The role of oxygen diffusion in the release of technetium from reducing cementitious waste forms

Conference · Fri Dec 31 04:00:00 EST 1993

OSTI ID:62206

Smith, R W ^[1]; Walton, J C ^[2]

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1. Idaho National Engineering Lab., Idaho Falls, ID (United States)
2. Southwest Research Institute, San Antonio, TX (United States)

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 the elemental sulfur present in BPS reacts with the highly mobile pertechnetate anion (TcO{sub 4}{sup -}) anion to form an insoluble technetium sulfide phase (Tc{sub 2}S{sub 7(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 Tc{sub 2}S{sub 7(a)} oxidizes to the mobile TcO{sub 4}{sup -} anion. The TcO{sub 4}{sup -} 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 Tc{sub 2}S{sub 7(s)} and subsequent release of TcO{sub 4}{sup -} 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.

• DOE Contract Number: AC07-76ID01570
• OSTI ID: 62206
• Report Number(s): CONF-921101--Vol.294
• Country of Publication: United States
• Language: English

Related Subjects

05 NUCLEAR FUELS
36 MATERIALS SCIENCE
99 GENERAL AND MISCELLANEOUS
CEMENTING
CEMENTS
GEOCHEMISTRY
LEACHING
LOW-LEVEL RADIOACTIVE WASTES
M CODES
RADIOACTIVE WASTE PROCESSING
REDUCING AGENTS
SLAGS
SOLIDIFICATION
WASTE FORMS