Reaction mechanisms in swelling clays under ionizing radiation: impact on the geological storage of the nuclear waste

Overview & Synopsis

The influence of ionizing radiation on clay mineraIs is poorly known, in spite of their use as a major component of the engineered barrier in High Level Nuclear Waste Repositories (HLNWR). In tbis context, the production of H$_2$ by clay minerals under ionizing radiation could be a real issue. It can, e.g., lead to the loss of radionuclide retention properties by creating cracks in the engineered barrier. It is thus important to determine H$_2$ formation reaction mechanisms and to understand the role of several parameters on this production, such as the water amount, the presence of impurities and the nature of the clay mineraI. We studied synthetic clay mineraIs (talc, which has no water molecules, montmorillonite and saponite which exhibit charge excess in different locations) [1-2] and also natural montmorillonites from Camp Berteau [3]. The reactivity under electron irradiation is deciphered thanks to H$_2$ production measurements at well-defined relative humidities together with electron spin resonance experiments that allow detecting the defects created upon irradiation. Comparing the reactivity induced in the synthetic samples enables understanding the impact of the presence of two-dimensionally confmed water molecules and the influence of the charge location in the sheets of the clay mineraI. Moreover, the comparison between synthetic and natural talc evidences the strong role played by impurities on the H$_2$ production yields [1]. We find that H$_2$ production is due to the presence of structural-OH groups and that the confmement of water molecules is a crucial factor. Moreover, the charge location in the sheets of the clay mineraIs plays only a marginal role [1-2]. AlI these results will be discussed and the corresponding reaction mechanisms will be presented. Lastly, the role of impurities in natural clay mineraIs such as Fe$^{3+}$ that strongly decrease the H$_2$ production will be discussed.