Measurement of radionuclide diffusion in ocean floor sediment and clay

The mobility of cationic neptunium, plutonium, americium, and sodium, and of the anionic species pertechnetate, TcO-4, has been determined in samples of various sediments from the ocean floor, and in bentonite and hectorite clay. The experiments were conducted at ambient temperatures (298 + or - 5 K), and the periods of observation ranged from several hours to ten months. All tests were carried out under static conditions permitting only molecular diffusion of the ionic species. Results indicate very low mobilities for the transuranium elements plutonium and americium, for which the upper limit of the effective diffusion coefficient is <10-10 cm2xs-1. Sodium, neptunium, and TcO-4 were found to have higher mobilities characterized by values for the effective diffusion coefficient of 3 X 10-6, 1.8 X 10-8, and 3.2 X 10-6 cm2xs-1, respectively. Some implications of the measured results for the assessment of barrier effectiveness are discussed