Evaluation of Cracking in Feedwater Piping Adjacent to the Steam Generators in Nine Pressurized Water Reactor Plants

摘要

Cracking in ASTM A106-B and A106-C feedwater piping was detected near the inlet to the steam generators in a number of pressurized water reactor plants. The authors received sections with cracks from nine of the plants with the objective of identifying the cracking mechanism and assessing various factors that might contribute to this cracking. The authors observed variations in (1) piping surface irregularities, (2) corrosion-product, pit, and crack morphology, (3) surface elemental and crystal structure analyses, and (4) steel microstructures and mechanical properties. However, with but two exceptions, namely, arrest bands and major surface irregularities, the authors were unable to relate the extent of cracking to any of these factors. Tensile and fracture toughness (J sub Ic and tearing modulus) properties were measured over a range of temperatures and strain rates. No unusual properties or microstructures were observed that could be related to the cracking problem. All crack surfaces contained thick oxide deposits and showed evidence of cyclic events in the form of arrest bands. Using transmission electron microscopy, the authors clearly observed fatigue striations on replicas of cleaned crack surfaces from one plant and possibly from three others. Calculations based on the observed striation spacings gave a value of Delta sigma = 150 MPa (22 ksi) for one of the major cracks. The direction of crack propagation was invariably related to the piping surface and not to the piping axis. These two factors are consistent with the proposed concept of thermally induced, cyclic, tensile surface stresses.