Research Activities in the Laboratory for Waste Management
Radioactive waste arising from nuclear power generation, as well as from medicine, industry and scientific research, poses a potential hazard over extended periods of time and must therefore be isolated from the human environment. The disposal of such wastes in engineered caverns far below the Earth's surface fulfills this purpose. The Laboratory for Waste Management (LES), in co-operation with the National Cooperative for the Disposal of Radioactive Waste (Nagra) carries out experiments and develops models which contribute to safety assessments of disposal concepts for radioactive waste. The staff consists of more than 30 scientists, from a broad range of natural sciences, and technicians. Full experimental demonstrations of the safety case are not possible because of the long timescales associated with the disposal of radioactive waste. The release of radionuclides into the human environment occurs only on time scales of tens of thousands to millions of years after the wastes have been placed in the subterranean repositories. For this reason, safety analyses rely on extrapolations and computations based on a sufficient understanding of the events and processes which influence the long term performance of the repository, and which can affect the transport of radionuclides from the repository back to the human environment. The necessary information is obtained from laboratory experiments and investigations on appropriate natural systems e.g. natural analogues and field experiments in underground research laboratories. On the basis of the results from such investigations, LES develops mathematical models which allow the safety of the disposal system to be numerically assessed.
The principal areas of research carried out by LES can be conveniently partitioned into three main parts covering the most important issues i.e. geochemical in situ conditions and their temporal evolution, radionuclide retention/retardation and transport mechanisms.
Radionuclide uptake on the materials in the near-field and the host rock is one of the main pillars upon which the safety case for a deep geological repository is built. The research activities aim at improving the understanding of the retention mechanisms and processes. One of the primary tasks of the Sorption Mechanism Group is to develop (and update) sorption data bases (SDBs) for the near-field (bentonite) and far-fields (Opalinus Clay, ‘Brauner Dogger’, Effingen Member and Helvetic Marl) for safety analyses. At the present time the SDBs consist of "smart" distribution ratios tailored to specific geochemical conditions. By definition the SDBs must be "state-of-the-art".
The "bottom up" strategy currently followed for the high clay mineral content argillaceous rocks is a key activity. The concept is to carry out experimental investigations on the major clay mineral near- and far-field repository components, elucidate the sorption mechanisms, and apply the “in house” developed 2 Site Protolysis Non Electrostaic Surface Complexation and Cation Exchange (2SPNE SC/CE) sorption model which can quantitatively describe the uptake of radionuclides over a wide range of conditions (pH, Eh, water chemistry, concentration). Since clay minerals are the main sorbing phases in the systems under consideration, the premise is that the sorption on argillaceous rocks and bentonite can be calculated by using the sorption models for the individual clay minerals modified by their modal compositions. This essentially wet chemistry/modelling approach is supported by surface spectroscopic techniques, e.g. X-ray absorption spectroscopy (XAS) which provide on a molecular level information on the sorbed species (coordination numbers, bond distances and system disorder).
Such a mechanistic modelling approach to sorption has the goal of developing a thermodynamic based sorption data base (TD-SDB). Such a TD-SDB readily lends itself to porting into a coupled code which can then be used to calculate radionuclide migration in PA studies without having to use Kd values directly.
Geosphere transport group uses interdisciplinary approach to merge experimental knowledge at the field and laboratory scale, geochemical and molecular modelling in order to assess geochemical and transport phenomena in geological repositories for radioactive waste. The major activities are focused on radionuclide transport modelling for needs of performance assessment, predictive modeling of the temporal and spatial evolution near- and far-fields in future underground repository. We combine simulation techniques at different scales to get mechanistic understanding of ion sorption and transport in heterogeneous porous materials.
The foci of the activities in the “Diffusion Processes” group are i) to understand the diffusion mechanism(s) of radionuclides in compacted argillaceous materials and ii) to measure diffusion parameters (effective diffusion coefficients and rock capacity values) which can be used in performance assessment studies. For this purpose laboratory experiments are carried out on (i) pure clay minerals and (ii) on heterogeneously composed clay rocks. The latter types of experiments are complemented by field studies carried out in the Underground Rock Laboratory of Mont Terri (North-Western part of Switzerland). Diffusion experiments carried out using highly compacted clay minerals or intact clay rocks also give valuable information regarding retention mechanisms under high solid to liquid ratios. They contribute thus significantly to an in-depth understanding of radionuclide uptake processes by the host rock material.
The experiences gained in these studies clearly showed that an in-depth understanding of the processes governing mass transfer processes in compacted argillaceous materials critically depends on the molecular dynamic properties of the diffusing species, on the geometric pore structure and on physico-chemical properties of the pore solution such as its composition. Diffusion can thus not be viewed simply as a physical transport process.
The Cement Systems group carries out an experimental research programme focusing on the sorption of radionuclides by cementitious materials and studies of near field processes related to the safe disposal of radioactive waste in a cement-based repository. The sorption project is performed with the aim of strengthening the credibility of sorption values recommended in cement sorption data bases, determining the influence of complexing agents (organic ligands, colloids) on sorption and developing a sufficiently detailed mechanistic understanding of sorption process by combining wet chemistry and spectroscopic techniques. Experimental studies on the porewater and phase compositions of cementitious materials and the fate of 14C containing organic compounds under the conditions of a cementitious near field are aimed at improving our understanding of the geochemical processes and the in situ conditions in a cement-based repository.