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概要

The experimental fast reactor Joyo is the first sodium cooled fast reactor (SFR) that was constructed in Japan. In 2007, 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 as an obstacle inside of the Joyo reactor vessel 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 case of SFR, because of the environment with high temperature, high radiation dose and remaining sodium, the demonstrated in-vessel repair techniques were not enough to secure the reliability. The experience and knowledge accumulated in this work provides valuable insights into further improvements for in-vessel repair techniques in SFRs.

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