PSI - Issue 60

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

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pressure tube failure accident in the CANDU type reactor and methods for improving reactor performance has been presented in Perryman (1978), Yoo and Chung (1998), and Langille et al. (2021). In case of PHWRs, a severe accident scenario can happen if a loss of coolant accident occurs and adequate cooling arrangements are not available to contain the temperature rise of the core components of the reactor. This can result in large temperature rise of different components including that of the PT-EF rolled joint region and this may lead to disassembly of the pressure tube from the end-fittings. As evaluation of residual stresses (after rolling process) and the pull-out strengths of these rolled joints at different temperatures is cumbersome, finite element analysis is often employed for this purpose. In this work, the residual stresses developed in the pressure tube after the rolling process (as well as after subjecting the rolled joint to high temperature environment) have been evaluated using 3D finite element analysis. Later, the pull-out strengths of the PT-EF rolled joints of 220 MWe Indian PHWRs have been evaluated at different temperatures (i.e., 27oC to 627°C). 2. Brief description of the rolling process to make rolled joint between pressure tube and end-fitting The rolled joint forming involves aligning of end fitting and pressure tube at the required location. Subsequently the mandrel with cage and roller assembly [Fig. 1] is employed at the inner surface of pressure tube to induce plastic flow in the pressure tube. The rollers having line contact with pressure tube cause the plastic flow of pressure tube material into the end fittings groove resulting in formation of leak tight mechanical lock between pressure tube and end fitting.

Fig. 1: Arrangement of rollers, mandrel, tube and end-fitting in a typical rolling process The circumferential residual stress generated during the rolling process helps in maintaining required contact pressure between the end fitting and pressure tube to ensure effective leak tightness during designed operating life. 3. Details of FE modelling of the rolling process The actual rolled joint forming procedure is a complex process involving various types of non -linearity such as material non-linearity, contact, friction, gap etc. In order to reduce computation cost, certain approximation are required to be incorporated at each modelling and solution procedure. The detailed description of modelling techniques and approximations employed is given below.

3.1. Geometrical modelling

The rolling assembly involves a tapered cylindrical shaft having cage to house the rollers (which have opposite taper) resulting in parallel expanding and reducing cylindrical surface during rolling [Fig. 2(a)]. An inward motion of shaft results in expansion of cylindrical roller surface whereas outward motion of the shafts results in reduction of the rolling surface diameter. The effect of roller assembly have been directly incorporated in the model by specifying a spiral