热老化对核电厂一回路主管道LBB分析失效模式的影响

Study of Influence of Thermal Aging on Failure Mode of Primary Water Pipes in LBB Analysis

核电厂一回路主管道的材料热老化对其"未爆先漏"(LBB)分析存在明显的影响,基于美国NUREG/CR 4513报告,分析了热老化对一回路主管道的结构塑性极限载荷、裂纹扩展稳定性图(J-T图)、弹塑性断裂失效(EPFM)临界裂纹尺寸和结构失效模式的影响。案例分析结果表明,热老化增加了材料的流变应力和结构塑性极限载荷(增加了11.78%)、降低了材料韧性(裂纹扩张量为5 mm时下降了62.65%)、同样的撕裂模量下热老化后允许的J积分降低了50%以上,热老化后结构的塑性极限载荷与EPFM极限载荷之比(对应于ASME规范Ⅺ卷的Z因子)明显增大。研究结果还表明,在较大的外部载荷作用下,J-T图的驱动力曲线不再是条直线,这与诸如ASME规范Ⅺ卷的简单分析假设不一致,采用ASME规范中的简单假设可能得到非精确的结论。主管道既可能发生EPFM失效,又可能发生塑性失稳失效,发生EPFM失效对应的载荷区间更大。

There is an obvious effect on the analysis of leak before break（LBB） for thermal aging of the primary water pipes. In this paper, based on the information in the report of NUREG/CR 4513, the effects of thermal aging on the plastic limit load, stability assessment diagram （J - T diagram）, critical crack size of the elastic - plastic fracture and the failure mode are analyzed. The results of the case study show that, the thermal aging increases the flow stress and the plastic limit load（ increased by 11.78% ）, decreases the material toughness（ decreased by 62.65% when the crack extension is 5 mm） , and the allowed J inte- gral was decreased half at the same tearing modulus. The Z factor in the ASME code will be increased sig- nificantly. Sometimes, the driving force curve is not a straight line which is not consistent with the assumption in the ASME code. The failure mode may be either the elastic plastic fracture or plastic collapse ,and the elastic plastic fracture relates to larger rang of the load.