Surveillance materials consisting of a SA-508 Class 2 forging, a Mn–Mo–Ni Linde 80 submerged-arc weld, and an SA-533, Grade B, Class 1, correlation monitor material were thermally aged on a commercial reactor pressure vessel. The materials were exposed to a thermal environment of 260°C for 209000h. This temperature is below the range (minimum of 370°C) where the effects of long-term thermal aging are typically considered relevant. Charpy impact, master curve transition temperature, upper-shelf fracture toughness, and tensile testing were conducted to evaluate the long-term thermal aging changes in material properties. Small changes in the impact properties were observed for all the materials, but were generally within the 95 % confidence bounds for typical Charpy data. Upper-shelf energy also showed small variations, but a general decrease for all materials was not seen. Fracture toughness testing at the upper shelf indicated that the upper-shelf toughness had increased, however the data is scattered. Master curve T0 testing in the transition region showed little change in the forging and plate results; however an improvement in the transition temperature of the weld metal was measured.

The Master Curve (MC) reference temperature, T0, characterizes the fracture performance of pressure vessel steels in the ductile-brittle transition region. An MC-based flaw-evaluation analysis procedure that accounts for the variations of stress intensity factor, temperature, constraint, and material conditions is presented. The analysis procedure is applied to an example problem of pressure-temperature curve construction for a Section XI, Appendix G, internal, axial, semi-elliptical surface flaw subjected to a simple cool-down transient. The results show that the enhancement in the fracture toughness due to constraint loss in the ductile-brittle transition region as seen in data from test specimen geometries does not always translate to similar benefits in structural applications. Analyses such as those carried out in this work are necessary to quantify the potential benefit for each application.

Discrete dislocation simulations of crack-tip plasticity are used to study the sharp increase in fracture toughness around ductile-brittle transition temperature of ferritic steels. The model used composed of a macrocrack with a microcrack ahead of it in its crack plane. The microcrack represents potential fracture sites at internal inhomogenities, such as brittle precipitates. The simulation has two stages: at first the fracture stress of microcrack, σF is calculated from dislocation simulation of microcrack-tip plasticity. In the next stage the fracture toughness is estimated by the macrocrack tip plasticity simulation; the fracture toughness is applied stress intensity at the macrorack when the tensile stress at the microcrack position attains σF. The brittle-ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with increasing temperature are found to be the increase in dislocation mobility, the decrease in tensile stress ahead of the macrocrack tip due to blunting and increase in mircocrack fracture stress due to increase in plasticity at the microcrack tips. The shape of the curve obtained is similar to the Master Curve.

The pressurized thermal shock (PTS) re-evaluation program has been developed at the US NRC with the interaction of the EPRI Materials Reliability Program to revisit the PTS issue using new technology developed over the last decade. The results are encouraging as more realistic assessments of reactor pressure vessel (RPV) integrity are now possible using the NRC probabilistic fracture mechanics code FAVOR. This technology is now being considered for application to the ASME Code Section XI Appendix G methodology for pressure-temperature limit curves. This paper is a first attempt to use a FAVOR-type code to evaluate the true safety margin of nuclear power plant operational pressure temperature limits, especially low temperature over pressurization events. Preliminary results indicate that there is ample room for relaxing these limits based on this probabilistic approach.