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Contribution to the study of tritium behaviour in beryllium (ITER context)

Laura Ferry has defended her thesis on 13th November 2017 in Saint-Paul-lez-Durance.​

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PSN-RES/SAG/LETR

Authors > FERRY L.

Publication Date > 13/11/2017

Summary

​Beryllium will be used as a plasma-facing material for the ITER vacuum vessel. Due to high plasma fluxes, significant amounts of hydrogen isotopes could be retained in the beryllium walls. From the safety point of view, it is important to assess the capability of devices which will be used to limit the tritium inventory in the tokamak in nominal conditions and secondly, to predict the behaviour of tritium in case of accident. The objective of this work is to evaluate within the framework of the Density Functional Theory the behaviour of tritium in beryllium in terms of retention and desorption mechanisms. Firstly, the stability of point defects in beryllium has been evaluated and compared to experiments.

Vacancies are shown to be the dominant defect in beryllium. Then, the most stable interstitial sites for tritium atoms and the most favorable migration pathways have been determined. This study has been extended to multiple-trapping phenomenon in monovacancy, in which up to five atoms can be trapped. These data have been used in a kinetic Monte-Carlo code to calculate the diffusion coefficient of tritium and a reaction-diffusion based model, which provides a good agreement with experimental thermal desorption spectra made at IEK-4 (Jülich – Germany). The emergence of desorption peak at low temperature under high fluence could be explained by the hydride formation. For future modelling improvements, some thermodynamic properties of beryllium hydride have been evaluated and compared to experimental data, showing a good agreement.

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