In this paper, work done for defining the lowest temperature for ductile fracture initiation in piping and fittings was extended to a sample reactor pressure vessel (RPV). The methodology used is called “Master Curve of Fracture Transition Temperatures” and was developed from correlations of thousands of laboratory tests and hundreds of pipe tests originally presented in 2005. Since then it has been extended to; low and high strength line-pipe steel base metals (oil/gas industry), blunt flaws as well as sharp cracks, girth welds for the oil/gas industry, fracture of pipe fittings/valves, and is the technical basis for the lowest temperature for ductile crack initiation in ferritic piping in Appendix C of ASME Section XI. Since the methodology is quite different than traditional approaches for nuclear component applications, the general methodology will be reviewed, as well as analysis results showing how surveillance capsule Charpy data could be used to predict the lowest temperature where ductile crack initiation would occur in RPVs. Once this temperature is established, then the upper-shelf toughness values can be used to determine if the failure is EPFM or limit-load, and the associated failure stresses. This temperature could be used for defining the pressure-temperature limits to assure that the RPV material has a high flaw tolerance. This methodology was proposed for the Doel 3 and Tihange 2 RPVs in Belgium.
One key concern for operation of RPVs is determining the lowest operating temperature where ductile crack initiation is still expected which can be based on studies of irradiation effects on the measured toughness from Charpy tests of material in surveillance capsules. With ductile crack initiation the flaw tolerance is quite high even with long-term irradiation damage to the material. In fact, if the toughness is closer to limit-load than LEFM in a FAD analysis, then as long as there is ductile initiation, the irradiation effects increase the strength which could increase the flaw tolerance. There are some ongoing efforts within the ASME Section XI activities to define the minimum temperature where ductile initiation occurs in fracture toughness testing (typically based on a mixture of Charpy and 1T CT specimen data); however, there are still thickness and constraint effects on the fracture toughness for more precise application to a thick-walled vessel with a postulated axial surface crack.
