Practical experiences of incident cause study by fractography analysis, 4; Cause analysis on Monju's sodium leak incident

Wada, Yusaku

In December 1995, a thermocouple well was broken and liquid sodium leaked out of the secondary heat transport system of Monju, which was operated in 40% power for the general plant performance tests. The sodium leakage was caused by the breakage of a thermowell that was installed on the main pipe. The thermowell had been suffered from flow induced vibration leading to high cycle fatigue failure. This vibration was an in-line oscillation associated with symmetric vortex shedding. The evidence of failure cause analysis was based on the fractographic examination, and microstructure of fracture surface showed the features of high cycle fatigue. Furthermore crack arrests were observed. The well was broken at the neck where a stress concentration was large by the geometric discontinuity of diameter transition. Crack initiation and growth analyses were carried out considering the deterioration of natural frequency of well with crack depth increase.