Assessment of Residual Stress on Expanded Steam Generator Tubes and Establishment of Fundamentals for Their Stress Corrosion Cracking Property

In Korean Standard Nuclear Power(KSNP) Plants, secondary circumferential outer diameter stress corrosion cracking(ODSCC) has occurred on the expansion transition region around the top of tube sheet. The circumferential ODSCC has been known to occur more often in CE type nuclear power plants(NPPs) compared to Westinghouse or Framatone type ones. It has been often reported that the compressive residual stress was developed on the transition regions expanded by the explosive and hydraulic expansion. However, circumferential ODSCC was observed to occur on the expansion transition region around the top of tube sheet(TTS) in operating NPPs, especially very often when sludge on TTS was piled up. This means that the residual stress state on the expansion transition region might be changed by the sludge piled up on TTS, such as from compressive to tensile stress, which has not been confirmed by laboratory experiment though. Thus, in this study, the residual stress state on the transition regions developed during explosive and hydraulic expansion was measured accurately using the strain gauge relaxation and X-ray diffraction methods. Any stress state changes due to the piled sludge on TTS will be checked experimentally in the laboratory. In order to due that, experimental facilities will be set up and experimental method will be developed in this study