title: Long-term diffusion of U(VI) in bentonite: Dependence on density.
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, Deand Da, decreased with increasing dry density. The Davalues for the out-diffusion of natural U(VI) were in good agreement with previously determined values. Surprisingly, Davalues 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.
authors.affiliation: 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.
authors.forename: Claudia
authors.initials: C
authors.lastname: Joseph
authors.affiliation: Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Mibus
authors.affiliation: Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany.
authors.forename: Paul
authors.initials: P
authors.lastname: Trepte
authors.affiliation: Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany.
authors.forename: Christa
authors.initials: C
authors.lastname: Müller
authors.affiliation: Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany.
authors.forename: Vinzenz
authors.initials: V
authors.lastname: Brendler
authors.affiliation: Biosciences and Biotechnology Division, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.
authors.forename: Dan M
authors.initials: DM
authors.lastname: Park
authors.affiliation: Biosciences and Biotechnology Division, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.
authors.forename: Yongqin
authors.initials: Y
authors.lastname: Jiao
authors.affiliation: Glenn T. Seaborg Institute, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.
authors.forename: Annie B
authors.initials: AB
authors.lastname: Kersting
authors.affiliation: Glenn T. Seaborg Institute, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA.
authors.forename: Mavrik
authors.initials: M
authors.lastname: Zavarin
keywords: Clay; Extended Archie's law; MX-80; Nuclear waste repository; Speciation; Uranium