Automated Robotic Grinding of the control rod drive mechanism J-Groove Weld at PWR nuclear power plants

Recent discovery of a cracked and leaking Reactor Pressure Vessel Head (RPVH) nozzles have raised concerns about the structural integrity of RPVH nozzles in the pressurized water reactor (PWR) nuclear power plants. In order to remove crack abnormalities in the areas of nozzle J-Groove Weld on reactor pressure vessel head, we have developed an “Automated Robotic Grinding” (ARG) system which removes the surface cracks of the J-Groove Weld nozzles. Cracks are removed by the means of grinding of the J-Groove Weld performed both automatically and remotely. A manipulator was designed to be attached to the nozzle of the reactor head and enable the grinder tool to move according to cylindrical coordinate system with three degrees of freedom. Control system allows a synchronized motion of the manipulator axes and remote operation of the ARG manipulator. The inputs for grinding are the surface area coordinates of the crack and the maximum depth of the crack. Due to the limited accessibility in the area around the CRDM nozzle, the system is designed to include combined sensor and actuator. The grinder tool is first used as a sensor in a procedure to automatically detect the surface topology in the given area of the J-Groove Weld and then as an actuator which grinds with predefined depth, leaving the surface smooth and free of sharp edges. In this paper we present main features of the ARG system as well as main features of the grinding procedure.