The Korea Superconducting Tokamak Advanced Research (KSTAR) device, constructed in 2008, is a world-class superconducting tokamak fusion research device for the development of fusion energy. The expected heating power goal has been set to 12 MW by using an additional heating system, i.e., the second neutral beam injection (NBI) system NBI-2. As the heating power increases, resistance to high heat flux and cooling capacity at the divertor should be improved to exhaust power in the scrape-off-layer domain. Therefore, an upgrade of the divertor system for KSTAR was launched in 2019, and the upgrade divertor will be installed by 2022. The peak heat flux on the divertor target in steady-state operation is set to 10 MW/m2, and the ITER-like divertor type, the water-cooled tungsten monoblock, has been applied.

The upgrade KSTAR divertor system comprises 64 cassette divertor modules. A divertor module consists of the inner target, the central target, the outer target, and the cassette body with supports to connect each part. In this study, thermal analyses were carried out to confirm the design’s thermal robustness for a whole divertor module. The temperature distribution and pressure drop were calculated by computational fluid dynamics analyses. Based on the response surface optimization method, the optimized tungsten monoblock design was derived. The optimized monoblock design showed that all materials, tungsten, Cu, and CuCrZr, comprising the divertor target, are operated within their allowable temperature windows. For the global divertor model applying the optimized monoblock design, steady-state and transient analyses were carried out for heat fluxes of 10 and 20 MW/m2. At 10 MW/m2, all composing materials were operated within the allowable temperature, while the maximum temperatures of tungsten, Cu, and CuCrZr exceeded the allowable temperature range of 20 MW/m2. However, the results were acceptable since the temperatures are sufficiently lower than the melting temperatures, and the slow transient case occurs quickly.