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The divertor is an essential component in fusion reactors and must operate under extreme thermal loads and intense particle flux conditions. Consequently, most divertor designs, including ITER, adopt a water-cooled tungsten (W) monoblock (MB) concept. For fusion devices currently under construction or planned, neutrons generated by fusion reactions represent a critical design factor alongside high heat load. Thus, the divertor must play a vital role in shielding the vacuum vessel and superconducting magnets. From this perspective, neutron streaming occurring through inherent geometric gaps such as the poloidal gaps between MBs and the toroidal gaps between divertor modules is a crucial point in the fusion devices considering burning plasma. These gap regions are not protected by W armor, leading to direct exposure of divertor components and the vacuum vessel to high energy neutrons. As a result, neutron flux in these regions is higher than in W shielded areas, causing increased neutron-induced damage such as displacements per atom and helium production. In addition, these conditions significantly affect material activation due to the high-energy neutrons of 14.1 MeV.
In this study, the impact of neutron streaming through MB and inter-module gaps on divertor activation was analyzed. A simplified divertor model based on the Compact Pilot Device (CPD), currently under conceptual design in Korea, was employed. The model consists of W armor, a copper (Cu) interlayer, a CuCrZr heat sink, water coolant, and a cassette body made of Advanced Reduced Activation Alloy (ARAA). Compared to ITER, ARAA, a reduced-activation ferritic/martensitic steel, is used for the cassette body to mitigate long-term activation. Neutron transport calculations were performed using MCNP6 with the FENDL-3.1 library, and activation analyses were conducted using FISPACT-II with the TENDL-2017 library.
The results indicate that neutron flux in the gap regions of W, Cu, CuCrZr, and the ARAA cassette body is approximately 6–11% higher than that beneath the MBs. When this flux distribution is applied to activation calculations over a 100-year cooling period, the activation level in gap regions is found to be 11–37% higher than in MB regions. The surface of the cassette body shows differences of up to 65%. These results demonstrate that exposure to high energy neutrons through gaps can significantly amplify activation, even for relatively small increases in neutron flux.
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