Evaluation of tritium burnup fraction for CFETR scenarios with core-edge coupling simulations

A key mission for the next-step fusion tokamak device China Fusion Engineering Test Reactor (CFETR) is demonstrating tritium self-sufficiency, which requires a sufficiently high tritium burnup fraction (f(burnup)) in order to match a practically achievable tritium breeding ratio (TBR) with the blanket design constraints. Core-edge coupling simulations are performed to investigate the dependence of f(burnup) on different controlling parameters for CFETR scenarios. Core plasma profiles with a range of pedestal densities are simulated by consistent iterative calculations of equilibrium, transport, auxiliary heating and current drives within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate f(burnup) with the OMFIT modelled core plasma parameters as input. According to the simulations, f(burnup) can be effectively increased with a higher pedestal density on account of the fusion power increasing faster than the fueling source required to maintain steady-state. Higher can also increase the f(burnup) due to increase of fuel recycling. Deeper pellet fueling deposition and lower ratio of particle to thermal diffusivities D/chi can both increase the effective particle confinement time and thus f(burnup). However, the effect of helium and other impurities (Ar and W) is shown to reduce f(burnup) for comparable impurity and main ion transport. Based on our analysis, using present pellet fueling technology, achieving f(burnup) > 3% for CFETR will be very challenging. This is a lower limit for the required TBR (>1) to match the achievable TBR for tritium self-sufficiency. Furthermore, our study suggests that if fueling can penetrate deeper than r/a < 0.8 under optimistic conditions, the required burnup fraction could be attainable. The modelling results thus provide important suggestions and implications for the optimization of CFETR scenarios and development of advanced fueling systems.