Radionuclide Sorption and Transport in Brine Solutions

Abstract

The Canadian Nuclear Waste Management Organization (NWMO) is responsible for implementing Adaptive Phased Management (APM), Canada's plan for the long-term management of used nuclear fuel produced by Canada's nuclear reactors. The goal of APM is a long-term containment and isolation of used nuclear fuel in a Deep Geologic Repository (DGR) constructed in a suitable crystalline or sedimentary rock formation. Sedimentary rocks in Canada are known to contain Na-Ca-Cl brine solutions with total dissolved solids (TDS) in the range of 200 to300 g/L. The sorption properties of shales and limestone and for clay-based materials used in engineered barriers are required in evaluations of the safety of a deep geological repository in a Canadian sedimentary setting. The process of defining the role of sorption in the transport of radionuclides in these rocks includes: (1) batch sorption experiments in brine solutions to develop an understanding of processes and mechanisms; (2) the establishment of a defensible database of sorption values; and (3) demonstration that sorption values can be applied to explain mass transport processes.

The sorption of Sr, Ni, Cu, Eu and U was measured on bentonite, shale and limestone in Na-Ca- Cl brine solutions with TDS values as high as 300 g/L. Initial results have confirmed that group 1 and 2 elements, which sorb by non-specific coulombic attraction, are not likely to sorb in brine solutions. Transition metals, such as Ni and Cu, and the trivalent Eu and hexavalent U sorb by surface complexation mechanisms to bentonite, shale and limestone. The high concentrations of Ca in the brine were found to compete with Ni for sorption sites and the formation of complexes with carbonate reduced the sorption of Eu and U.

The development of a sorption database for sedimentary rocks (shale and limestone) and bentonite, in a setting that would include Na-Ca-Cl brine solutions at near neutral pH, was initiated. The elements of interest for performance assessment included C, Cu, As, Se, Zr, Nb, Mo, Tc, Pd, Sn, Pb, Bi, Ra, Th, Pa, U, Np, Pu and Am. The process included a search of the international literature for data relevant for bentonite, shale, limestone, illite, chlorite and calcite in high salinity water, and a consideration of results from new batch experiments. The sorption values determined as part of this process are intended as a starting point for the development of a Canadian sorption database for sedimentary rocks. Future work will include batch and mass transport experiments, combined with modelling to fill data gaps and improve understanding of sorption processes during mass transport, and continued development of a sorption database for sedimentary rocks.