Fluidelastic instability analysis of operating nuclear steam generator U-tubes Original Research Article

This paper presents a systematic assessment methodology of the potential for steam generator tube failures caused by fluidelastic instability in operating nuclear power reactors and provides the results of assessment for the U-tube steam generator (UTSG) model being employed at Kori units 2, 3 and 4, and Yonggwang units 1 and 2 in Korea. The assessment process involves evaluation of anti-vibration bar insertion conditions for the UTSG, three-dimensional thermal-hydraulic analysis of the steam generator, determination of flow distributions along the length of a specific U-tube, calculation of natural frequencies and mode shapes of the tube, and fluidelastic tube instability analysis. The thermal-hydraulic analysis for providing the detailed three-dimensional two-phase flow field in the secondary side of the steam generator model was accomplished using the ATHOS3 steam generator thermal-hydraulic analysis code. The UTVA code designed for calculating both the free vibration responses and fluidelastic stability ratio of a specific U-tube was used to assess the potential for fluidelastic instability of the steam generator U-tubes at various conditions of anti-vibration bar (AVB) inactive modes. In addition, the effects of tube plugging on the forced response of either plugged or intact tubes were discussed.