P. Vilks et al., The release of organic material from clay based buffer materials and its potential implications for radionuclide transport, RADIOCH ACT, 82, 1998, pp. 385-391
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.