(84g) Impact of Soret Effect on Hydrogen and Helium Retention in Plasma-Facing Tungsten Under ELM-like Conditions

A uniquely harsh operating environment of a fusion power plant with combined stressors makes harnessing fusion energy a challenging materials problem, and is a key factor for considering fusion to be one of the major grand challenges for engineering in the 21^st century. Combined stressors like extremely high particle fluxes and heat loads that modify the plasma-facing components (PFCs) materials microstructure are a major concern. Another such critical issue, an operational challenge related to D‒T fuel fusion plants, is to minimize tritium retention within materials to keep total onsite tritium inventories in the ~1 kg per plant range with environmental releases below ~1 g/year. For its many advantageous properties, including low tritium retention, tungsten is chosen as the PFC material for ITER (International Thermonuclear Experimental Reactor). One such critical PFC is the divertor, the exhaust system used for removing helium produced in the D‒T fusion reaction, which is expected to undergo severe structural damages due to its exposure to helium flux under typical operating conditions. Helium is insoluble in metals and will form clusters/bubbles in PFC tungsten; these bubbles are formed close to the plasma-exposed surface and trap hydrogenic species. On the other hand, a steep temperature gradient developed in the divertor tungsten under extreme transient heat loads, characteristic of plasma instabilities that induce edge localized modes (ELMs), would promote thermal-gradient-driven diffusion of helium and hydrogenic species and, thus, influence the deuterium and tritium retention in the PFC. Taking all the aforementioned physics into account, we focus here on developing a predictive model for hydrogenic species and helium retention under fusion reactor relevant conditions.

To fundamentally understand the impact on PFC tungsten of thermal-gradient-driven diffusion, commonly called ‘Soret effect’, we have used nonequilibrium molecular-dynamics (NEMD) simulations to analyze the transport of He, mobile helium clusters, H, and self-interstitial atoms (SIAs) in the presence of a thermal gradient in pristine tungsten. We have found that all the species examined tend to migrate from the cooler to the hot regions of the tungsten sample, characterized by a negative heat of transport [1,2]. The findings of our thermal and species transport analysis have been implemented in our cluster-dynamics code, Xolotl, which has been used to compute temperature and species profiles over spatiotemporal scales representative of PFC tungsten under typical reactor operating conditions, including extreme heat loads at the plasma-facing surface characteristic of plasma instabilities that induce ELMs [2]. Xolotl simulations of individual species transport predict that accounting for temperature-gradient-driven species transport has a very significant effect on the steady-state species profiles in plasma-facing tungsten. We will also present the cluster-dynamics simulation results that include self-clustering of helium and/or tritium partitioning toward helium clusters and discuss the influence of the Soret effect on helium and hydrogenic species retention inside PFC tungsten, as well as plasma fueling and tritium flux at the cooling tube side. All these results and analysis are crucial for fuel cycle assessment.

[1] E. Martinez, N. Mathew, D. Perez, S. Blondel, D. Dasgupta, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 130, 215904 (2021).

[2] D. Dasgupta, S. Blondel, E. Martinez, D. Maroudas, and B. D. Wirth, Nucl. Fusion 63, 076029 (2023).