DIFFUSION OF RADIONUCLIDES IN CONCRETE AND CONCRETE-BENTONITE SYSTEMS

Various construction materials are under consideration for nuclear was te repositories. Two important materials are concrete and bentonite cl ay, which will act as mechanical barriers and prevent convective water flow. These barriers will also retard transport (diffusion controlled ) of dissolved radionuclides by a combination of mechanical constraint s and chemical interactions with the solid. An important issue is the possible change of the initial sodium bentonite into the calcium form due to interaction with calcium from the concrete. The initial leachin g of concrete was studied using radioactive spiked concrete in contact with compacted bentonite. Measurement were made of the diffusion of C s, Am and Pu into 5 different types of concrete in contact with pore w ater. The diffusivity measured for Cs agrees reasonably well with data found in the literature. No movement could be measured for Am and Pu (< 0.2 mm), even though the contact times were extremely long (2.5 and 5 yr, respectively). The diffusion of Na, Ca and Cs from concrete int o bentonite was also measured.