The reactor of FUGEN NPP consists of many number of double-tube arranging a pressure tube (P/T) and a calandria tube (C/T) coaxially. In the case of dismantling both a P/T and a C/T simultaneously, it is preferable to use the mechanical cutting method because it does not apply the heat to highly activated P/T and C/T, as a result, it would be suppressed to release activated particles into surrounding environment. The method is also required to use a smaller cutting device for permitting the remote control of it in a narrow space, and to have a relatively long standoff for cutting the double-tube simultaneously. The authors chose the abrasive water jet (AWJ) cutting method as one of the mechanical cutting method for dismantling the reactor. The authors have also developed the smaller AWJ cutting device and confirmed that the device is applicable to the cutting of the double-tube. On the other hand, it is an issue to reduce an amount of abrasive because it becomes the secondary radioactive waste after the cutting work. It is also difficult to survey the cutting condition by any visual methods like a TV camera in the water becoming cloudy by both used abrasive and cut metal grit. For solving these issues, the following two studies were conducted. (1) The recycle of used abrasive was examined. And it was also confirmed to be able to predict an optimal supply rate of abrasive by considering the conservation of momentum between the water jet and the abrasive. (2) It was possible to judge whether the material could be cut or not by detecting the change in the frequency characteristics of vibration or sound caused during the cutting process.

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