Clay mineralogical-organic processes in barrier systems

Country / Region: Worldwide

Begin of project: January 1, 2002

End of project: July 31, 2014

Status of project: July 23, 2010

Fig.1: Bentonite application in in barrier systems for high level radioactive waste (HLRW) confinement Source: BGR

The aim of the study is to investigate the properties of bentonites which are planned to be used in barrier systems for high level radioactive waste (HLRW) confinement (Figure 1). Based on systematic investigation of different bentonites and their performance in laboratory tests an optimized bentonite barrier will be suggested.

Clay minerals of the bentonite barriers are possibly altered with respect to their arrangement (microstructure) or even chemically modified under the conditions expected. The aim of the study is to identify the optimum bentonite mainly with respect to its stability. This will finally lead to an optimized barrier system. Hence the study is focussed on “mineralogical-geochemical-geotechnical investigations of bentonites for the production of optimized HLRW bentonite barriers”.

The project attempts to answer the question which bentonite properties are most important with respect to the desired properties of the bentonite barrier. First, the actually relevant range of the different important bentonite properties was identified. Secondly a significant set of different bentonites samples from all over the world was chosen.

Several studies are available dealing with the applicability of bentonites in HLRW repositories. Such studies, commonly, focus on a few different materials only. In addition, the commonly varied parameter is the type of exchangeable cation. In fact several parameters of bentonite exist each showing a wide range. This results in rarely foreseeable properties of different bentonites, a problem which is particularly known from daily praxis of the bentonite producing industry. Therefore a common praxis in the bentonite industry is to try to identify the most suitable bentonite for a given application which is usually done by screening tests. In the present study we additionally try to identify the reason for the different performance of different bentonites. In this respect particular focus is on the rarely considered bentonite parameters: microstructure (arrangement of minerals and aggregates), layer charge density distribution and clustering of atoms (non-statistical distribution of elements within the smectite crystal). These parameters are believed to be helpful for the identification of the reason for different bentonite performances and will lead to an improved understanding of the system bentonite-water-contaminant.

The scientific work of Dr. Kaufhold in the field of clay-radwaste was honoured 2012 with the Karl-Jasmund-Preis of the German Clay Group DTTG

Project results

In the frame of international research projects many different concepts are tested for the safe disposal of high level radioactive waste (HLRW) mostly considering the isolation of the HLRW in underground waste repositories. In most of these concepts an additional barrier is included to separate the metal canister containing the radwaste from the host rock. Such a barrier may be particularly useful in the case of fractured host rocks such as granite. Scientific and engineering tests yet indicate the suitability of a highly swelling clay as additional barrier. This type of clay is called bentonite and used for sealing of landfills or contaminated sites since decades. The bentonites´ swelling capacity results in a low hydraulic conductivity and both pore structure and surface charge render it an effective adsorbent for the retention of different hazardous substances. Bentonites are produced from different deposits. Its composition and performance in different applications significantly varies from one deposit to another. Therefore, not any bentonite can be used for the production of a specific product (for a specific application).

Fig. 2: Schematic representation of the bentonite barrier enclosing the metal canister. The 10 key requirements or key issues were sketched additionally Source: BGR

Bentonite industry first identifies raw materials suitable for a specific application before supplying the market. However, the application of bentonite as barrier in radwaste repositories is new, valuable experience for the selection of a suitable bentonite, therefore, is missing. As a result of a 12 years project BGR summarized the first criteria to distinguish suitable from less suitable radwaste bentonites. These conclusions were drawn based on both laboratory and in-situ tests in underground rock laboratories. First 10 important requirements the bentonite has to fulfil were identified. Secondly these requirements were compared with the variance of bentonites from a set of different deposits. Some parameters, such as hydraulic and thermal conductivity turned out to depend more on the way of producing the bentonite barrier elements than on the choice of material. Stability related parameters, in contrast, differed significantly from one type of bentonite to another. In conclusion, a suitable radwaste bentonite should contain no or few soluble and/or reactive components such as organic matter or sulfur-phases such as pyrite and gypsum. Because of lower tendency towards detachment of colloidal particles which may be relevant for erosion processes Ca/Mg-bentonites should be preferred over Na-bentonites. Also Fe-poor bentonites should be preferred because of larger chemical and thermal stability. Finally, bentonites containing smectites with low layer charge density result in higher corrosion rates in contact with metal-Fe and hence should be avoided if Fe is used as canister material. These quality determining parameters will be complemented by future research finally leading to the possibility to select an ideal bentonite.

