Numerical Calculation on Recycling Ratio of Tritium from Tungsten Wall Used in Current CFETR Design

Abstract

The recycling of tritium from plasma facing wall is an important neutral fuel source (in atomic and molecular form) for plasma confinement and particle control. In this study, the recycling process at tungsten wall based on current CFETR design was modeled. Monte Carlo code SRIM was used to model the implantation of energetic tritium ions into pure tungsten and to get back-scattering fraction of ions and the distribution of implanted tritium ions. The diffusion process of atoms in materials, with recombination at surface as boundary condition, was simulated using numerical approach for both stead and transient state. The total recycling ratio was contributed by fast process (implantation and back scattering) and slow process (diffusion and recombination) and its value nearly equals to 1 for stead state. Temporal dependence of total recycling ratio mainly depended on the slow process and was limited by diffusion coefficient in the bulk near surface and existence of traps in material. For tungsten material with good surface condition, the time of 90% recycling was characterized as 1 ms and affected by temperature, recombination coefficient and concentration of traps while the thickness of material had less affection. Isotope effect that recycling ratio of tritium was larger than that of deuterium at the same situation was also found in the simulation and this effect may affect particle balance and fueling in D-T plasma operation. A collection of theoretical models to estimate the recycling ratio and its time dependence were also summarized and validated by the simulation results.