Development of repair techniques in the reactor vessel of the experiment fast reactor 'Joyo'. Replacement of upper core structure

In the experimental fast reactor 'Joyo', it was confirmed that the top of the irradiation test sub-assembly of 'MARICO-2' (material testing rig with temperature control) had been broken and bent onto the in-vessel storage rack as an obstacle and had damaged the upper core structure (UCS). This incident necessitated the replacement of the UCS and the retrieval of the MARICO-2 sub-assembly for 'Joyo' restoration. This paper describes the in-vessel repair techniques for UCS replacement, which are developed in Joyo. The UCS replacement was conducted along with four stages involving (1) jack-up of the damaged UCS, (2) retrieval of the damaged UCS, (3) installation of O-ring, and (4) insertion of the new UCS. Since the UCS replacement was not anticipated in the original design, the work conditions at Joyo were carefully investigated, and those results were applied to the design of special handling equipment. The UCS replacement was successfully completed in 2014. (author)