Literature:

• Kaufhold, S., Pohlmann-Lorz, M., Dohrmann, R., Nüesch, R. 2007. About the possible upgrade of compacted bentonite with respect to iodide retention capacity. Applied Clay Science, 35, 39-46
• Plötze, M., Kahr, G., Dohrmann, R., Weber, H. 2007. Hydro-mechanical, geochemical and mineralogical characteristics of the bentonite buffer in a heater experiment. The HE-B project at the Mont Terri rock laboratory. Physics and Chemistry of the Earth, 32, 730–740
• Kaufhold, S., Dohrmann, R. 2008. Detachment of colloids from bentonites in water. Applied Clay Science, 39, 50-59
• Kaufhold, S., Dohrmann, R., Koch, D., Houben, G. 2008. The pH of aqueous bentonite suspensions. Clays and Clay Minerals, 56 (3), 338–343.
• Kaufhold, S., Dohrmann, R. 2008. Comparison of the traditional Enslin-Neff method and the modified Dieng method for measuring the water uptake capacity. Clays and Clay Minerals, 56 (6), 686–692
• Kaufhold, S., Dohrmann, R. 2009. Stability of bentonites in salt solutions I sodium chloride. Applied Clay Science, 45, 171-177
• Kaufhold, S., Stührenberg, D., Dohrmann, R. 2009. Water redistribution between bentonite and salt at elevated temperature. Applied Clay Science, 46, 245–250
• Kaufhold, S., Dohrmann, R., Klinkenberg, M. 2010. Water uptake capacity of bentonites. Clays and Clay Minerals, 58, 37–43
• Klinkenberg, M., Kaufhold, S., Dohrmann, R., Siegesmund, S. 2009. Abrasivity by bentonite dispersions. Applied Clay Science 46, 37–42
• Kaufhold, S., Dohrmann, R. 2010. Effect of extensive drying on the cation exchange capacity of bentonites. Clay Minerals, 45, 441-448.
• Kaufhold, S., Dohrmann, R. 2010. Stability of bentonites in salt solutions II. Potassium chloride solution — Initial step of illitization? Applied Clay Science, 49, 98-107.
• Kaufhold, S., Dohrmann, R., Klinkenberg, M. 2010. Water uptake capacity of bentonites. Clays and Clay Minerals, 58, 37–43.
• Kaufhold, S., Dohrmann, R., Klinkenberg, M., Siegesmund, S., Ufer, K. 2010. N2-BET specific surface area of bentonites. Journal of Colloid and Interface Science, 349, 275-282
• Kaufhold, S., Dohrmann, R. 2011. Stability of bentonites in salt solutions III Ca-hydroxide solutions. Applied Clay Science, 51, 300-307
• Kaufhold, S., Dohrmann, R., Ufer, K., Kleeberg, R., Stanjek, H. 2011. Termination of swelling capacity of smectites by Cu-trien exchange. Clay Minerals, 46, 411–420
• Kaufhold, S., Stanjek, H., Penner, D., Dohrmann, R. 2011. The acidity of surface groups of dioctahedral smectites. Clay Minerals 46, 583-592.
• Dohrmann, R., Genske, D., Karnland, O., Kaufhold, S., Kiviranta, L., Olsson, S., Plötze, M., Sandén, T., Sellin, P., Svensson, D., Valter, M. 2012. Interlaboratory CEC and ex-changeable cation study of bentonite buffer materials: I. Cu(II)-triethylentetramine method. Clays and Clay Minerals, 60, 162–175.
• Dohrmann, R., Genske, D., Karnland, O., Kaufhold, S., Kiviranta, L., Olsson, S., Plötze, M., Sandén, T., Sellin, P., Svensson, D., Valter, M. 2012. Interlaboratory CEC and ex-changeable cation study of bentonite buffer materials: II. Alternative methods. Clays and Clay Minerals, 60, 176-185.
• Dohrmann, R., Olsson, S., Kaufhold, S., Sellin, P. 2013. Mineralogical investigations of the first package of the alternative buffer material test – II. Exchangeable cation population rearrangement. Clay Minerals, 48, 215–233.
• Kaufhold, S., Dohrmann, R., Sandén, T., Sellin, P., Svensson, D. 2013. Mineralogical investigations of the first package of the alternative buffer material test – I. Alteration of bentonites. Clay Minerals, 48, 199–213.
• Kaufhold, S., Plötze, M., Klinkenberg, M., Dohrmann, R. 2013 Density and porosity of bentonites. Journal of Porous Materials, 20, 191–208.
• Kaufhold, S., Dohrmann, R. 2013. The variable charge of dioctahedral smectites. Journal of Colloid And Interface Science 390, 225–233.
• Kaufhold, S., Klinkenberg, M., Dohrmann, R. 2013. Comparison of the dry densities of highly compacted bentonites. Clay Minerals, 48, 105–115.
• Landais, P., Kaufhold, S., Dohrmann, R. 2013. Overview of the clay mineralogy studies presented at the 'Clays in natural and engineered barriers for radioactive waste confinement' meeting, Montpellier, September 2012. Clay Minerals, 48, 149-152
• Dohrmann, R., Kaufhold, S. 2014. Cation exchange and mineral reactions observed in MX 80 buffers samples of the prototype repository in situ experiment in Äspö, Sweden. Clays and Clay Minerals, 62, 357–373.
• Kaufhold, S., Baille, W., Schanz, T., Dohrmann, R. 2015. About differences of swelling pressure - dry density relations of compacted bentonites. Applied Clay Science, 107, 52–61
• Kaufhold, S., Sanders, D., Hassel, A.-W., Dohrmann, R. 2015. Corrosion of high-level ra-dioactive waste iron-canisters in contact with bentonite. Journal of Hazardous Materials, 285, 464–473