RESIDUAL STRESSES IN CANDU FEEDER BENDS - EFFECT OF BEND RADIUS

摘要

Since 1997, sections of nine feeder pipes have been removed at the Point Lepreau Generating Station (PLGS) because of cracking. All PLGS feeder cracks are axial in orientation and are located in the feeder bends. In all cases, cracks were either at the inside surface of the bend flanks (approximately 60° from the intrados symmetry plane) or at the outside surface of the bend extrados.Root cause analyses indicated that the residual stress has a significant role in these failures. In a typical feeder, there are a number of bends and welds, which are potential locations of high residual stresses. To reduce inspection scope, or to identify the highest risk components, a relative ranking of crack susceptibility is needed. This can be achieved using the residual stress data of these components.This paper compares the measured residual stress data in tight-radius and large-radius CANDU feeder bends. It was found that residual stresses are significantly higher for the bends with small bend radius (r) over diameter (D) ratios (for example, r/D = 1.5) as compared to those bends with large r/D ratios (r/D > 4). The differences in the magnitude of residual stress are consistent with the measured and calculated level of cold work in the two types of bends. It was concluded that the likelihood of cracking in large-radius bends is significantly smaller than that in tight-radius bends.