Large Deflection 4 Elastic-Mastic Piping Stress Analysis of Feeder and Feeder Supports used in a CANDU Reactor

Abstract

The piping stress analysis of the feeder and feeder supports used in a CANDU reactor is carried out using the finite element method for the combined effect of the fuel channel thermal and axial creep movement. With the large fuel channel pressure tube creep elongation (approximately 150 mm (6 inches) over its design life), the behaviour of the feeder and feeder supports falls into the class of geometric and material nonlinearities. The design adequacy of the linked welded eye-rod mechanism of the lower feeder support becomes a concern. The ordinary small deflection linear elastic analysis would not give realistic and acceptable results. Proper analysis of the system must consider large deflection and material strain hardening. The "large deflection" capability of the finite element program ANSYS is used to determine the displacements and forces in the feeder supports. This is an iterative process in which changes in geometry are taken into consideration by continuously updating the structural stiffness as the channel axial movement is increased. The large deflection elastic analysis is performed for the channel creep movement from 13 mm (0.5 inch) to 63 mm (2.5 inches). In addition, the large deflection elastic-plastic analysis is performed for the channel axial movement larger than 67 mm (2.75 inches) as the lower eye-rod begins to yield at about 75 mm (3.0 inches) of channel axial movement. By this analysis, the adequacy of the lower feeder support design is demonstrated. The nonlinear elastic-plastic finite element analysis indicates that the axial stress of the eye-rod is significantly higher than that obtained from conventional linear elastic analysis. The behaviour of the swing mechanism of the eye-rod predicted by the nonlinear analysis is also more appropriate than that predicted by the linear analysis.