PSI - Issue 60

R.P. Pandey et al. / Procedia Structural Integrity 60 (2024) 324–334 R.P. Pandey/ StructuralIntegrity Procedia 00 (2023) 000 – 000

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either using the rolling process or hydraulic expansion (especially for the smaller diameter tubes) and these can be either with or without the presence of grooves. A detailed review of the tube to tube-sheet joint is presented in Abselesalam and Dokainish (1993). Parametric studies during numerical simulation of mechanical roller expansion of tube-tubesheet joints are presented in Kalnins et al. 1991, Manu et al. 2005 and Alexouli et al. 2017. It was observed from these literature that the magnitude of residual stress is mainly dependent upon the percentage of thickness reduction of tube during rolling and the initial clearance/interference between tube and tube- sheet. The groove depth doesn’t have much effect on development of residual stress. The effects of initial tube-to tube-sheet clearance, tube wall reduction and material strain hardening have been discussed in detail in Al-Aboodi et al. 2006, 2009a,b and Merah et al. 2009. Higher tube to tube-sheet clearance has a detrimental effect of joint strength and leak tightness. The strength of the joint is linearly dependent on the material strain hardening. The effects of groove geometry and coefficient of friction have also been studied in these works. The details of experimental evaluation of residual stresses in roll expanded joints in CANDU nuclear reactors have been presented in Paley and William, 2012. The evolution of residual stresses in the mechanical roll expansion of heat exchanger tubes into TEMA specified grooves are presented in Williams (2017). The elastic-plastic deformation behavior of the tube during the mechanical expansion process is presented in Wilson (1978). In India, majority of the nuclear reactors are of the type known as pressurized heavy water reactors (PHWR). In PHWRs, the primary heat transport systemconsists of horizontal pressure tubes (PT) through which the coolant (heavy water) flows over the fuel bundles at high pressure. At its ends, the pressure tubes are connected to components known as the end fittings (EF), which passes through the calandria end shield. The joint between the end fittings and pressure tubes are leak tight joints which must withstand the high pressure of the flowing coolant. The NDT method used for qualification of rolled joint is leak tightness test through the standard method of helium gas leak test. The leak tightness is ensured by controlling the rolling parameters such as percentage reduction of thickness of pressure tube through rolling (usually 13 to 15% thickness reduction is ensured)and making a 100 micron radial interference between pressure tube outer diameter and inner diameter of end-fitting. By ensuring these parameters during rolling, the leak tightness and strength of the joint are assumed to be satisfactory according to the current s tandard procedure. The end fittings are manufactured from stainless steelSS403, while the pressure tubes are made of Zr2.5Nb alloy. Since these are made of two different grades of materials, joining methods like welding cannot be used to make the joint between the pressure tube and the end-fitting. In order to accomplish a leak tight joint between the pressure tube and end fittings, a mechanical rolled joint is used in the PHWRs. The process of rolling during manufacturing of the rolled joint has to be designed in such a way that the residual compressive hoop stress should be greater than pD/4t, where ‘p’ is design pressure,‘D’ is mean diameter of the pressure tube and ‘t’ is the tube thickness. This shall ensure the leak tightness of the joint. This magni tude of residual compressive hoop stress should be generated at the inner surface of the end-fitting in the joint region. Of course, the effect of residual stress relaxation at the design/operating temperature should be taken into account as residual stress magnitude at design/operating temperature is not same as the residual stress magnitude at rolling temperature. The specified pull-out strength is 700 kN (minimum) at room temperature and 300 kN (minimum) at the design and operating temperature of the reactor. The rolled joints derives its strength from contact pressure between tube and tube sheet as well as due to the flow of material of tube into grooves of tube-sheet (wherever present) producing a mechanical locking type effect. After the rolling operation is over, the compressive residual stresses are generated between tube and tube-sheet. The strength of the rolled joint is usually evaluated experimentally using the pull-out tests, in which the tube is pulled out from the rolled joint and the load at which the joint fails, is measured. The minimum required pull out strength of the rolled joints depends on the type of reactor, i.e., it depends upon diameter of tube and the type of clearance or interference between the pressure tube and end-fitting. Carrying out pull-out tests experimentally is very difficult as specialized setups are required. Moreover, high temperature pull-out tests require specialized machines with large diameter furnaces and hence, the experimental data regarding high temperature pull-out strengths of the rolled joints between pressure tube and end-fittings (i.e., PT-EF rolled joint) is very scarce in literature. After the Fukushima incident in 2011, there has been renewed emphasis regarding the study of failure behavior of nuclear reactor components under severe accident scenario. A review of