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

Claudia Joseph; Jens Mibus; Paul Trepte; Christa Müller; Vinzenz Brendler; Dan M Park; Yongqin Jiao; Annie B Kersting; Mavrik Zavarin

doi:10.1016/j.scitotenv.2016.10.005

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

Sci Total Environ. 2017 Jan 1:575:207-218. doi: 10.1016/j.scitotenv.2016.10.005. Epub 2016 Oct 12.

Authors

Claudia Joseph¹, Jens Mibus², Paul Trepte², Christa Müller², Vinzenz Brendler², Dan M Park³, Yongqin Jiao³, Annie B Kersting⁴, Mavrik Zavarin⁴

Affiliations

¹ Glenn T. Seaborg Institute, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA. Electronic address: joseph20@llnl.gov.
² Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany.
³ Biosciences and Biotechnology Division, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.
⁴ Glenn T. Seaborg Institute, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.

PMID: 27741456
DOI: 10.1016/j.scitotenv.2016.10.005

Abstract

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.

Keywords: Clay; Extended Archie's law; MX-80; Nuclear waste repository; Speciation; Uranium.