HR: 0800h
AN: H21B-1330    [Abstracts]
TI: Immersion Experiment of Commercial Concrete in Groundwater (Kanamaru, Japan)
AU: * Bros, R
EM: regis-bros@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AU: KANAI, Y
EM: y.kanai@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AU: OKUSAWA, K
EM: koichi-okuzawa@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AU: SEKI, Y
EM: yoji-seki@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AU: SUZUKI, M
EM: masaya-suzuki@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AU: Watanabe, Y
EM: Yoshio.Watanabe@aist.go.jp
AF: AIST, Research Center for Deep Geological Environments, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, IBARAKI, 305-8567 Japan
AB: In the case of low and intermediate level radioactive wastes, most current repository designs envisage the use of large volumes of cementitious materials to immobilise the wastes and to backfill the repository. Tackling the difficult problems of evaluating the long-term alteration of concrete by groundwater after disposal, the generation of high pH in cement pore waters, the interaction between cement leachates and the host rock and the related transport of radionuclides are commonly achieved using a combination of laboratory, URL and natural analogue studies. Our group is developing an experimental approach which consists to immerse cylindric commercial concrete in existing wells and let it in contact with groundwater for several days to several months. Groundwater quality is monitored before and after immersion at regular time-scale intervals. Such experiments are conducted under realistic in situ conditions with regards to host rock geology and groundwater flow rate and chemistry, therefore approaching repository conditions. The aquifer is located within a cretaceous granite (15-30 m) characterized by a low permeability (10^-9-10^-8 m/s) and overlain by tertiary fluviatile sediments. It is representative of surface to sub-surface repository conditions subjected to inflow of fresh and oxidizing water (depth: 30 m; pH: 5.20; EC: 0.047 mS/cm, T: 9.91; Eh: +594 mV). It corresponds to a mixture of deep granitic Ca-HCO₃ type (Ca: 1.44 mg/l ; HCO₃: 36 mg/l) and shallow Na-Cl type (Na: 5.54 mg/l; Cl : 5.73 mg/l) groundwater having low pH and low salinity. After immersion of concrete, rapid changes were observed; pH increases to 5.37, 6.54 and 10.22 after 64 hours, 72 hours and 61 days, respectively. EC increases to 0.048, 0.072 and 0.155 mS/cm respectively, together with the concentration of dissolved Ca (1.88 mg/l after 4 days of immersion). These changes are related to the release of Ca(OH)₂ in cement pore water. Ca was partially coprecipitated as gypsum filling fractures within concrete. The rate of release of OH^- per concrete surface unit was estimated to 0.03 mol/cm²/day. The gradual decrease of pH with decreasing depth suggests diffusion-type upward migration of hydroxile ions released from cement through the water column and the inferred distance of migration is 2.5 m. The temporal evolution of C/C₀ which represents the concentration of dissolved nuclides normalized to the concentration before immersion was determined for several elements; in case of U, C/C₀ do not change substantially during the first days then dramatically increase at higher pH after several weeks. Mo concentration displays progressive depletion with increasing pH. Alkaline elements show differential behaviour i.e gradual increase with time (Li) and serrated patterns (Rb, Cs). Cu and Zn display solubility limit controled behaviour with significant leaching from the cement phase at pH 6.5 followed by precipitation at pH 10. Other transition elements (Ni, Co, Cr) evolution patterns suggest their good stability and the absence of significant release from the cement phase. These preliminary results are promising and show the validity of the immersion experimental method which might be applied to other HLW and LLW-ILW disposal related materials.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1834 Human impacts
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005