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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1. Introduction Pressurized Heavy Water Reactors (PHWRs) are natural Uranium based pressure tube (PT) type reactors. The Zr 2.5 Nb pressure tubes are part of the primary pressure boundary and span across the reactor core. PTs are subjected to harsh loading and environment conditions, and are crucial from the operability and safety point of view of the reactor. PT failure would not only hinder the reactor operations, but may act as an initiating event for loss of coolant accident leading to irreversible large-scale damage to the reactor. Therefore, detailed insight about the various structural degradation mechanisms/ phenomena such as delayed hydride cracking, residual stresses, irradiation creep and growth etc., that could affect the structural health of PT are required for design, qualification and life assessment activities.

Nomenclature BP

Bearing Pad

K

Empirical wear coefficient for Archard’s wear model

LES Large Eddy Simulation PHWR Pressurized Heavy Water Reactors PT Pressure Tube V̇ Material volume removal rate Ẇ N Normal work rate

In PHWRs, a typical PT houses a fuel string composed of 12 or 13 fuel bundles. These bundles rest on PT inner surface through the bearing pads which prevent fuel pins from touching the PT thus creating space between PT and outer fuel pins to ensure flow of coolant and heat removal. High velocity coolant flows through the PT, carrying away the heat generated from nuclear fission in the fuel elements. The fuel bundles, although located firmly inside PT, are subjected to highly turbulent coolant flow which may lead to excitations of fuel pins locally, i.e. flow induced vibration of the fuel pins. Excitation of few fuel pins at and near the bundle resting locations, may cause attached bearing pads to impart rubbing or impacting motion intermittently/ continuously on the PT, leading to fretting of PT. Local material removal due to fretting may lead to stress concentration and associated structural degradations in PT. In view of its potential to cause further damage, it is required to assess and gain insight about the fretting damage of pressure tube. In early 1990s, in some of the pressure tubes of CANDU reactors, the fret marks were observed between the fuel bundle bearing pads and pressure tubes as reported by Stewart (1992) and Yetisir and Fischer (1997). Detailed root cause investigations revealed that the random turbulent flow excitation inside the bundle sub-channels under normal operating conditions alone could not produce the level of bundle vibrations and work-rate required to cause observed fretting damage. It was concluded that the additional factors such as pressure pulsations and acoustics were at play that enhanced the driving force behind fretting the vibrations and hence fretting wear, as also noted by Norsworthy and Ditschum (1995). The material degradation due to fretting (repeated rubbing between the two surfaces) wear is an important concern in systems with relative sliding between the bodies. It is a complex phenomenon and its study requires careful consideration of many aspects. A number of models spanning from empirical to semi-analytical and mechanics based have been proposed by the researchers. However, no unique model, applicable in all cases has been established so far. The Archard’s wear law (1953), based on the asperity defor mation considerations, is one of the most famous and frequently used equation for estimating the fretting wear. It correlates the volumetric material removal rate with contact pressure and sliding distance at the contact interface. Recent studies such as Shiva et al. (2023) have employed finite element method (FEM) to evaluate the resultant stresses and strains in the contacting bodies. With FEM, the contact parameters such as contact pressure, shear stresses and relative sliding distance etc. under the given loading and boundary conditions can be evaluated. The p resent work is aimed at devising a methodology based on Archard’s wear model for assessment of fretting wear of pressure tube using FEM. A case study has been taken to assess the fretting wear in a typical pressure tube of a large Indian PHWR for (a) no pulsation (normal operating conditions), and (b) assumed pulsating flow conditions based on the developed methodology.