Spatially dependent cluster dynamics model of He plasma surface interaction in tungsten for fusion relevant conditions

Abstract

In fusion reactors, plasma facing components (PFC) and, in particular, the divertor will be irradiated with high fluxes of lowenergy (∼100 eV) helium and hydrogen ions. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behaviour of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The PFC response and performance changes are intimately related to microstructural changes, such as the formation of point defect clusters, helium and hydrogen bubbles or dislocation loops. Computational materials' modelling results are described here that investigate the mechanisms controlling microstructural evolution in tungsten. The aim of this study is to understand and predict sub-surface helium bubble growth under high flux helium ion implantation (∼10 22 m −2 s −1 ) at high temperatures (>1000 K). We report results from a spatially dependent cluster dynamics model based on reaction-diffusion rate theory to describe the evolution of the microstructure under these conditions. The key input parameters to the model (diffusion coefficients, migration and binding energies, initial defect production) are determined from a combination of atomistic modelling and available experimental data. The results are in good agreement with results of an analytical model that is presented in a separate paper. In particular, it is found that the sub-surface evolution with respect to bubble size and concentration of the helium bubbles strongly depends on the flux and temperature.

Influence of exposure conditions on helium transport and bubble growth in tungsten

“… He clusters concentration profiles in the tendril at 500 K, 1000 K and 1500 K. Comparison between the current implementation (solid) and Faney’s results 30 (dashed). Discrepancies at high temperature are likely due to the use of a different set of dissociation energies.…”

“…In this section, the current implementation is first compared with the one from Faney 30 to ensure the additional assumptions do not produce different results. A standard half-slab case is then described and a parametric study is performed by varying the exposure conditions.…”

“…An effort has been made to assess He transport in W using atomistically informed macroscopic models called cluster dynamics models implemented using finite differences. Some examples of such implementations are the work of Faney et al 29 , 30 and the Xolotl code 31 . The simulated results are promising but these codes require substantial computational resources considering the thousands of equations that need to be solved.…”

Temperature dependence of underdense nanostructure formation in tungsten under helium irradiation

“…Along these lines, a four-step process has been proposed by Ito et al [22,23] at 2000 K: (i) He ions penetrate the W surface; (ii) He diffusion and agglomeration even at interstitial sites occurs; (iii) He bubbles grow at both vacancy and the interstices and (iv) based on hybrid MD-MC simulations, they explained how He bubbles burst, forming fuzzy nanostructures. Further, the dynamics of small He clusters formed near the W surface has been extensively studied [24][25][26][27][28][29]. Nevertheless, no quantitative studies were found in the literature addressing why fuzz formation occurs only in this limited temperature window.…”

Object kinetic Monte Carlo simulations of irradiated tungsten for nuclear fusion reactors

“…Along these lines, a four-step process has been proposed by Ito et al [67,195] at 2000 K: (i) He ions penetrate the W surface; (ii) He diffusion and agglomeration even at interstitial sites occurs; (iii) He bubbles grow at both vacancy and the interstices and (iv) based on hybrid Molecular Dynamicskinetic Monte Carlo simulations, they explained how He bubbles burst, forming fuzzy nanostructures. Moreover, the behaviour of small He clusters when forming near the W surface has been extensively studied [190,[196][197][198][199][200]. Nevertheless, no quantitative studies were found in the literature addressing why fuzz formation occurs only in this limited temperature window.…”