Demonstration Results of the Robotized Repair of Underclad Cracks in the Main Vessel

The increase in nuclear power plants lifetime is a major issue for EDF. But the subsequent ageing of main components requires new maintenance strategy development especially for the reactor pressure vessel. The R&D division of the French electricity company EDF is working on robotized repairing feasibility. This approach can be more relevant than the replacement of components or the development of dedicated machines for costs, delays and dosimetry reasons. In this context, EDF R&D has engaged a feasibility demonstration of robotized controls and repairs of underclad cracks in this vessel. Stable underclad cracks in the main vessel have been known for many years. Anticipating the ageing of components, EDF R&D is currently running a full repairing feasibility demonstration. The complete repair consists in installing a robot dedicated to the carrying out of the following processes: • Precise localization of the defects using UT testing, • Elimination of the defects by high speed machining, • Control that the defects were eliminated using penetrant testing, • Geometry recording of the removed volume by laser scanning, • Weld metal refilling of the cavity with TIG tempered bead process, • UT control of the repair. Two main repairing strategies have been developed. The first one consisted in removing the defect without refilling. This option simplifies the repairing operation but requires extra justifications on corrosion created by the contact between carbon steel and primary circuit water. The second option with all the processes developed above provides a complete protection against corrosion but presents specific issues mainly coming from welding without pre and post heat-treatment. The welding procedure and the mechanical and metallurgical substantiation of the complete repair have been developed in 2005. The main work carried out in 2006 consisted in developing high speed machining and UT control procedures and in measuring some mechanical parameters resulting from this repair. The complete repair demonstration including all of the complementary processes (positioning and localization of the arm at the beginning of the repair, penetrant testing, video inspections, laser scanning) was performed on a mock-up at the beginning of 2007.