Study of Burst Pressure of Heat Exchange Tube with Defect in Sodium-cooled Fast Reactor

[Background]: Heat exchange tubes of sodium-cooled fast reactor may develop various corrosion-induced defects during prolonged service. These defects may lead to the rupture of the heat exchange tubes due to the high-pressure environment inside the tubes, resulting in a serious sodium-water reaction accident and affecting the safe operation of the reactor. [Purpose]: This study aims to investigate the burst pressure of heat exchange tubes with various defect types which has significant engineering value for plugging criteria. [Methods]: Firstly, in order to simulate the fatigue cracks and pitting defects caused by corrosion in the heat exchange tubes, axial grooves, circumferential grooves and flat-bottom holes with different depths (depth of 20%, 40%, 60%, and 80%) were designed and machined at the axial center positions of the inner and outer walls of the heat exchange tubes. Thus, utilizing an independently developed high-temperature burst experimental facility for heat exchange tubes, burst experiments of tubes with axial groove, circumferential groove, and flat-bottomed hole defects under room temperature and high-temperature (515 ℃) conditions were conducted to investigate the burst pressures. Finally, the fracture morphology of the heat exchange tubes after the burst experiment was observed. [Results]: The burst pressures of the heat exchange tubes without defects show a certain degree of dispersion under room temperature and at 515 ℃. The burst pressure of heat exchange tubes (515 ℃) with axial groove defects does not show any significant decline in 20% defect depth, while exhibiting a significant reduction to about 80% pressure of raw tubes with increasing defect depth to 80%, and the decline displays an accelerating trend. The burst pressure of heat exchange tubes with circumferential groove and flat-bottomed hole defects does not change significantly with increasing defect depth from 20% to 60%. All the ruptures extend along the axial direction and exhibit the characteristics of tearing. The width and length of the ruptures of heat exchange tubes with axial groove defect generally decrease as the defect depth increases (from 40% to 80%) and the burst pressure decreases. Similar to axial groove defects, the ruptures of the heat exchange tubes with circumferential groove and flat-bottom hole defects mostly extend along the axial direction. It’s worth noting that the burst position of the heat exchange tube sample with an outer wall thickness of 3 mm and a defect depth of 20% in the axial groove is not at the location of the defect processing, indicating that burst pressure not significantly decrease at a 20% defect depth, which consistent with the relationship between burst pressure and axial defect depth. The rupture positions of the burst experimental samples for circumferential groove and flat-bottom hole defects do not all occur at the defect processing sites (some of the ruptures occur near the defects), indicating that the influence of circumferential groove and flat-bottom hole defects on the burst pressure of the heat exchange tubes in this study is indeed not significant. [Conclusions]: The burst pressure results clearly indicate that, under the same defect depth conditions, different types of defects have significantly different effects on the burst pressure of the heat exchange tubes. Axial groove defects have a significant impact on the burst pressure of heat exchange tubes. Consequently, particular attention should be devoted to axial defects during non-destructive examination of heat exchange tubes.