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

摘要

One of the methods being considered at the Hanford site in the state of 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 currentplan 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 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 migrati on. Because of their anionic nature in aqueous soluti ons, the radionuclides, ~(99)Tc and ~(129)I were identified as long-term dose contributors in LLW. The leachability or diffusion of these radionuclide species must be measured to assess the long-term performance of waste grouts when contacted with vadose zone pore water or groundwater. To supplement the previously obtained data, a set of experiments were conducted using ~(99)Tc-spiked concrete (with 0 or 4% metallic Fe additi ons) in contact with Hanford soil at extremely low moisture content by mass). The ~(99)Tc diffusion profiles in the soil half-cells were measured aftera time lapse of -1.9 yr and the diffusion coefficient was calculated to be -1.0 x 10~(-9) cm~2 s~(-1).