Fatigue Behavior And LBB Assessment Of Primary Circuit Piping Materials In Nuclear Power Plants

The design of nuclear power station in early period used to set double-ended guillotine break (DEGB) as the benchmark in order to ensure structure safety. However, later theoretical research and experiments found that the occurrence probability of DEGB accidents was very little, thus the leak-before-break (LBB) assessment was proposed. In the LBB assessment, the leakage crack length and the critical crack length are calculated based on the crack growth of structural components. The nuclear primary pipes are often subjected to cyclic loads as a result of startup and shutdown of nuclear power stations or temperature and pressure gradients during operating conditions. Therefore, the fatigue properties and fatigue crack propagation behaviors become the major factors in the LBB assessment of nuclear primary pipes. In this paper, the fatigue properties and fatigue damage mechanisms of 316LN austenitic stainless steel and Z3CN20-09M cast duplex stainless steel were studied through low cycle fatigue tests and fatigue crack propagation tests. The LBB assessment for both unaged and thermally aged nuclear pipes was also conducted. The main work and conclusion are as follows:Low cycle fatigue behavior and dislocation structures of 316LN stainless steel exhibit different characteristics at different strain amplitudes. At low strain amplitude, cross slip of dislocations is restricted because of the low stacking fault energy (SFE) and short-range ordering (SRO) structure, and the dislocation structures are mainly composed of typical planar dislocation structures. The material displays a very small initial hardening followed by a continuous and slow softening period. At medium strain amplitude, along with the increase of cycles, SRO structures are destroyed and cross slip is activated, which leads to the formation of clustered dislocation walls. The material manifests an initial cyclic hardening followed by a pronounced cyclic softening. At high strain amplitude, integrated dislocation cell structures develop. The material shows an initial cyclic hardening followed by a fast cyclic softening, and then a cyclic saturation. The different cyclic stress-strain features and hysteresis loop shapes of 316LN under different strain amplitudes primarily result from the variation of dislocation configurations.The crack growth behaviors of 316LN stainless steel were investigated using compact tension (CT) specimens. The results from the constant amplitude experiments show a sensitivity of the crack growth rate to the stress ratios, and the crack growth rate increases with the increase of stress ratios under the identical stress intensity factor range. Within a single overload condition, the fatigue crack growth rate experiences a significant decrease followed by a gradual increase, which results in the crack growth retardation phenomenon. The retardation effect of crack growth becomes more and more remarkable along with the increase of overload ratio and stress intensity factor range and the decrease of stress ratio.After long-term thermal aging, spinodal decomposition leads to the increase of hardness and decrease of plastic deformation capacity in aged ferrite of Z3CN20-09M cast duplex stainless steel. Meanwhile, thermal aging has significant influences on the low cycle fatigue and fatigue crack growth behaviors. At low strain amplitude and stress intensity factor range, microcracks initiate in austenite, and then pass through the ferrite phases in the unaged materials, whose responses are described as "austenitic-ferritic like" cyclic deformation mechanisms. For the aged materials, microcracks also initiate in austenite, and then grow along the ferrite-austenite interfaces, because of the cracks cannot pass through the hardened ferrite phases, and whose responses are described as "austenitic-like" cyclic deformation mechanisms. As a consequence, the aged materials exhibit lower crack growth rates and longer fatigue lives than those of the unaged materials. At high strain amplitude and stress intensity factor range, microcracks initiate in both of austenite and ferrite in unaged materials, and then pass through the ferrite phases, whose responses are described as "austenitic-ferritic like" cyclic deformation mechanisms. For the aged materials, microcracks almost initiate in ferrite, and the ferrite fracture by the way of cleavage crack, which increases the crack growth rate and decreases the fatigue life. However, the high content austenite takes plastic deformation, and the aged materials are also described as "austenitic-like" cyclic deformation mechanisms at high strain amplitude.Three-dimensional finite element analysis models were built for cracked pipes, and the effects of the crack positions and material properties on the elastic-plastic fracture parameters were analyzed. The results show that the circumferential cracked Z3CN20-09M pipes are dangerous because of their larger elastic-plastic fracture parameters. Based on the elastic-plastic fracture mechanics theory, the detectable leakage crack lengths were obtained based on a two-phase critical flow model, and the critical crack lengths were calculated by the J-integral stability assessment diagram method. The LBB curves and the LBB assessment diagrams for circumferential cracked Z3CN20-09M pipes were constructed, and the effect of thermal aging on the LBB behavior was analyzed. The results show that the ligament instability line and critical crack length line for the thermally aged pipes move downward and to the left, respectively, and unsafe LBB assessment results will be produced if thermal aging is not considered. With increasing applied bending moment, the degree of unsafety in the LBB assessment becomes larger.