Root cause analysis of SG tube leakage at Fessenheim unit 2 in 2008

In February 2008, a primary-to-secondary leak caused an unscheduled shutdown at Fessenheim Unit 2 NPP. A circumferential crack was observed just above the top support plate of Row 12 Column 62 U-bend tube on Steam Generator (SG) number 3, which has been attributed to high cycle fatigue. This tube was pulled out in 2011, just before the SG replacement at the third decenal outage, in order to perform exhaustive metallurgical investigations. The destructive examinations revealed that the circumferential crack (70 degrees of extension) was due to high cycle fatigue, with several external initiation areas associated with the presence of small piles of Intergranular Attack (IGA) (600 MA tube) and with very low stress intensity factors ΔK (close to the non-propagating threshold region). This paper complements the metallurgical investigations by carrying out numerical analyses (thermal-hydraulic computation, fluid-elastic instability evaluation, tube vibratory response analysis and fatigue evaluation). The first objective of the study is to attempt to clarify the effect of IGA and the role of several competing factors that could be involved in the tube vibration induced fatigue failure. From these results, a root cause analysis of the R12C62 tube fatigue failure is then provided. It appears that a combination of various factors led to the failure of the tube