Clay minerals naturally found in deep geological formations (like the Callovo-Oxfordian argillite, a potential host formation to dispose high-level radioacive waste in France) or used in constructing engeenered geochemical barriers at the waste storage sites are particularly benefitial for such applications because of their many favorable properties (low permeability, high soprtion and ion exchange capacity, swelling in the presence of water…) that makes them capable to considerably reduce the mobility of radioactive substances in the environment. This mobility reduction involves adsoprtion and/or retention mechanisms through a combination of various physical and chemical processes taking place at the clay-water interface. The molecular mechanisms controlling these phisico-chemical processes are still insufficiently understood. Computational molecular modelling techniques have already become an important tool in the study of thermodynamic, structural and transport properties of hydrated clays. We are applying some of these techniques to bring out quantitative molecular scale information related to the interactions of hydrated clay particles and some radoactive substances.
The generalized mineralogical composition of the Callovo-Oxfordian argillite shows about 41% of clay minerals (23% of interstratified illite/smectite (I/S), 14% of illite-type minerals, 2% kaolinite and 2% chlorite) and a non-negligible amount of organic matter is also present (~1%). From a qualitative perspective, there are many different minerals in the Callovo-Oxfordian argillite. One may then think that Callovo-Oxfordian argillite properties are averaged contributions of all the properties of the mineral species found in its composition. For this reason, we consider important to study the phisico-chemical properties of each of these minerals species separately. From a quantitative perspective, it also appears to be important to first look at the most abundant minerals in view of their contribution to the behavior of the total complex mixture. In this sense interstratified I/S and illite-type minerals have the largest proportion. Illite (non swelling) and smectite (swelling) clays (making up interstratified I/S minerals) also have cationic adsoprtion capacities among the highest as compared with other clay minerals, and the adsorbing surfaces of the I/S particles are known to be smectitic. For these reasons, we decided to focus on smectite clay minerals first.Organic molecules, constituting about 1% of COx composition can be potential vectors for the transport of radionuclides in the Callovo-Oxfordian argillite according to previous works. Temperature effects should also not be neglected because of the heat generated in geological repositories of radiacotive waste during the residual decay processes.
Our objective in this research project is to use molecular dynamics method to get detailed quantitative microscopic scale information on the structural, dynamic, and energetic aspects of the interactions of some important waste radionuclides (Cs+, Sr2+, Ni2+, UO22+) with a smectite-clay called montmorillonite. At the same time we will be looking at the effect of organic molecules and temperetaure on these interactions.