title: Diffusivity measurement of heavy ions in Wyoming montmorillonite gels by X-ray computed tomography.
abstract: Medical X-ray computed tomography (CT) was applied to the measurement of the diffusion coefficients of heavy ions in an artificial barrier material for the disposal of nuclear wastes. Cs(+), Sr(2+), I(-), and Br(-) are the heavy ions measured and the barrier used is the water-rich gel of Wyoming montmorillonite (86.5-100 wt.% H(2)O). X-ray CT yields an inevitable artifact (beam-hardening) in the obtained images. Before the diffusion experiments, the polychromatic primary X-ray spectrum of the CT scanner was measured by a CdZnTe detector, and the effects of the artifact were examined for an aqueous CsCl solution sample. The results show that the beam-hardening artifact derived from the polychromatic photon energy distribution can be suppressed by applying a special image reconstruction method assuming the chemical composition of samples. The transient one-dimensional diffusion of heavy ions in a plastic container filled with the gel was imaged nondestructively by the X-ray CT scanner with an in-plane resolution of 0.31 mm and slice thickness of 2 mm. The results show that diffusivities decrease with increasing clay weight fraction. The degree of the diffusivity decrease was high for cations (Cs(+) and Sr(2+)) and low for anions (I(-) and Br(-)). The quantitative decomposition of the contribution of the geometrical tortuosity and of the sorption to the diffusivity was performed by subtracting the diffusivity of nonsorbing I(-) from the measured diffusivities. The results show that the contribution of the sorption is large for Cs(+), Sr(2+) and small for Br(-). Because X-ray CT allows nondestructive and quick measurements of diffusivities, the technique would be useful particularly for measuring the diffusive migration of harmful radioactive elements.
authors.affiliation: Exploration Geophysics Research Group, National Institute of Advanced Industrial Science and Technology, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan. nakashima.yoshito@aist.go.jp
authors.forename: Yoshito
authors.initials: Y
authors.lastname: Nakashima
mesh_terms: D001546:Bentonite / Q000737:chemistry*; D004058:Diffusion; D005782:Gels; D011850:Radioactive Waste; D011868:Radioisotopes / Q000032:analysis*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D018505:Waste Management
chemical_list: D005782:Gels; D011850:Radioactive Waste; D011868:Radioisotopes; D001546:Bentonite