压水堆控制棒驱动机构CANOPY密封焊缝服役环境与失效

The Service Environment and Failure of the CANOPY Seal Weld on the Control Rod Drive Mechanism in PWR

综述了国际上对PWR核电站控制棒驱动机构密封焊缝(CSWs)结构、载荷、应力状态、失效分析的研究。研究认为CSWs的结构设计易造成高溶解氧服役环境,在CSWs死水区空腔存在微量氯离子污染的可能,导致较大应力腐蚀倾向。目前研究确认的失效模式有穿晶应力腐蚀(TGSCC)、沿晶应力腐蚀(IGSCC)、SCC+点蚀。计算出CSWs死水区空腔理论浓度可达230×10^(-6),分析认为溶解氧和低浓度水平的氯污染是可引起上述腐蚀的环境因素;CSWs应力腐蚀裂纹扩展模式与材料敏化、服役溶液环境、初始应变、应力状态等因素相关;点蚀可能成为应力腐蚀的起源也可发展成为独立的破坏形式。

The international research status of the structure, load, stress state and failure analysis of the CANOPY seal welds （CSWs）, which are belong to the control rod drive mechanism （CRDM） in PWR nuclear power plant, are reviewed in detail. Research shows that CSWs backwater cavity is easy to form high dissolved oxygen （DO） and chloride ion contamination possibly exist under service environment, which both lead to high susceptibility of stress corrosion crack（SCC）. The failure modes of CSWs confirmed by previous research are among transgranular stress corrosion cracking（TGSCC）, intergranular stress corrosion crack（IGSCC）,and SCC＋pitting. We calculate the theoretical concentration of CSWs backwater cavity which is up to 230×10-6, and conclude that the DO and chlorine pollution are the environmental factors that can cause the above corrosion modes. The review also suggested that the crack growth mode of CSWs′ stress corrosion depends on sensitization of material, service solution environment, initial strain, stress state and so on. In addition, pitting may become crack origin of SCC or evolves into independent damage as well.