(305b) Modeling of Surface Morphological Evolution of Plasma-Facing Tungsten in Fusion Reactors

Tungsten (W) and tungsten alloys are widely viewed as the most promising plasma facing material (PFM) candidates for divertor and first-wall systems in a nuclear fusion reactor. However, experiments aiming to investigate PFM performance under continuous operation and high first-wall temperature have shown that a nanostructure with a fuzz-like morphology develops on the W surface under the operating conditions of temperature, helium (He) impact energy, and He flux expected for ITER’s divertor. Formation of such fuzz nanostructure may adversely influence the reactor performance and operation. The aim of our work is to obtain a fundamental understanding of the initial stage of fuzz formation by modeling the surface morphological evolution of helium-ion-irradiated tungsten considered as a PFM.

We have developed an atomistically-informed (with constitutive equations parameterized using results of large-scale molecular-dynamics simulations [1]), continuous-domain model to describe the surface morphological evolution of helium-ion-irradiated tungsten. Based on this model, we have conducted self-consistent numerical simulations of the dynamics of the irradiated W surface morphology and benchmarked the simulation results by systematic comparisons with experimental measurements. Under the experimental conditions, namely, an RF plasma source (~3×10²⁰m^-2s^-1) exposure of 75-eV He on ITER-grade W at 840^°C, the surface morphology predicted by our simulations is in good qualitative agreement with the experimental observations for the early stage of nanotendril formation on the W surface, a precursor to fuzz-like surface growth. Additionally, quantitative comparisons between the experimental data and the simulation results show that our model predicts the growth rate of the nanotendrils reasonably well, and provides reasonable approximate estimates for the nanotendril width and arrangement on the W surface. Two plausible factors affecting the predictive capabilities of our model are hypothesized: subsurface bubble dynamics and bubble bursting/pinhole formation. Future model extensions to test these hypotheses, as well as the anticipated divertor performance are discussed. Furthermore, we have investigated the dependence of the nanotendril width and nanotendril arrangement on the W surface on the He fluence and the W surface temperature.

[1] K. D. Hammond, S. Blondel, L. Hu, D. Maroudas, and B. D. Wirth, Acta Materialia144, 561-578 (2018).