Effect of thermal aging on the leak-before-break analysis of nuclear primary pipes

• Thermal aging embrittlement is considered in LBB assessment of nuclear pipe.
• Effect of thermal aging on growth behavior of partial-through crack is not obvious.
• Detectable leakage crack length of thermally aged material is slightly increased.
• Critical crack length of thermally aged material is significantly reduced.
• Ignorance of thermal aging produces less conservative LBB results.

Three-dimensional finite element analysis (FEA) models were built for pipes with circumferential cracks and the effect of thermal aging embrittlement on the leak-before-break (LBB) behavior was analyzed according to the Level 2 and Level 3 safety assessments. The detectable leakage crack length obtained using the two-phase critical flow model and the critical crack length calculated by the J-integral stability assessment diagram method were carried out to assess the LBB behavior. The propagation behavior of partial-through circumferential cracks for both unaged and thermally aged materials was estimated by testing fatigue crack growth rate. The results show that the effect of thermal aging on detectable leakage crack length is not obvious, whereas the critical crack length after thermal aging significantly decreases due to degradation of fracture toughness. The increments of partial-through cracks are insignificant after 40 years of service. In the Level 2 and Level 3 safety assessments for nuclear piping, LBB is shown to have sufficient safety margins, while it is suggested to decrease in the case of thermal aging. This work demonstrates that less conservative LBB assessment results will be produced if thermal aging embrittlement in piping steels is not taken into consideration.