Development of repair techniques in the reactor vessel of the experimental fast reactor Joyo; Summary of Joyo restoration works

Ashida, Takashi ; Takamatsu, Misao ; Ito, Hideaki; Okawa, Toshikatsu; Yoshihara, Shizuya

In the experimental fast reactor Joyo, it was confirmed that the top of the irradiation test sub-assembly (S/A) 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 summary of Joyo restoration works. In-vessel repair techniques for sodium cooled fast reactors (SFRs) are important in confirming safety and integrity of SFRs. However, the demonstrated techniques under the actual reactor environment with high temperature, high radiation dose and remaining sodium are not enough to secure the reliability of these techniques. The experience and knowledge accumulated in the UCS replacement provides valuable insights into further improvements for in-vessel repair techniques in SFRs.