Helium bubble formation in Tungsten: from nanoscience understanding to macroscale impact – HEBUTERNE

The HEBUTERNE project is linked to the development of nuclear fusion for energy production. In tokamaks, the walls of the plasma chamber will be subjected to extreme operating conditions. In particular, the tungsten chosen to cover a critical component of the machine will be exposed to a high flux of helium which may degrade its structure. This project aims at understanding of the He-W interaction by studying the formation and growth of helium bubbles in tungsten, an open question that needs to be solved in order to improve the codes that allow the prediction of the behavior of components in tokamaks. For this purpose, we propose to use state-of-the-art techniques developed in nanoscience, an innovative approach in the field of nuclear fusion. We will first characterize experimentally the dynamics of He-W interactions under model conditions, and then to approach the conditions expected in tokamaks. Six partners with complementary expertise are joining forces to carry out this project, which is divided into several tasks involving experimentation and modeling based on the results obtained: - WP1: Dynamics of the He-W interaction and in situ characterization. The objective is to describe the fundamental processes from the very beginning of the He bubble formation via the study of W single crystals of different crystallographic orientations submitted to an exposure of monokinetic He ions and characterized in operando by central X-ray scattering at grazing incidence (GISAXS) and X-ray diffraction thanks to synchrotron radiation (collaboration in progress with the BM32 line at ESRF). The energy of He implantation will be varied in order to evaluate the impact of the creation of defects during He implantation (i.e. below and above the minimum energy of displacement of the W by He). A particular focus will also be put on the impact of temperature during He implantation and post-implantation (annealing). -WP2: Towards real Plasma Facing materials interactions: larger range of He exposure and impact of grain boundaries. The study of polycrystalline samples is planned in order to evaluate the impact of grain boundaries on bubble formation and mobility. Exposures to various He plasmas will be carried out on the PHISIS and PIMAT facilities, in order to estimate the impact of the type of irradiation (higher flux and fluence) on the evolution of the morphology of the bubbles. Some exposures will use 3He for irradiation to allow the detection of small quantities accessible via nuclear reaction analysis (NRA). An important focus will be kept on a parametric study for the impact of temperature on the various mechanisms observed, once again during He implantation and after annealing. A methodology similar to the WP1 one will be adopted: characterization of the bubbles formed by microscopy (size, shape, distribution), estimation of the He inventory, impact of temperature parameters during or after implantation. - WP3: Integration of the experimental results into a multi-scale model, from the atomic level to the continuous medium, a crucial step towards the extrapolation of the mechanisms at the origin of the expected evolutions in lTER. A thermomechanical finite element model, based on Abaqus software, will be used to predict the He bubble pressure at rupture depending on temperature, depth and bubble size. The results will be used to propose a bursting model including all these dependencies. To better understand the role of dislocations, atomistic simulations will allow an accurate description of the interatomic binding energy landscape using the TAMMBER code. These new data will then be included in a cluster dynamic code. Last but not least, the evolution of the bubble shape will be modeled, based on experimental observations, by integrating the effect of plasticity on the appearance of facets.