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 3 mm, 1.5 mm, and 1 mm into the clay plug at rho = 1.3, 1.6, and 1.9 g/cm(3), 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, De 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. (C) 2016 Elsevier B.V. All rights reserved.