Extended title: Tritium retention inside the ITER facility: from monitoring of tritium inventory to the source term evaluation during accidents

The ITER facility aims at demonstrating the feasibility of obtaining a net amount of energy from a magnetically confined plasma of deuterium and tritium. The choice for the protective materials for the vacuum vessel is limited due to the fact that several criteria must be fulfilled, e.g. resistant to extreme temperature and neutron fluxes during normal operations, having a low impact on the plasma, as well as a limited capacity to retain tritium for safety objectives. Considering these different constraints, beryllium has been selected for the first walls of the vacuum vessel.

The aim of this present work is the study of the tritium interaction with the extended defects of beryllium which will be created in a large amount in the material due to irradiation. These defects can be dislocations, voids, gas bubbles or precipitates like beryllium oxide. Some of them could trap a large amount of tritium. This work should support the improvement of the model which will be employed to assess the efficiency of the protective equipment of the ITER facility. The thesis work is then divided into two different topics as following explained.

Investigations of inter-granular extended defects

The beryllium microstructure is an important feature of material because it directly impacts the ductility of material and its swelling under irradiation. As first step, an in-depth bibliography analysis will be carried out in order to define the geometries of beryllium grains and especially their orientations in relation to each other. Then, the second step will be focused on the simulation of the grain boundaries behavior by molecular dynamic calculations. From these previous steps, the behavior of hydrogen isotopes will be studied to highlight the role of grain boundaries on the behavior of tritium in beryllium.

Investigations of tritium behaviour inside beryllium oxide

The second topic will be focused on the beryllium oxide. During the normal operation of the ITER facility, the formation of oxidised film onto the first walls as well as at grain boundaries is expected. Therefore, the understanding of the tritium behaviour in the beryllium oxide is of importance. The diffusion process will be studied either by atomic scale method like DFT or by a mesoscale approach as molecular dynamic. The obtained data will support the simulations, performed with a coupled reaction diffusion code, devoted to the calculations of the tritium release. Furthermore, the critical review related to diffusion process of tritium in metallic beryllium could be improved by taking into account the effect of thin oxidized film which always exists onto the beryllium samples. Finally, tritium reactivity with the beryllium oxide should allow exploring the formation mechanism of beryllium hydroxides.

Knowledge requirements:

-MSC in chemistry, physics or material science.

-Computational skills are required.

-Basic knowledge of ab-initio method like DFT as well as molecular dynamic would be better.

Contact: François Virot, [email protected]

Working place:  Cadarache (PACA district, France), a training period of several months at the CIRIMAT institute (Toulouse, France) is planned at the beginning of the thesis. Please visit our website https://www.irsn.fr/EN/Pages/home.aspx IRSN for more information.