The Role of Oxygen Diffusion in the Release of Technetium from Reducing Cementitious Waste Forms
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THE ROLE OF OXYGEN DIFFUSION IN THE RELEASE OF TECHNETIUM FROM REDUCING CEMENTITIOUS WASTE FORMS ROBERT W. SMITH* AND JOHN C. WALTON** *Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 83415 "**Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78023 ABSTRACT Cementitious 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 (TcO 4 ) anion to form an insoluble technetium sulfide phase (Tc2 S7(S)). 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 Tc2 S7 (s,)oxidizes to the mobile TcO4 -anion. The TcO 4-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 Tc2 S7(s) 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. INTRODUCTION
In the United States and other countries cementitious materials are used to immobilize lowlevel radioactive waste. Although concrete waste forms are used primarily for their physical properties, such as low permeability, cementitious materials can provide an ideal geochemical environment (e.g., high pH pore fluids and large surface areas for sorption) for immobilizing nuclear waste. Including reducing agents can immobilize radionuclides, such as 99Tc by forming of solid sulfide phases and reducing multivalent radionuclides to lower oxidation states that are less mobile. Blast furnace slag (BFS), which is a widely available glassy by product of pig iron production, is a commonly used reducing agent in cement mixes. Under oxidizing conditions, aqueous technetium occurs in the heptavalent oxidation state as the pertechnetate anion (TcO4-). Heptavalent technetium solids have high solubilities leadi
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