Influence of dry density on HTO diffusion in GMZ bentonite

Effect of compaction on bisulfide diffusive transport through MX-80 bentonite

Canada's deep geological repository (DGR) design includes an engineered barrier system where highly compacted bentonite (HCB) surrounds the copper-coated used fuel containers (UFCs). Microbial-influenced corrosion is a potential threat to long-term integrity of UFC as bisulfide (HS-) may be produced by microbial activities under anaerobic conditions and transported via diffusion through the HCB to reach the UFC surface, resulting in corrosion of copper. Therefore, understanding HS- transport mechanisms through HCB is critical for accurate prediction of copper corrosion allowance. This study investigated HS- transport behaviour through MX-80 bentonite at dry densities 1070-1615 kg m-3 by performing through-diffusion experiments. Following HS- diffusion, bromide (Br-) diffusion and Raman spectroscopy analyses were performed to explore possible physical or mineralogical alterations of bentonite caused by interacting with HS-. In addition, accessible porosity ε was estimated using extended Archie's law. Effective diffusion coefficient of HS- was found 2.5 × 10-12 m2 s-1 and 5.0× 10-12 m2 s-1 for dry densities 1330 and 1070 kg m-3, respectively. No HS- breakthrough was observed for highly compacted bentonite (1535-1615 kg m-3) over the experimental timeframe (170 days). Raman spectroscopy results revealed that HS- reacted with iron in bentonite and precipitated as mackinawite and, therefore, it was immobilized. Finally, results of this study imply that HS- transport towards UFC will be highly controlled by the available iron content and dry density of the buffer material.

A sensitive improved method for analyzing diffusion coefficients of Cs in compacted bentonite with different lengths

Based on the one-dimensional diffusion theory, the diffusion parameters were obtained from numerical fitting by the Cyclic Initial Value (CIV) program written by MATLAB. Taking the through-diffusion experimental of cesium (stable isotope 133Cs) as an example, on the premise of ensuring accuracy, fitting calculation was used to obtain the diffusion equilibrium time of Cs in different lengths bentonite column. The fitting results of diffusion test for tritium water (HTO) and Cs are both very well. The calculation results of the equilibrium time for Cs diffusion show that the equilibrium time obtained by CIV is less than the experimental period in both groundwater (GW) and seawater (SW). In GW environment, when the sensitivity is at the maximum setting value, the diffusion coefficient of Cs in 1.5 cm bentonite column could be calculated in a shorter period of time. Compared with the experimental period, the time was shortened by 110 days. The main purpose is to verify the feasibility of CIV through the experimental data of Cs in different column lengths. The CIV program can also be used to fit and calculate the experimental data of other ongoing diffusion experiments and get the equilibrium time required for diffusion experiments. It shows that the proposed models offer the advantages of saving experimental time and reducing experimental waste.

Investigation of Re(VII) diffusion in Tamusu clayrock core by through-diffusion method

Tamusu area is the primary pre-selection site of clayrock disposal repository for high-level radioactive waste (HLW) in China. However, the research on the migration behavior of nuclides in Tamusu clayrock is still in its infancy. For the first time in laboratory, the diffusion behavior of Re(VII) in Tamusu clayrock core was studied by means of through-diffusion method. The effects of pH, ionic strength and humic acid on the diffusion behavior of Re(VII) in clayrock were studied. The effective diffusion coefficient, apparent diffusion coefficient and rock capacity factor value were obtained. All the experimental conditions of Re(VII) diffusion in Tamusu clayrock are compared with other geological samples under the same conditions in literature data. The diffusion mechanism of radionuclide in Tamusu clay is discussed, which can provide experimental data for site selection and safety assessment of high-level radioactive waste repository in China. The experimental results showed an effective application and reference for the countries disposed HLW in mudrocks or clayrocks, such as France, Belgium etc. in Europe. Moreover, this research can provide the original data support for the metallogenic regularity and prospecting prognosis of rare element rhenium in different geological environments.

Long-term diffusion of U(VI) in bentonite: Dependence on density

As a contribution to the safety assessment of nuclear waste repositories, U(VI) diffusion through the potential buffer material MX-80 bentonite was investigated at three clay dry densities over six years. Synthetic MX-80 model pore water was used as background electrolyte. Speciation calculations showed that Ca₂UO₂(CO₃)₃(aq) was the main U(VI) species. The in- and out-diffusion of U(VI) was investigated separately. U(VI) diffused about 3mm, 1.5mm, and 1mm into the clay plug at ρ=1.3, 1.6, and 1.9g/cm³, respectively. No through-diffusion of the U(VI) tracer was observed. However, leaching of natural uranium contained in the clay occurred and uranium was detected in all receiving reservoirs. As expected, the effective and apparent diffusion coefficients, D_e and D_a, decreased with increasing dry density. The D_a values for the out-diffusion of natural U(VI) were in good agreement with previously determined values. Surprisingly, D_a values for the in-diffusion of U(VI) were about two orders of magnitude lower than values obtained in short-term in-diffusion experiments reported in the literature. Some potential reasons for this behavior that were evaluated are changes of the U(VI) speciation within the clay (precipitation, reduction) or changes of the clay porosity and pore connectivity with time. By applying Archie's law and the extended Archie's law, it was estimated that a significantly smaller effective porosity must be present for the long-term in-diffusion of U(VI). The results suggest that long-term studies of key transport phenomena may reveal additional processes that can directly impact long-term repository safety assessments.