AbstractThe development of a locking structure of in-vessel components such as blanket vessels is one of the key issues for a tokamak fusion reactor. A concept of a locking structure with a cotter (wedge) and cotter driver has been proposed. We considered the transformation of a metallic balloon (MB) with thin wall and flat shape cross-section very promising as a cotter driver. We tried to fabricate the MB made of different kinds of titanium alloy by superplastic forming (SPF). Several mockups of the MB were successfully fabricated and used for performance tests. A mockup of the locking structure with blanket vessels, cotter structures and MBs has also been fabricated. Using this mockup, the locking mechanism was verified through an experimental study.