(497a) Hydrogen and Helium Retention in Plasma-Facing Tungsten

A uniquely harsh operating environment of a fusion power plant with combined stressors makes harnessing fusion energy a challenging materials problem. Combined stressors such as extremely high particle fluxes and heat loads that modify the microstructure of plasma-facing components (PFCs) are a major concern. Another such critical issue, an operational challenge related to deuterium–tritium (D‒T) fuel fusion plants, is to minimize tritium retention within materials to keep the total onsite tritium inventories below the regulatory safety threshold. For its many advantageous properties, including low tritium retention, tungsten is the leading candidate for the PFC material. The divertor, a critical PFC used for removing helium produced in the D‒T fusion reaction, is expected to undergo severe structural damages due to its exposure to very high helium fluxes under typical operating conditions. Helium (He) is insoluble in metals and will form clusters and 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 tungsten divertor 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, hydrogen isotopes, 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. 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 the plasma instabilities that induce ELMs. We will also present cluster-dynamics simulation results that include self-clustering of helium and/or tritium partitioning toward helium clusters and will 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 in fusion power plants.