Current status of restoration work for obstacle and upper core structure in reactor vessel of experimental fast reactor "JOYO", 3-2; Replacement of upper core structure

Ushiki, Hiroshi; Ito, Hiromichi; Okuda, Eiji; Suzuki, Nobuhiro; Sasaki, Jun; Ota, Katsu; Kawahara, Hirotaka ; Takamatsu, Misao ; Nagai, Akinori; Okawa, Toshikatsu

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 bent onto the in-vessel storage rack as an obstacle and had damaged the upper core structure (UCS) in 2007. As a part of the restoration work, UCS replacement was begun at March 24, 2014 and was completed at December 17. In-vessel repair (including observation) for sodium-cooled fast reactors (SFRs) is distinct from that for light water reactors and necessitates independent development. Application of developed in-vessel repair techniques to operation and maintenance of SFRs enhanced their safety and integrity. There is little UCS replacement experience in the world and this experience and insights, which were accumulated in the replacement work of in-vessel large sturucture (UCS) used for more than 30 years, are expected to improve the in-vessel repair techniques in SFRs.