Advanced Preventive/Proactive Maintenance and Repair Techniques for Nuclear Power Plants by Controlling Weld Residual Stress Fields in Embedded Flaw Repair Process

In this paper, numerical investigation was systematically performed using thermal elastic-plastic analysis to study the effect of welding conditions in embedded flaw repair technique, such as weld heat input, crack depth and thickness of structural member, on the welding-induced residual stress at repair welds of nuclear power plants. The stress intensity factor (SIF) was also estimated based on the calculated distribution of residual stress around the remaining crack within repair parts. Meanwhile, the essential variable of welding conditions described above was theoretically derived from welding thermal conduction theory as representative of the temperature distribution along the crack to conduct the unified evaluation on the relation between the residual stress and the SIF around the remaining crack and welding conditions. As the results, it was concluded that the residual stress and the SIF at the downside crack tip region could be arranged by a unified curve associated with the essential variable derived. Besides, the residual stress becomes compressive and the SIF becomes zero when the essential variable is smaller than approximately 1.5. Further investigation was numerically performed for multi-layer and multi-pass repair welding process to study the effect of weld pass sequence on the residual stress and the SIF around the remaining crack. Then, it was clarified that the appropriate weld pass sequence condition was dependent on value of the essential variable described above. With the right selection of weld pass sequence, the residual stress and the SIF after multi-layer and multi-pass repair welding is possible to be compressive and zero respectively even when the essential variable is vastly larger than approximately 1.5 to introduce the highly tensile stress at the downside crack tip region after single-pass repair welding. Thus, the methodology to improve structural soundness for preventive maintenance at repair welds of nuclear power plants was developed on the basis of the essential variables of welding heat input and pass sequence conditions in embedded flaw repair technique.