Study on LBB Behavior of Nuclear Primary Pipes After Long-Term Thermal Aging

Abstract

The leak-before-break (LBB) analysis is a high level safety assessment method for pressurized vessels and pipes containing defects. The key technology of LBB safety assessment is the crack stability analysis and the determination of critical crack length. In this paper, on the basis of experimental data of a pipe material, the evolution of the fracture toughness after long-term thermal aging at 280 and 330 °C were estimated using the Argonne National Laboratory (ANL) procedure. Three-dimensional finite element analysis models were built to make elastic-plastic fracture mechanics analysis for the postulated circumferential through-wall cracks with different sizes in the nuclear primary pipe. According to the J-integral tearing modulus (J-T) method, the critical crack length were obtained and LBB curves of circumferential through-wall cracked pipes were constructed. Effects of the fracture toughness and operating temperature of nuclear primary pipe on critical crack length and LBB curves were also investigated. The results show that the pipe material is sensitive to thermal aging embrittlement during long-term service, which results in the critical crack length and LBB curve significantly decreasing due to degradation of fracture toughness. The non-conservative (unsafe) results will be produced if LBB analysis does not take into account of the effect of thermal aging. Compared to the results at 280 and 330 °C, the pipes suffer more serious thermal aging embrittlement and the degree of non-conservative in LBB analysis become larger when operating at a higher temperature.