Effects of fracture toughness curves of ASME Section XI–Appendix G on a reactor pressure vessel under pressure–temperature limit operation

• The state-of-the-art PFM technique is employed to analyze a reactor pressure vessel.
• Various P–T limit transients are established as the loading condition.
• Novel flaw model and embrittlement correlation are considered in the study.
• The critical values of RTNDT for a postulated flaw under P–T limit transients are determined by deterministic minimization analysis.
• KIC based P–T curve which can increase the operational margin of nuclear power plant is recommended.

According to the Code Case N-640 issued in 1999, the fracture toughness requirement of reactor pressure vessel materials in ASME Section XI–Appendix G was amended to the KIC curve. In Taiwan, the present pressure–temperature limit operation curves of normal reactor startup (heat-up) and shut-down (cool-down) for the reactor pressure vessel is still calculated per the KIa curve in 1998 or earlier editions. In the paper, the failure risks of a Taiwan domestic reactor pressure vessel under various pressure–temperature limit operations were analyzed. First, the pressure–temperature limit curves of the reactor pressure vessel based on KIa and KIC curves, and various levels of radiation embrittlement, were established. Then, the ORNL's probabilistic fracture mechanics code, FAVOR, and the PNNL's flaw model were employed to assess the failure probabilities of the reactor pressure vessel under such pressure–temperature limit transients. Further, the deterministic analyses of FAVOR code were also conducted. It is found that under the pressure–temperature limit transients based on KIC curves, the reactor pressure vessel presents higher failure probabilities, but are all below the allowable risk. The present results indicate that using the KIC curve the pressure–temperature limits can increase the operational margin as well as maintaining the sufficient stability of the analyzed reactor pressure vessel.