PSI - Issue 60

Rahul Mittal et al. / Procedia Structural Integrity 60 (2024) 535–540 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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In older PHWR units (220 MWe and 540 MWe PHWRs), snubber functionality viz. free operability, drag force and sensitivity parameter is ensured by dedicated snubber ISI program during plant BSDs. In addition, snubber movement of representative snubbers is also measured manually during plant start-up after BSD completion (PHT system heat up). This ensures no jamming/locking of snubbers during installation/handling or due to any other issues. 5. Degradation of Snubbers in Indian PHWRs and Assessment for Fitness for Service High drag force and high/low acceleration threshold are the major failure modes observed in Mechanical Snubbers. The primary contributors associated with degradation of mechanical snubber performance are vibration, load transients, elevated temperature and moisture. Mechanical snubbers, particularly of the smaller sizes are subjected to degradation from high/low-amplitude vibration, if snubber is placed in vibration prone locations. This type of vibration can result in localized fretting and wear of mating parts, such as the lead screw, thrust bearing and capstan spring. It can also result in an increase in drag force, an increase in mechanical clearances, jamming, and/or an increase in the acceleration threshold (NUREG/CR-5870, PNL-8051, RV, R9, Results of LWR Snubber Ageing Research). Elevated temperature has often caused solidification/caking of lubricants used in mechanical snubbers. This effect increases friction and results in an increase in drag force and also lead to jamming of snubbers. Moisture can cause internal corrosion that in turn can lead to increasing drag force, jamming, and/or a decrease in the snubber's acceleration threshold as a result of a build-up of rust between the capstan and capstan spring. In some of the older Indian PHWRs units, degradations in mechanical snubbers viz., high drag force (higher than the acceptable limit), shift in dynamic characteristics of snubbers (locking characteristic), jamming of snubbers have been observed. One of the reasons of these degradations is solidification of lubricants of mechanical snubbers installed at locations subjected to harsh environment (elevated temperatures). These snubbers were either repaired or replaced with spare snubbers. Typical performance of mechanical snubbers subjected to periodic functional testing is showed in Figure-3. In Indian PHWR, the performance of hydraulic snubbers is found satisfactory as shown in Figure-4.

Fig. 3. Performance of Mechanical Snubbers in Indian PHWRs (a) Drag force; (b) Locking Acceleration.