Development of the in-vessel repairing technology with friction stir welding method for FBR

Application of Friction Stir Welding (FSW) method to in-vessel repairing of FBR has been studied. Material and shapes for the FSW tool were surveyed from the viewpoint of the in-vessel repairing of the reactor vessel and the in-vessel components made of stainless steel. It was also found that load control method was more preferable rather than position control method for the in-vessel repairing machine. Repair on SS316L austenitic stainless steel plates with a slit-type artificial crack has successfully demonstrated in both argon gas environment and in liquid sodium. Plunging downforce for FSW was not more than 30 kN, which is within realistic force capability of the in-vessel repairing machine. The welding speed had to be much slower than that in the argon environment to input enough friction heat for compensating the heat removed to the liquid sodium. Tensile strength of the repaired specimens was the same level as the base metal after a heat treatment, which corresponds to the 40 years of FBR operation. Weld bead and HAZ, which are more likely to have defects compared to the base metal, were also FSWed in the argon environment and it was found that the optimum process conditions were the same as the base metal. Considering the repairing on the curved in-vessel components, FSW on the tilted plate and curved specimen have been performed. Allowable tilt angle has been studied and corresponding length on the in-vessel component has been estimated. A concept of the in-vessel repairing machine has been established. The machine is inserted through the ISI hole to the inside of the reactor vessel and reaches to the place to be repaired with jointed-robotic arm. (author)