Design, manufacturing and repair of tube-to-tubesheet welds of steam generators of CPR1000 units

Highlights: • A repairing method is conducted to restore tube-to-tubesheet welds damaged by impact of loose parts, after applying which a hydraulic-test is performed. • The risk of PWSCC due to the change of residual stresses of the welds by the impact damage is evaluated. • The structural integrity of the damaged welds is reestablished. • Welding repair procedure applicable to the condition that vapor exists around the weld root is developed. - Abstract: This paper focuses on tube-to-tubesheet welds of Steam Generators by examining aspects of their design, materials and manufacturing to find solutions for restoring welds that had suffered from internal impact damage by loose parts. After looking into three designs of the weld, a flush-type design was adopted since it optimizes properties of the weld. Also the minimum size of the weld required in RCC-M was verified to be sufficient to sustain all of the loads and cycles involved in operation for 40 years. Then the materials and the manufacturing procedures were examined with regard to their effect on the susceptibility of the welds to Primary Water Stress Corrosion Cracking, and suitable resistance to such cracking was demonstrated. In addition, general welding defects which often existed during manufacturing were evaluated, and their repair methods by welding, including welding repairs after hydraulic test, were introduced. Ultimately, a repair method was developed to make the internal-impact-damaged SGs satisfy all structural requirements and resistant to Primary Water Stress Corrosion Cracking.