Repair of Steam Generator Heating Tubes by Roll-Expanded Plugs - Approach to Cover Multiple National Regulations

Abstract

During operation, steam generators in nuclear power plants are subject to degradation mechanisms which have an impact on the component life-time. Most affected are the heating tubes which constitute the barrier of the contaminated primary cycle to the secondary side. Various corrosive attacks may cause wall thinning which requires tube repair. A common repair method is to plug the tubes by roll expanded plugs. This is a fast method, easily applicable and requires less equipment or personnel qualification as needed for weld plugs. After insertion, the plugs act as a pressure boundary from primary to secondary side. Although the function of the roll plug is simple, the different national regulations define the requirements which need to be fulfilled by a roll plug differently. In order to reduce the tooling as well as the plug types to a minimum, an approach according to one common design for different regulations and steam generator types is profitable. It was found, that the regulations according to the ASME Boiler and Pressure Vessel code in combination with the German Safety Standards of the German Nuclear Safety Standards Commission covers the regulations of the majority of utilities- To develop a roll plug which suits the different regulatory demands, efforts were made to consider all technical and regulatory boundary conditions implied on roll expanded plugs. This covering approach had an impact on the plug design, which was required to be Helium tight after installation and suitable for a 40 year component lifetime also in accident and emergency conditions. To prove the suitability of the plug design a comprehensive testing programme of the mechanical and chemical properties of the designed roll-expaned plug was launched. A summary of the plug design and testing as well as the main test results are described.