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Étude par DFT statique et dynamique de l'adsorption de H2O/O2/H2 dans une zéolithe LTA, en vue de la recombinaison des gaz issus de la radiolyse de l'eau tritiée en entreposage.

Abstract : O2 and H2 molecules are amongst the main stable products of the water radiolysis. Previous experimental studies on the radiolysis of water in 4A zeolite [L. Frances et al, J. Phys. Chem. C, vol. 119, no. 51, pp. 28462–28469, 2015] have shown the gradual disappearance of O2 and H2 molecules from the gaseous formation above the studied system, several hundred days after radiolysis has begun. Two interesting points were noticed from the follow-up of O2 and H2 gas production: 1) the decrease in their observed amount occurs synchronously but increasingly later, after the start of radiolysis, as the water loading rate increases, 2) there is no more sight of new gas production once the two molecules have disappeared from the gaseous formation.The hypothesis of a H2/O2 recombination to form H2O can be advanced to explain this disappearance. We, thus, studied through static and dynamic simulations, the adsorption of these three molecules in the Z4A structure to describe in detail their confined behavior: ---equilibrium positions of each molecule have been optimized on their adsorption sites to establish the important role of Na+ cations in the preparation of the adsorbates to radiolysis and recombination, ---their adsorption energies have been calculated to define a hierarchy in the adsorbates-zeolite affinity H2O>>O2>H2, ---and finally, the adsorption of the mixtures have been studied to explain the role of water on the delays, observed experimentally, before the decrease of the quantity of O2 and H2 molecules.Regarding the weak inter-adsorbates interactions, our results have led us to conclude that the water regulates the access of O2 and H2 molecules to the cationic sites, which is however a required condition to lead the two molecules towards a recombination reaction. It is then possible to put forward the hypothesis that it cannot takes place as soon as the cationic sites are no longer accessible to H2 and O2 molecules, i.e. after the loading rate at ~13% and until saturation. Our numerical studies describe the path of all the components of the system, towards the recombination reaction. They establish the conditions under which this reaction will take place. The results obtained during this work lead to the search for one (or several) event(s) triggering the reaction, which will be followed by the description of its mechanism.
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Submitted on : Thursday, June 23, 2022 - 11:19:13 AM
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  • HAL Id : tel-03702600, version 1

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Joharimanitra Randrianandraina. Étude par DFT statique et dynamique de l'adsorption de H2O/O2/H2 dans une zéolithe LTA, en vue de la recombinaison des gaz issus de la radiolyse de l'eau tritiée en entreposage.. Radiochimie. Université Bourgogne Franche-Comté, 2021. Français. ⟨NNT : 2021UBFCD037⟩. ⟨tel-03702600⟩

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