwoodite, (K,Na)[(UO 2)(SiO 3OH)](H 2O) 1.5, will form under repository conditions when groundwater contacts the spent fuel waste form. Cs-exchanged boltwoodite, Cs[(UO 2)(SiO 3OH)], has been obtained from single crystals of natural boltwoodite by ion exchange with 2 M CsCl solutions at 90°C (14 days) and 185°C (24 h). Exchange of Cs + for K + and Na + in the interlayer of the structures was demonstrated by single-crystal structure determinations for exchanged crystals. This is the first demonstration of ion exchange in a single crystal of a uranyl phase, as well as the first evidence that Cs can be incorporated into the structure of a uranyl phase that is expected to form due to alteration of spent fuel under repository conditions. Boltwoodite that crystallizes under repository conditions is likely to incorporate Cs into its structure, thereby inhibiting Cs mobility. Earlier-formed Na- and K-rich boltwoodite may also exchange Cs with fluids that contact the crystals. The strongly bonded sheets of uranyl and silicate polyhedra in the structure of boltwoodite are only relatively weakly bonded to the interlayer constituents, permitted exchange of the interlayer cations without the loss of structural integrity. The structures of many uranyl phases are based upon similar sheets of polyhedra of higher bond-valence, suggesting that ion exchange may be possible in many other uranyl phases.