Generally, when we need repair welding due to the reduction of the thickness generated by corrosion in the inside of tanks, usually people enter and repair after the inner liquid is drained. However, the direction that performed repair work, without draining intension liquid may be desirable on work. Especially, for the radioactive waste (RW) tanks in operating nuclear plants, since there are danger that be accompanied by the risk of exposure, it is desired that to shorten their work time (abbreviation of drainage) and to use the robot system. As the welding repair method that can be performed underwater, there is underwater welding technology by YAG laser. So far, we developed the remote underwater YAG laser repair-welding robot. A repair welding work to the robot is performed as following steps: 1. Degradation area detection. 2. Crack removal and edge preparation. 3. Repair welding, and inspection. And we found that the application of the repair welding to use the robot at the base metal and butt joint. Furthermore, we will indicate the wide range application of the remote underwater YAG laser repair-welding robot to make sure the appropriate work of welding at fillet groove joint, in this time.

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