The Role of Oxygen Diffusion in the Release of Technetium from Reducing Cementitious Waste Forms

Abstract

ABSTRACTCementitious materials provide an ideal geochemical environment (e.g., high pH pore fluids and large surface areas for sorption) for immobilizing nuclear waste. The inclusion of reducing agents, such as blast furnace slag (BFS) can immobilize radionuclides by forming of solid sulfide phases. Thermodynamic calculations using the MINTEQ geochemical computer code indicate that elemental sulfur present in BFS reacts with the highly mobile pertechnetate anion (TcO4−) anion to form an insoluble technetium sulfide phase (Tc2S7(8)). Initially, the waste form very effectively immobilizes technetium. However, as oxygen diffuses into the waste form, an outer zone of oxidized concrete and a shrinking core of reduced intact concrete develops. Oxidation of sulfur in the outer zone results in increased technetium concentrations in the pore fluid because Tc2S7(8) oxidizes to the mobile TcO4− anion. The TcO4− anion can then diffuse from the waste form into the environment.A mathematical model that accounts for diffusion of oxygen into concrete coupled with oxidation of sulfur and sulfide to sulfate has been developed. This model assumes the existence of an oxidized outer layer of concrete surrounding a shrinking core of reducing intact concrete. A sharp boundary between the two zones moves slowly inward resulting in oxidation of Tc2S7(8) and subsequent release of TcO4− via aqueous diffusion in the concrete pore fluids. The model indicates that this mechanism results in a linear dependance of release with the square root of time similar to pure diffusion. In addition, the release of technetium is related to the inverse of the square root of the concentration of BFS, indicating that performance will significantly increases with the addition of approximately 20 percent BFS to the cement mix.