SA508 low-alloy steel for a reactor vessel was exposed to primary water in a pressurized water reactor (PWR) plant because the cladding layer of type 309 stainless steel for a reactor pressure vessel (RPV) was peeled off owing to an accident in which the thermal sleeve was detached. The advantage of electrochemical deposition (ECD) Ni plating techniques is that the RPV can be repaired without significant thermal effects, and Ni has sound corrosion resistance that can withstand exposure in primary water. The corrosion rate of the damaged part was assessed, and its trend was analyzed. The essential variables of Ni plating for repair of the damaged part were derived. These conditions are applicable variables for the repair plating device and have been carefully adjusted using the repair plating device. The process for establishing ASME technical standards called code case N-840 is described. The process for developing Ni-plating devices and the electrodeposition procedure specifications are described as well.