Inspection and repair techniques in reactor vessel of the experimental fast reactor JOYO. Investigation and design for retrieval device of bent MARICO-2 test subassembly

In the experimental fast reactor Joyo, the disconnecting of an irradiation test subassembly MARICO-2 (Material Testing Irradiation Rig with Temperature Control) from its holding mechanism was conducted in March 2007. In those operations, the rotating plug was rotated despite the fact that the test subassembly was not disconnected completely. Consequently, top of wrapper tube of the MARICO-2 test subassembly was bent onto the in-vessel storage rack. Since the overhanging part of the test subassembly is in the height in which contacts with the bottom of the Upper Core Structure (UCS), the operation area of a rotating plug for the fuel exchange is limited at present. Retrieving the bent MARICO-2 test subassembly is required for Joyo restration. Since MARICO-2 retrieving has to be conducted safely and without failure, the retrieval device needs performance warranty and reliable design. In order to obtain information for the design of retrieval device, the bending test and the re-bending test of a wrapper tube using the same material with the MARICO-2 test subassembly were conducted. Confirmed re-bending strength of a bent wrapper tube was enough to hold directly and lift up the test subassembly. As the MARICO-2 lifting up test, the wrapper tube of the MARICO-2 test subassembly in a reactor vessel was directly gripped, and tried to lift in order to investigate the state and lifting load. This test revealed that the bent wrapper tube is stuck to a transfer pot of the in-vessel storage rack and those were lifted up together. Therefore, the MARICO-2 test subassembly was decided to retrieve together with the transfer pot which contains it. And the detail design of the retrieval device was started. This report summarizes the design of the retrieval device optimized by reflecting in-reactor vessel observations, out-pile mockup tests and lift-up tests of MARICO-2 test subassembly. MARICO-2 retrieving is planned in 2014. Hereafter, the retrieval device will be fabricated for the resumption of Joyo on the basis of this design. (author)