Development of inspection and repair techniques in reactor vessel of experimental fast reactor 'Joyo'. Retrieval of the bent test subassembly

In the experimental fast reactor Joyo, it was confirmed that the top of the irradiation test subassembly of the material testing rig named 'MARICO-2' had been broken and bent onto the in-vessel storage rack as an obstacle and had damaged the upper core structure (UCS). As a result, to restart Joyo, it was necessary to replace the damaged UCS and to retrieve the bent subassembly. This paper describes the in-vessel repair techniques performed in the retrieval of the obstacle inside the reactor vessel. The retrieval was mainly conducted in five stages: (1) installation of a temporary shielding plug and the formation of a temporary boundary, (2) installation of the retrieval device, (3) retrieval of the bent subassembly, (4) storage of the bent subassembly in a cask, and (5) transportation of the cask to a postirradiation examination facility. The devices which were prepared for this work demonstrated the expected performance under the environmental conditions of a sodium-cooled fast reactor (SFR) such as a high temperature and radiation dose, and the work was completed in 2014. The successful operation to retrieve the damaged component inside the reactor vessel will contribute to the development of in-service inspections and repair techniques in SFRs. (author)