A Recent Look at CANDU Feeder Cracking: High Resolution Transmission Electron Microscopy and Electron Energy Loss Near Edge Structure (ELNES)

Abstract

The primary heat transport system of modern CANDU® (CANDU is a registered trademark of Atomic Energy of Canada Limited) reactors uses A106B piping (i.e., feeder pipes). Feeder cracking has only affected tight-radius bends at outlet feeders (higher temperature), and cracking is limited to regions with high residual stress suffering from wall-thinning by flow accelerated corrosion. To date, the mechanism of feeder cracking has not been identified. This paper includes high-resolution transmission electron microscopy and electron energy loss near edge structure characterization of inside and outside surface intergranular cracks from ex-service CANDU feeders. Prior to this work, no high resolution characterization has been performed for CANDU feeder cracking. All intergranular cracks show evidence of cementite decomposition, leading to decoration of grain boundaries with amorphous carbon, and carbon diffusion along un-cracked boundaries ahead of crack tips. Sulfur has been found on the oxide-metal interface of all intergranular cracks, but is not observed ahead of the crack tips. Sulfur is believed to be from the breakdown of manganese sulfides during service. The cementite decomposition and breakdown of manganese sulfides are believed to be accelerated in the presence of hydrogen produced from the flow accelerated corrosion. Small (<15 nm) voids are also present ahead of some intergranular crack-tips along the ferrite-ferrite boundaries, indicating that hydrogen enhanced, low temperature creep-cracking, may also contribute to intergranular fracture.