Difference in Distribution and Flux of Tritium and Deuterium in Fusion Power Plant First Wall

Plasma-facing materials and components in a fusion power plant are subjected to a high flux of a mixture of hydrogen isotopes, deuterium, and tritium. They penetrate the first wall, and while most of them return to the plasma, a substantial amount diffuses through the wall and can either reach the opposite side or get trapped in structure defects created by neutron irradiation.Usually, it is considered that the distribution of deuterium and tritium in the bulk of the first wall is the same as in the incoming flux, i.e. equal concentrations of each isotope. However, the mass difference between deuterium and tritium leads to differences in the concentrations. This is related to the different reflection ratios and penetration depths of incoming ions, the difference in the diffusion coefficients, and the different detrapping energies due to zero-point energy contributions.We summarize the processes influencing the difference between the isotopes and use the open-source finite element code FESTIM to obtain numerical results for the time-dependent and steady-state distribution of isotopes in the first wall, as well as the flux of these isotopes across the wall and into the coolant. We show that relatively small differences in the properties of the isotopes of hydrogen can have an observable influence on their retention and flux in plasma-facing surfaces. Deuterium flux across the first wall was found to be higher than the flux of tritium, while tritium inventory in the wall can be up to 10% higher than that of deuterium in the case of a traps-free first wall.