Development of manufacturing technology of radial plate in superconducting coil for fusion reactor by diffusion bonding by hot isostatic pressing

Abstract

The radial plates (RPs), which are used in the toroidal field (TF) coil in the International Thermonuclear Experimental Reactor (ITER), are quite large, 13 m tall and 9 m wide, but thin, 10 cm thick, and are made of stainless steel. Even though they are very large structures, they require very high manufacturing tolerances and high mechanical strength at 4 K. There will be similar requirements in the next-generation fusion reactor. Therefore, the authors intend to develop efficient manufacturing methods in parallel with ITER TF coil RP manufacture. The authors therefore performed trial manufacture of the RP segments using a diffusion bonding method, namely hot isostatic pressing (HIP). As a result of trials, it was clarified that even when HIP is applied, the mechanical characteristics of the base metal does not deteriorate. The machining period can be reduced by about 1/3 compared with the traditional manufacturing method. On the other hand, mechanical strength at 4 K is degraded due to weak bonding, that is no grain growth through joint, by HIP. However, an additional test indicates the promising possibility of a much better joint by higher temperature and joint surface-treated HIP. These results justified that RP segment manufacturing is not only possible, but it is a technically valid manufacturing method that satisfies all requirements.