Effects of Near-Surface Redox Potentials on Radioactive Selenium, Iodine and Technetium Mobility in Soils.
George Shaw, Univ of Nottingham, University Park, University Boulevard, Nottingham, United Kingdom
The problem of radioactive waste disposal currently confronts many countries worldwide and is inextricably bound up with the sustainable exploitation of nuclear energy on a global basis. Geological disposal of radioactive wastes is the most likely solution to this problem, although it is expected that low level leakage from such repositories will lead to contamination of soils and associated biota over time scales of 1,000 to 100,000 years. A critical step in the movement of long-lived radionuclides from the sub-surface to the surface soil environment is the transition from the saturated to the unsaturated zone, with an associated change from anoxic to oxic conditions. This step may either reduce or enhance the long-term transport of key radionuclides from sub-surface repositories into the biosphere, depending on the chemistry of the individual elements concerned. Se-79, I-129 and Tc-99 are important long-lived and potentially mobile radionuclides within radioactive wastes, each of which is sensitive to changes in oxidation-reduction (redox) potential. Both temporal and spatial changes in soil wetting can lead to significant fluctuations in the speciation of these elements and hence in the degree of sorption which they experience within the soil. We have undertaken a series of studies to determine the effects of changes in redox potential in soils contaminated with these radionuclides (or isotopic surrogates) on their mobility and bioavailability. In the case of radioactive selenium and technetium, permanently saturated subsoils were found to present a significant redox barrier to vertical migration within soil columns, although small quantities of each element were found to migrate past this barrier and into oxic surface soil. There is evidence that processes such as biological transformation and translocation may enhance this small degree of transport which may be significant over the time scales considered in radioactive waste safety studies. Radioactive iodine was found to migrate readily in a reduced form within the anoxic sub-surface, although significant accumulation occurred at the boundary between anoxic and oxic soil as oxidation of iodine occurred. For I-129, with a 15.7 million year half life, this represents a very substantial accumulation mechanism which could result in significant localised sinks of radioactive iodine in locations with appropriate hydrological conditions. These studies have been designed to provide information to help improve quantitative models of radionuclide migration and impact in the context of radioactive waste disposal in the UK and France.