T. Missana

Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland: impact of reaction kinetics and speciation on radionuclide migration

Summary The influence of smectite colloids on the migration behaviour of U(VI), Th(IV), Pu(IV), Am(III), Np(V), Sr(II) and Cs(I) is investigated within the Colloid and Radionuclide Retardation experiment (CRR). Two in situ experiments in a well-characterized granitic fracture zone are carried out in presence and absence of bentonite colloids. Radionuclide retardation observed in the field studies increases in the sequence Np(V)∼U(VI)<Sr(II)<Cs(I), where a small fraction of colloid borne breakthrough is only stated for Cs(I) in presence of bentonite colloids. Am(III) and Th/Pu(IV) mainly migrate as colloids without retardation in the presence and absence of smectitic colloids. The radionuclide migration behaviour is discussed on the basis of results obtained in laboratory batch sorption experiments and spectroscopic studies. Consistent with the field observation, laboratory derived Kd values increase in the order Np(V)∼U(VI)<Sr(II)<Cs(I). Significant kinetic hindrance for the sorption to fault gauge minerals is observed for Sr(II) and Cs(I), but notably for Am(III) and Pu(IV). The slow sorption reaction of tri- and tetravalent actinide ions is explained by their kinetically hindered dissociation from colloidal species. In order to explain the colloidal behaviour of tri- and tetravalent actinides even in absence of bentonite colloids, ultracentrifugation and spectroscopic experiments are performed. It is found that up to 60% of Pu(IV) and Am(III) species can be centrifuged off. Adding Cm(III) (5×10-8 mol L-1) into both injection solutions instead of Am(III) allows for a spectroscopic study by using the time resolved laser fluorescence spectroscopy (TRLFS). Peak position and fluorescence lifetimes (λ=604 nm, τ=110-114 μs) together with the fact that Cm(III) can be widely separated by ultracentrifugation, suggest the existence of inner-sphere surface complexes on groundwater and bentonite colloids. Carbon K-edge XANES analysis of the bentonite colloids reveal the presence of natural organic constituents. They are mainly of aliphatic nature containing high fractions of carboxylate groups. A contribution of these organic species towards the bentonite colloid stability and sorption of actinides is assumed to be likely.

Experimental study and modelling of selenite sorption onto illite and smectite clays.

This study provides a large set of experimental selenite sorption data for pure smectite and illite. Similar sorption behavior existed in both clays: linear within a large range of the Se concentrations investigated (from 1x10(-10) to 1x10(-3) M); and independent of ionic strength. Selenite sorption was also analysed in the illite/smectite system with the clays mixed in two different proportions, as follows: (a) 30% illite-70% smectite and (b) 43% illite-57% smectite. The objective of the study was to provide the simplest model possible to fit the experimental data, a model also capable of describing selenite sorption in binary illite/smectite clay systems. Selenite sorption data, separately obtained in the single mineral systems, were modeled using both a one- and a two-site non-electrostatic model that took into account the formation of two complexes at the edge sites of the clay. Although the use of a two-site model slightly improved the fit of data at a pH below 4, the simpler one-site model reproduced satisfactorily all the sorption data from pH 3 to 8. The complexation constants obtained by fitting sorption data of the individual minerals were incorporated into a model to predict the adsorption of selenium in the illite/smectite mixtures; the models predictions were consistent with the experimental adsorption data.

Solute transport properties of compacted Ca-bentonite used in FEBEX project

The present Spanish concept of a deep geological high level waste repository includes an engineered clay barrier around the canister. The clay presents a very high sorption capability for radionuclides and a very small hydraulic conductivity, so that the migration process of solutes is limited by sorption and diffusion processes. Therefore, diffusion and distribution coefficients in compacted bentonite (i.e. in realistic liquid to solid ratio conditions) are the main parameters that have to be obtained in order to characterise solute transport that could be produced after the canister breakdown. Through-Diffusion (TD) and In-Diffusion (ID) experiments with HTO, Sr, Cs and Se were carried out using compacted FEBEX bentonite, which is the reference material for the Spanish concept of radioactive waste disposal. Experiments were interpreted by means of available analytical solutions that allow the estimation of diffusion coefficients and, in some cases, distribution coefficients. Analytical solutions are simple to use, but rely on hypotheses that do not hold in all the experiments. These experiments were interpreted also using an automatic parameter estimation code that overcomes the limitations of analytical solutions. Numerical interpretation allows the simultaneous estimation of porosity, diffusion and distribution coefficients, accounts for the role of porous sinters and time-varying boundary concentrations, and can use different types of raw concentration data.

Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal

Thermodynamic sorption models (TSMs) offer the potential to improve the incorporation of sorption in environmental modelling of contaminant migration. One specific application is safety cases for radioactive waste repositories, in which radionuclide sorption on mineral surfaces is usually described using distribution coefficients (Kd values). TSMs can be utilised to provide a scientific basis for the range of Kd values included in the repository safety case, and for assessing the response of Kd to changes in chemical conditions. The development of a TSM involves a series of decisions on model features such as numbers and types of surface sites, sorption reactions and electrostatic correction factors. There has been a lack of consensus on the best ways to develop such models, and on the methods of determination of associated parameter values. The present paper therefore presents recommendations on a number of aspects of model development, which are applicable both to radioactive waste disposal and broader environmental applications. The TSM should be calibrated using a comprehensive sorption data set for the contaminant of interest, showing the impact of major geochemical parameters including pH, ionic strength, contaminant concentration, the effect of ligands, and major competing ions. Complex natural materials should be thoroughly characterised in terms of mineralogy, surface area, cation exchange capacity, and presence of impurities. During the application of numerical optimisation programs to simulate sorption data, it is often preferable that the TSM should be fitted to the experimentally determined Kd parameter, rather than to the frequently used percentage sorbed. Two different modelling approaches, the component additivity and generalised composite, can be used for modelling sorption data for complex materials such as soils. Both approaches may be coupled to the same critically reviewed aqueous thermodynamic data sets, and may incorporate the same, or similar, surface reactions and surface species. The quality of the final sorption model can be assessed against the following characteristics: an appropriate level of complexity, documented and traceable decisions, internal consistency, limitations on the number of adjustable parameter values, an adequate fit to a comprehensive calibration data set, and capability of simulating independent data sets. Key recommendations for the process of TSM development include: definition of modelling objectives, identification of major decision points, a clear decision-making rationale with reference to experimental or theoretical evidence, utilisation of a suitable consultative and iterative model development process, testing to the maximum practicable extent, and thorough documentation of key decisions. These recommendations are consistent with international benchmarks for environmental modelling.

Determination of Granites' Mineral Specific Porosities by PMMA Method and FESEM/EDAX

Over extended periods, long-lived radionuclides (RN) or activation products within geologic disposal sites may be released from the fuel and migrate to the geo/biosphere. In the bedrock, contaminants will be transported along fractures by advection and retarded by sorption on mineral surfaces and by molecular diffusion into stagnant pore water in the matrix along a connected system of pores and micro-fissures. The objective of this paper was to determine the connective porosity and mineral-specific porosities for three granite samples by {sup 14}C methyl-methacrylate ({sup 14}C-PMMA) autoradiography. Scanning electron microscopy and energy-dispersive X-ray analyses (FESEM/EDAX) were performed in order to study the pore apertures of porous regions in greater detail and to identify the corresponding minerals. Finally, the porosity results were used to evaluate the diffusion coefficients of RNs from previous experiments which determined apparent diffusion coefficients for the main minerals in three granite samples by the Rutherford Backscattering technique. The total porosity of the Grimsel granite (0.75%) was significantly higher than the porosities of the El Berrocal and Los Ratones granites (0.3%). The porosities of the Grimsel granite feldspars were two to three times higher than the porosities of the El Berrocal and Los Ratones granites feldspars. However, there wasmorexa0» no significant difference between the porosities of the dark minerals. A clear difference was found between the various quartz grains. Quartz crystals were non-porous in the El Berrocal and Los Ratones granites when measured by the PMMA method, but the quartz crystals in the Grimsel granite showed 0.5% intra granular porosity. The apparent diffusion coefficients calculated for uranium diffusion within Grimsel granite on different minerals were very similar (2.10{sup -13} {+-} 0.5 m{sup 2}/s), but differences within both Spanish granites were found from one mineral to another (9 {+-} 1.10{sup -14} m{sup 2}/s in feldspars and 4.5 {+-} 0.5.10{sup -14} m{sup 2}/s in quartz) - always presenting lower diffusion values than in the Grimsel granite. (authors)«xa0less

A clay permeable reactive barrier to remove Cs-137 from groundwater: Column experiments

