Development of Inspection and Repair Techniques for Reactor Vessel of Experimental Fast Reactor “Joyo” Replacement of Upper Core Structure

Abstract

 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” was broken and bent onto the in-vessel storage rack as an obstacle, damaging the upper core structure (UCS). In this paper, we describe the in-vessel repair techniques for UCS replacement, which are developed in Joyo. The UCS replacement was conducted in the following four stages: (1) jack-up of the existing damaged UCS, (2) retrieval of the existing damaged UCS, (3) installation of the 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 the obtained results were applied to the design of special handling equipment. The UCS replacement was successfully completed in 2014. In-vessel repair techniques for sodium-cooled fast reactors (SFRs) are important in confirming the safety and integrity of SFRs. However, the techniques demonstrated in the actual reactor environment with high temperature, high radiation dose, and remaining sodium are insufficient to secure the reliability of these techniques. The experience and knowledge accumulated in the UCS replacement provide valuable insights into further improvements of in-vessel repair techniques for SFRs.