Repairability of Underclad Cracks in the Main Vessel and of BMI Leakages Using Robots

The increase in the authorized lifetime of nuclear power plants and the subsequent ageing of the main primary system require a new maintenance strategy. 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, dosimetry. EDF R&D has engaged two anticipating studies of feasibility demonstrations: repair of underclad cracks in the main vessel and repair of the Bottom Mounted Instrument penetration (BMI). The repair of underclad cracks in the main vessel is composed of different processes including machining and welding. The main work carried out in 2005 was on the welding procedure and on the mechanical and metallurgical substantiation of this repair. Robotized machining tests have also been done. A complete repair demonstration will be performed on a mock-up in 2006. The BMI leakages reparation consists of making a weld overlay between the penetration tube and the vessel external surface. The project aims at minimizing the number of adjoining guiding tubes that will have to be removed, and at using a robot and tools able to directly operate on all the BMI.