Surface conditions of W components : Impact on hydrogen inventory
Mykola IALOVEGA
Mardi 23/02/2021, 09h30-11h30
Salle René GRAVIER 506 rdc, CEA Cadarache

AVIS DE SOUTENANCE

Monsieur Mykola IALOVEGA

Soutiendra publiquement ses travaux de thèse intitulés :

Surface conditions of W components : Impact on hydrogen inventory

Soutenance prévue le mardi 23 février 2021 à 9H30

Lieu :   IRFM - René Gravier - Centre de Cadarache 13115 Saint paul lez Durance

Et par SKYPE (voir lien ci-dessous)

Composition du jury proposé :

M. Thierry Angot

Aix Marseille Université

Directeur de thèse

Mme Anne-Lise Thomann

Université d’Orléans

Rapporteur

M. Wolfgang Jacob

Max Planck Institute

Rapporteur

M. Christian Linsmeier

Forschungszentrum Jülich

Examinateur

Mme Sabina Markelj

Jozef Stefan Institute

Examinatrice

M. Christian Grisolia

CEA Cadarache

Examinateur

Mme Céline Martin

Aix Marseille Université

Examinatrice

M. Régis Bisson

Aix Marseille Université

Invité

Mme Elodie Bernard

CEA Cadarache/IRFM

Examinatrice, encadrant CEA 

Résumé :

Investigations of hydrogen isotopes and helium retention in plasma facing components (PFC) that are exposed to various plasma conditions are important for future fusion devices such as ITER and DEMO. Due to its favorable physical properties, in particular its high melting point, tungsten (W) has been chosen as the plasma-facing material of the ITER divetor. In the deuterium/tritium (D/T) phase of ITER, W PFC will be subjected to intense fluxes composed of hydrogen isotopes (HI), helium (He), impurities and neutrons.

In particular, it has been found that He significantly affects W PFC near surface, with the formation of dislocation loops, bubbles, or even W-fuzz. The presence of impurities in the edge plasma may cause redeposition or codeposition of mixed layers on the surface of the PFC W, and in the presence of residual oxygen, surface oxidation is possible due to the high temperature of the ITER divertor. Such structural modifications of W PFC may considerably modify the properties of the material, and therefore its life expectancy, as well as its hydrogen retention, which arises safety concerns as tritium is radioactive.

In this PHD thesis, we used laboratory experiments involving ion implantation and thermal desorption spectrometry (TDS) technique to investigate the fundamental retention properties of HI in W PFC due to different surface conditions: 1) Presence of an oxide layer formed on the surface of polycrystalline W (PCW) in ITER relevant conditions, 2) Presence of structural changes due to an impact of ITER relevant helium irradiation.

The TDS measurements were coupled with microscopy observations in order to characterize the modifications occurring on the surface and in the bulk of the material at different scales: scanning electron and confocal laser scanning microscopy techniques were used for surface observations from micrometer to nanometer scale; transmission electron microscopy was used for cross-sectional observations. Raman and X-ray spectroscopy techniques were used to characterize the structure and chemical composition of the samples.

TDS experiments involving low energy D ion implantation or T gas loading on oxidized PCW samples highlighted drastically different retention mechanisms of HI in tungsten oxides as compared to pristine PCW. The initial surface condition of the material (for example the presence of adsorbed water) can also significantly modify the outgassing of HI. We have also evidenced a change in the color of the oxide following HI implantation, that suggests tungsten bronze formation. The fact that the oxide grown on PCW was not modified upon D implantation and TDS experiments (the surface morphology and elemental composition were restored), raises additional concerns for the safe long-term operation of W PFC in ITER.

In the other set of experiments, we exposed PCW to helium plasma in fluence conditions relevant to ITER and WEST both at low and high flux. Surface and bulk morphology observations combined with sequences of low flux and low fluence 250 eV D ion implantations and TDS measurements up to 1200-1350 K revealed interesting effects of the thermal cycling on He-induced defects evolution and deuterium retention. D implantation and thermal cycling experiments highlighted a major impact of thermal cycling on the evolution of defects created by He and deuterium retention. D retention in the He bubbles enriched near-surface is increased 3 to 8-fold as compared to non-damaged PCW. The evolution of D retention upon thermal cycling appears to be linked with the density and size of various He bubbles. Furthermore, we show that high temperature annealing induces He bubbles faceting into a polyhedron shape as well as a faceting of holes into a rectangular shape on

the surface.

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