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Radiolysis in 2D-and ID-confining materials

Sophie Le Caer 1
1 LIONS - Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
Abstract : 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).1 In this context, the production of H 2 by clay mineraIs 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. Moreover, clay mineraIs provide a unique and very interesting system to study radiolysis in a model 2D-confining materiaI. Indeed, clay mineraIs are hydrous silicates of aluminum, magnesium, or iron displaying a layered structure. In the sheets, T0 4 tetrahedral (T) sites usually occupied by Si 4 +, AI 3 +, or Fe 3 + cations interconnected by three vertices, combine themselves into pseudo-hexagons (T04)6 linked to a sheet of octahedral (0) sites, usually occupied by AI 3 +, Fe 3 +, Mg 2 +, or Fe 2 + cations and form 2D sheets (Figures la and lb). By changing the nature of the atoms which are present, the amount of water can be tuned: in talc, no water is present (Figure la) whereas its amount can be varied in swelling clays such as montmorillonite or saponite for example (Figure lb) due to the presence of cations in the interlayer space. We have shown that under irradiation by accelerated electrons, the dihydrogen production in synthetic talc, which is solely due to structure hydroxyl groups, is of the same order of magnitude as the one obtained in liquid water? This yield is divided by 30 in the case of natural talc from Luzenac, evidencing the importance of the impurities as scavengers of the precursors of dihydrogen. In the case of synthetic montmorillonite and saponite, the radiolysis of water confined in the interlayer space, that has a thickness of a few A, leads to H 2 yields which are two to three times higher than the one measured in water? Moreover, these yields are similar for montmorillonite and saponite, evidencing that the charge location only plays a minor role in the H2 production. Layered double hydroxides (LDH) are analogous of clay mineraIs (Figure lc). They are also a layered material, where the positive charges of the sheets are compensated by the presence of anions in the interlamellar space. In this case, the anion can play a role in the radiation chemistry, contrary to the case of clay mineraIs for which the cation (Na+ in the present work) is inert towards ionizing radiation. We have shown that the nature of the anion controls the H2 production in LDHs. A last system of interest is imogolite 4 (Figure Id) which is an aluminosilicate nanotube. It enables understanding the radiation chemistry of water confined in an ID geometry. Parallel to the H 2 production measurements, other characterization techniques such as electron paramagnetic experiments have enabled proposing reaction schemes. Reaction mechanisms accounting for H 2 production in confined media will be proposed and discussed according to the nature of the confining matrix, the amount of water .... AlI these results are of interest in the context of the disposaI of radioactive waste.
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Sophie Le Caer. Radiolysis in 2D-and ID-confining materials. 2nd International Conference on Ionizing Processes (ICIP 2018), Jul 2018, Annapolis, United States. ⟨cea-02327886⟩

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