Clay minerals are reputed sorbents for Cs-137 and can be used as a low-permeability material to prevent groundwater flow. Therefore, clay barriers are employed to seal Cs-137 polluted areas and nuclear waste repositories. This work is motivated by cases where groundwater flow cannot be impeded. A permeable and reactive barrier to retain Cs-137 was tested. The trapping mechanism is based on the sorption of cesium on illite-containing clay. The permeability of the reactive material is provided by mixing clay on a matrix of wood shavings. Column tests combined with reactive transport modeling were performed to check both reactivity and permeability. Hydraulic conductivity of the mixture (10(-4) m/s) was sufficient to ensure an adequate hydraulic performance of an eventual barrier excavated in most aquifers. A number of column experiments confirmed Cs retention under different flow rates and inflow solutions. A 1D reactive transport model based on a cation-exchange mechanism was built. It was calibrated with batch experiments for high concentrations of NH4+ and K+ (the main competitors of Cs in the exchange positions). The model predicted satisfactorily the results of the column experiments. Once validated, it was used to investigate the performance and duration of a 2 m thick barrier under different scenarios (flow, clay content, Cs-137 and K concentration).

Experimental study of colloid interactions with rock surfaces

In subsurface environments, colloids may play a role in the transport of contaminants to the biosphere because of their high capacity to adsorb them. The electrostatic interactions between colloids and the heterogeneous rock surfaces will determine the fate and transport of both colloids and contaminants adsorbed on the particles. Thus, the aim of the present work was to study the electrostatic interactions between colloidal particles and a crystalline rock surface. Since different minerals are expected to act as preferential reactive areas, the spatial distribution of colloids on the mineral surfaces was studied, accounting several conditions, by means of the nuclear ion beam technique μ-Particle Induced X-ray Emission (μPIXE).

Study of the Uranium Heterogeneous Diffusion through Crystalline Rocks and Effects of the “Clay-Mediated” Transport

Rock matrix diffusion is one of the possible mechanisms for radionuclide retardation in a deep geological high-level radioactive waste repository, and it is usually considered that radionuclides diffuse as solutes through the rock. Nonetheless, the potential effects that clay, from the bentonite barrier, may induce on the radionuclides migration should be taken into account. Furthermore, transport models generally assume that the whole mineral surface is accessible to transport, whereas transport is highly conditioned by the heterogeneous mineral distribution, since different minerals may act as preferential pathways, while others may present higher sorption capability. It is therefore necessary to determine the actual surface area accessible to transport. The aim of the present work is the identification of the uranium preferential pathways to the granite, both in presence or absence of bentonite clay. Results showed that uranium as solute diffused in specific mineral areas, indicating that the actual surface area accessible to matrix diffusion, and/or sorption on the surface, is significantly lower than the whole mineral surface. By the other hand, the uranium in presence of the clay was randomly distributed on the surface, and penetrated into the granite mainly through “defects” (as fractures or grain boundaries); its migration being enhanced on specially fractured or disturbed areas.

Migration experiments in compacted Ca-bentonite

Migration experiments were carried out in compacted Ca-bentonite as a complement of the FEBEX [Full-Scale Engineered Barriers Experiment in Crystalline Host Rock] project large scale tests. Through diffusion and permeation experiments with triated water were used to perform a deep characterization of the clay diffusion parameters (diffusion coefficient, effective porosity and permeability). In through diffusion experiments, a variation of tracer concentration in both reservoirs was allowed. Since in these experimental conditions an analytical solution of the diffusion problem does not exist, an analytical approximation was used and the results compared with a numerical model which takes into account the presence of the sintered steel filters confining the clay plug. The migration behavior of Se in this compacted bentonite has been studied and the retardation factor obtained. The calculated Kd is at least one order of magnitude lower than the one obtained by means of batch sorption experiments due to a lower availability of sorption sites when increasing the compaction degree.

Ion beam analyses of radionuclide migration in heterogeneous rocks

The migration of radionuclides (RN) in the environment is a topic of general interest, for its implications on public health, and it is an issue for the long-term safety studies of deep geological repositories (DGR) for high-level radioactive waste. The role played by colloids on RN migration is also of great concern. Diffusion and sorption are fundamental mechanisms controlling RN migration in rocks and many experimental approaches are applied to determine transport parameters for low sorbing RN in homogeneous rocks. However, it is difficult to obtain relevant data for high sorbing RN or colloids, for which diffusion lengths are extremely short, or within heterogeneous rocks, where transport might be different in different minerals. The ion beam techniques Rutherford Backscattering Spectrometry (RBS) and micro-Particle Induced X-Ray Emission (μPIXE), rarely applied in the field, were selected for their micro-analytical potential to study RN diffusion and surface retention within heterogeneous rocks. Main...