Structural Behavior of Tc and I Ions in Nuclear Waste Glass

Abstract

Technetium-99 (Tc) and iodine-129 (I) are two long-lived fission products of high volatility, which makes their study in glass structure challenging. Both technetium and iodine have broad ranging multivalent chemistry and complex reactivity dependent on redox conditions; technetium and iodine redox may vary from Tc0 to Tc7+ and from I- to I7+. Relatively few studies have been done on their speciation in glass, in part because of their low retention at the temperatures required for glass melting. To better understand the redox and structural behavior of Tc and I in various nuclear waste glasses, a series of technetium- and iodine-containing borosilicate glasses of varied chemistry were prepared at scales ranging from a few grams to hundreds of kilograms. Technetium was included in both high-level and low-level nuclear waste glass formulations under a variety of redox conditions at concentrations ranging from 0.003 wt% to 0.06 wt%. Non-radioactive iodine glass samples were prepared in crucible melts using excess amounts of sodium or potassium iodide salts or ammonium iodate that resulted in concentrations ranging from 0.04 to 1.27 wt% iodine. These samples were also compared to glasses prepared in pilot-scale experiments in which the overall retentions reached 48% and 34% for technetium and iodine, respectively. Tc and I speciation in the resulting glasses were determined by X-ray absorption spectroscopy (XAS). While technetium was found as Tc0, Tc4+, and Tc7+, only I- was identified in these glasses. Previous studies of Tc local environment information inferred from K-edge XAS and Raman spectroscopy identified pertechnetate tetrahedra surrounded by network-modifying cations in oxidized glasses and octahedral TcO6 units in glasses prepared under reducing conditions. Conversely, iodine K-edge XAS of all glasses studied indicate iodide environments with lithium or sodium nearest-neighbors resembling disordered versions of octahedral sites in crystalline lithium or sodium iodide.