Montmorillonite is swelling clay consisting of negatively charged aluminosilicate layers held together by interlayer cations. It was selected as the most suitable buffer material for the repository for high level radioactive waste. The main function of the clay buffer material is to retard the migration of the radio nuclides in the case of corrosion of the waste package material. The neutron diffraction experiment was curried out with using diffractometer D4 at the Institute Laue-Langevin (Grenoble, France). The aim of this experiment was to investigate by means of neutron diffraction with isotopic substitution the structural parameters of the hydration, hydrolysis and sorption of Sm and Eu, in the interlayer space of the montmorillonite and their dependence on chemical and physical factors, which are important for radioactive waste management. The experimental results are obtained for the hydrated samples of Sm-montmorillonite, prepared at low pH and pH>7 in order to find out whether Sm is present as aqueous Sm(OH)^o₃, Sm³⁺, or intermediate hydrolyzed species at pH>7, and how it is connected to the clay surface. The measurements with hydrated samples of Eu-montmorillonite, prepared at low pH were carried out for the estimation of the feasibility of our future neutron diffraction experiments, having the aim to study the possible changes of Eu coordination in clay due to the M³⁺ → Mn²⁺, Fe²⁺ → Fe³⁺ reaction. It was found that Sm³⁺ is bind to the clay surface, most probably in the syloxane cavities. In the case of pH>7 the slightly shorter Sm – O distance is indicated in comparison to the low PH samples. This might be interpreted as an indication of the fact that at least some of Sm is present as hydrolysed species. For the Eu-montmorillonite samples we obtained curves very similar to those for Sm. This result provides the proof of the feasibility of our future experiments on Eu – Fe-doped montmorillonite systems.

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