The release of organic material from clay based buffer materials and its potential implications for radionuclide transport

In the Canadian nuclear fuel waste disposal concept used fuel would be plac ed in corrosion resistant containers which would be surrounded by clay-base d buffer and backfill materials in an engineered vault excavated at 500 to 1000 m depth in crystalline rock formations in the Canadian Shield. Organic substances could affect radionuclide mobility due to the effects of redox and complexation reactions that increase solubility and alter mobility. The purpose of this study was to determine whether the buffer and backfill mat erials, proposed for use in a disposal vault, contain organics that could b e leached by groundwater in large enough quantities to affect radionuclide mobility within the disposal vault and surrounding geosphere complex. Buffe r material, made from a mixture of 50 wt. % Avonlea sodium bentonite and 50 wt. % silica sand, was extracted with deionized water to determine the rel ease of dissolved organic carbon (DOC), humic acid and fulvic acid. The eff ect of radiation and heat from the used fuel was simulated by treating samp les of buffer before leaching to various amounts of heat (60 degrees and 90 degrees C) for periods of 2, 4 and 6 weeks, and to ionizing radiation with doses of 25 kGy and 50 kGy. The results showed that groundwater would leac h significant amounts of organics from buffer that complex with radionuclid es such as the actinides, potentially affecting their solubility and transp ort within the disposal vault and possibly the surrounding geosphere. In ad dition, the leached organics would likely stimulate microbial growth by sev eral orders of magnitude. Heating and radiation affect the amount and natur e of leachable organics.