PSI - Issue 60

Keshav Mohta et al. / Procedia Structural Integrity 60 (2024) 402–410 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Results and Discussion It is noted from the two cases that in case of pressure/ flow pulsation in coolant, the average transient hydrodynamic forces on a fuel bundle may not be much different than those acting under coolant with no pulsation. On an individual fuel pin, the average forces may remain nearly same, but there is larger variation in hydrodynamic forces under pulsation. The fluctuating force is more likely to cause vibration of fuel pin than the steady force. This in turn may cause fretting of bearing pad and PT. The input excitation frequency of 150 Hz is clearly seen in the contact force time history at the bearing pad locations. Table-1 indicates higher average normal work rate at bearing pad- pressure tube contact locations under flow pulsation conditions. This is attributed to large relative sliding in case of pulsation (Fig. 7(b)), as the normal work rate is dependent on normal contact force and incremental sliding displacement. The higher normal work rate leads to higher fretting damage, as seen from Table-2. The average saturated fret depth is roughly three times higher for case of 40 kPa amplitude pressure pulsation than in the case with normal operating conditions and no pressure pulsation. Higher amplitude of pressure pulse and increase in degree of angular misalignment is likely to increase the turbulent excitations and resultant fretting damage. Although, the estimated fret depths are insignificant and do not pose major concern to the structural integrity of PT, it is evident that the fretting is higher in case of pressure pulsation. This observation is in agreement with the those made regarding CANDU reactors. It may also be noted for the cases considered here that the evaluated values are indicative in nature and are directly dependent on the empirical wear coefficient, K, as well as the normal work rate decay curve. These factors are themselves governed by local contact conditions, materials, temperature, surface conditions etc., and need careful consideration. Also, the acoustic characteristics of coolant circuit or individual channels were not analyzed in present study. The pressure pulse amplitude is assumed to be uniform throughout. Fretting damage is likely to be higher in case of pressure pulse amplification, as has been observed in few acoustically active channels for few CANDU reactor units as noted by Norsworthy and Ditschun (1995). 5. Conclusions Fretting wear assessment has been carried out for pressure tubes of a large Indian PHWR using Archard’s wear model. The evaluated fret depth for considered cases was insignificant and did not pose threat to structural integrity of pressure tube. The analysis revealed that the fretting damage was roughly three times higher in the case when 40 kPa amplitude pressure pulse were considered present in the system. The fretting damage may increase in case of higher-pressure pulse amplitude. Archard J. F., 1953, Contact and rubbing of the flat surfaces, Journal of Applied Physics, 24, 981-988 CFD-ACE+ V2014.0 Modules Manual, Part 1. Argatov I., Chai Y. S., 2020, Contact geometry adaptation in fretting wear: A constructive review, Frontiers in Mechanical Engineering 2020, Volume 6, Article 51 Norsworthy A, Ditschun A., 1995, Fuel bundle to pressure tube fretting in Bruce and Darlington, CNS Proceedings of the 16th Annual Canadian Nuclear Society Conference, Saskatoon, SK (Canada). Stewart W., 1992, Darlington NGS Unit 2 Fuel Damage Investigation, Proceedings of the 13th Annual Canadian Nuclear Society Conference, Saint John NB (Canada), p. 379-394. Shiva V., Christopher J., Veerababu J., Parthasarathi N. L., Kannan R., Nagesha A., Vasudevan M., 2024, Effect of maximum applied cyclic stress on fretting fatigue stress distribution of flat-on-flat modified 9Cr-1Mo steel contact: Finite element analysis, Nuclear Engineering and Design 417, 112883 Suh Y K, Lightstone M F., 2003, Numerical simulation of turbulent flow and mixing in a rod bundle geometry, Proceedings of the 24th Annual CNS Conference, Toronto, Ontario (Canada). Yetisir M, Fischer N., 1997, Prediction of pressure tube fretting-wear damage due to fuel vibration, Nuclear Engineering and Design 176, 261 – 271. 6. References