An Experimental Study of Diffusivity of Technetium-99 in Hanford Vadose Zone Sediments

Journal Article · Thu Nov 01 04:00:00 EDT 2012 · Vadose Zone Journal, 11(4)

DOI:https://doi.org/10.2136/vzj2011.0184· OSTI ID:1060656

Mattigod, Shas V; Bovaird, Chase C; Wellman, Dawn M; Parker, Kent E; Wood, Marcus I

One of the methods being considered at the Hanford site in Washington for safely disposing of low-level radioactive wastes (LLW) is to encase the waste in concrete and entomb the packages in the Hanford vadose zone sediments. The current plan for waste isolation consists of stacking low-level waste packages on a trench floor, surrounding the stacks with reinforced steel, and encasing these packages with concrete. Any failure of the concrete encasement may result in water intrusion and consequent mobilization of radionuclides from the waste packages. The mobilized radionuclides may escape from the encased concrete by mass flow and/or diffusion and move into the surrounding subsurface sediments. It is therefore necessary to conduct an assessment of the performance of the concrete encasement structure and the surrounding soil’s ability to retard radionuclide migration. The retardation factors for radionuclides contained in the waste packages can be determined from measurements of diffusion coefficients for these contaminants through concrete and fill material. Because of their anionic nature in aqueous solutions, the radionuclides, 99Tc and 129I were identified as long-term dose contributors in LLW. The leachability and/or diffusion of these radionuclide species must be measured in order to assess the long-term performance of waste grouts when contacted with vadose-zone porewater or groundwater. To measure the diffusivity, a set of experiments were conducted using 99Tc-spiked concrete (with 0 and 4% metallic iron additions) in contact with unsaturated soil half-cells that reflected the typical moisture contents of Hanford vadose zone sediments. The 99Tc diffusion profiles in the soil half cells were measured after a time lapse of ~1.9 yr. Using the concentration profiles, the 99Tc diffusivity coefficients were calculated based on Fick’s Second Law.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1060656
• Report Number(s): PNNL-SA-84094; 830403000
• Journal Information: Vadose Zone Journal, 11(4), Journal Name: Vadose Zone Journal, 11(4)
• Country of Publication: United States
• Language: English

Related Subjects

techetium
diffusion
vadose zone sediments