Engineering evaluation of the upgrade KSTAR divertor system

Abstract

KSTAR (Korea Superconducting Tokamak Advanced Research) plans to upgrade the external heating power to 24 MW by improving heating systems such as NBI, ECH, ICRF, and so on. The upgrade divertor system should guarantee resistance to high heat flux and cooling capacity for the exhausting power in the scrape-off layer domain. The activity for the upgrade of the KSTAR divertor was initiated in 2019, and the upgrade divertor was successfully manufactured and installed in October 2023.The upgraded KSTAR divertor system employs the water-cooled tungsten monoblock, ITER-like divertor type. Tungsten is the most robust and promising plasma-facing material under high heat flux plasma circumstances, and the combination of CuCrZr heat sink and pressurized water coolant is the effective cooling method. The upgraded KSTAR divertor system has a single null configuration and 64 cassette divertor modules placed at the bottom of the vacuum vessel. A divertor module consists of the inner target, the central target, the outer target, and the cassette body, with supports to connect each part. CFD analysis was carried out in the previous study to confirm the thermal stability of a whole divertor module. The result showed the design could be operated within a thermal allowable range in 10 MW/m² heat flux. The temperature distribution from CFD analysis is applied to the thermo-mechanical analysis. Based on the ASME code, the upgrade KSTAR divertor was estimated for plastic collapse, ratcheting, fatigue, and buckling. The result showed the upgraded KSTAR divertor is reliable from the thermal and mechanical points of view. In the KSTAR divertor, tungsten monoblocks were used only in the straight section due to space constraints. However, in anticipation of a new Korean fusion device following KSTAR, we also explored the production technology for a divertor target that includes a curved section. While the hot isostatic pressing (HIP) process was utilized for dissimilar metal bonding in the KSTAR divertor, we applied the hot radial pressing (HRP) process for manufacturing the curved section target to diversify our production methods. A small mock-up sample of the curved section, manufactured using the HIP and HRP, underwent testing for 20 MW/m² during 1,000 high-heat flux tests. Both the HIP and HRP samples were completed without any issues. The result confirms the design and quality of the KSTAR divertor target were reliable enough to withstand the heat load, although the recrystallization of tungsten occurred.