Tritium and Iodide Diffusion through Opalinus Clay
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Tritium and Iodide Diffusion through Opalinus Clay A. Yllera de Llano, M. Mingarro Sainz-Ezquerra and M. García Gutiérrez CIEMAT, Dpto. de Impacto Ambiental de la Energía. Avda. Complutense, 22, 28040 Madrid, Spain Email: [email protected] ABSTRACT The International Mont Terri Project [1, 2] started in 1995 under the patronage of the Swiss National Hydrological and Geological Survey (SNHGS), and has the authorization of the République et Canton du Jura. The underground rock laboratory is located at the northwestern part of Switzerland (Canton Jura), in and beside the reconnaissance gallery of the Mont Terri motorway tunnel, one of the several tunnels of the A16 “Transjurane” motorway. The depth of overburden above the rock laboratory is approximately 300 meters. The project is aimed to investigate the geological, hydrogeological, geochemical and rock mechanical properties of the Opalinus Clay for assessing the feasibility and safety of a repository for radioactive waste placed in this type of host rock. One of the issues under study is radionuclide migration by diffusion through clays. As a part of this investigation, an interlaboratory comparison on small-scale diffusion experiments was carried out by three research laboratories: AEA Technology (UK), SCK·CEN (Belgium) and CIEMAT (Spain). The radionuclides investigated were tritium and iodine. This paper concerns to the methodological approach and results of the experiments undertaken by CIEMAT. The effective diffusion coefficients were measured for tritiated water and iodine (as I-), resulting larger for tritium [(1.7±0.4)×10-11 m2/s] than for iodide [(2.7±0.3)×10-12 m2/s]. The porosity available for diffusion was calculated by using the time-lag method, but some results seemed unrealistic and showed a large variability. In general, tritium exhibited higher values of porosity than iodide (17 to 26% and 12 to 17%, respectively), which were consistent with the anion exclusion affecting the distribution of iodide into the clay pores. INTRODUCTION One key issue concerning the radwaste disposal safety is the potential release and transport of radionuclides from the canister to the environment. Natural occurring dense clay formations are considered amongst the candidate materials for the safe disposal of high level radioactive wastes because of their very low permeability, which ensures a small natural hydraulic gradient, and their high sorption capability. From the long-term disposal of nuclear spent fuel point of view, the long-lived fission product 129I draws special attention because of its very long half-life (1.59·107 yr). Moreover, iodine is highly soluble in groundwater, existing as I- under a wide range of environmental conditions [3]. Because of the negative charge, iodide ions usually exhibit very low sorption values when interacting with the clay surfaces and, therefore, it is expected that diffusive transport from the breached canister to the biosphere will be the dominant migration process. In
these conditions, the diffusion coefficient is an essentia
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