STABILITY AND CATION EXCHANGE DYNAMICS OF γ-IRRADIATED U60

The world at the nanometer level (10^-9 m) is vastly unexplored and is rapidly changing the way we think about uranium mobility in the environment. Nanomaterials often exhibit markedly different properties than their larger, bulk counterparts and can sometimes be tuned to yield beneficial physical or chemical properties. The chain-structured uranyl peroxide mineral studtite comprises one of the common ‘bulk’ (sensu lato) alteration phases of used nuclear fuel, due to the radiolysis of water it is in contact with.¹ Under such intense fields of ionizing radiation, studtite is comparatively more susceptible to alteration with respect to other uranyl minerals; amorphization is complete after absorbing 7.3 x 10⁶ Gy from an electron beam.² However, when a similar uranyl peroxide topology is instead assembled into the spherical nanocapsule U₆₀, its rigid capsule-like form, and crystallinity are preserved in the solid state and as discrete macroanions in solution after receiving comparable doses of gamma radiation. Ultra-small angle X-ray scattering (USAXS), spallation source neutron diffraction, and magic-angle spinning NMR spectroscopy techniques allow us to examine changes brought about by cation exchange during exposure. Though not yet observed in the field, uranyl peroxide clusters may form near the surface of used nuclear fuel where the concentration of radiolytic peroxide and uranium is high.² Cluster formation is favored in the absence of excess peroxide and suggests studtite is a metastable phase in the presence of cations.³ Understanding the dynamics that affect cluster solubility and stability under intense radiation will, in part, dictate uranium mobility. A newly discovered, naturally occurring uranyl-carbonate cluster mineral supports the concept that actinide macromolecules are an important feature to the speciation of actinides in the environment.

[1] Hughes-Kubatko, K. A., et al. Science. 2003, 302, 1191-1193. [2] Rey, A., et al. Stability of uranium (VI) peroxide hydrates under ionizing radiation. Am. Mineral., 2009, 94, 229-235. [3] Armstrong, C. R., et al. Uranyl peroxide enhanced fuel corrosion in seawater. PNAS, 2012, 109, 1874-1877.