Stress Analysis Of Heat Transfer Tube For PWR Steam Generator

The steam generator is the hub of both the primary and the secondary systems in a nuclear power plant. The root of the heat transfer tube is accident-prone areas of the steam generator. The wall thickness of heat transfer tube is generally 1mm～1.5mm. Therefore, the heat transfer tube is the weakest part of the whole primary coolant boundary. Stress corrosion and fretting wear are main reasons for a broken pipes accident. In this paper, ANSYS finite element software was used to carry out the thermal-structure coupling analysis, modal analysis, harmonic response analysis, and the fluid-solid two-way coupling analysis for steam generator tube of the nuclear power plant. The stress corrosion and fretting properties of heat transfer tube were analyzed using finite element software.The presence of tensile stress is a key factor in the material to stress corrosion damage. Firstly, coupled field analysis of thermal and structure of the heat transfer tube have been done. The calculation results show the inner wall surface of heat transfer tubes mainly with compressive stress, and the outer wall of the heat transfer tube surface is distributed with large tensile stress, especially in the vicinity of the root of the heat transfer tube. Analysis shows that the inner wall surface of the heat transfer tube is difficult to occur stress corrosion damage, while it is somewhat easy for the outer wall heat transfer tube surface especially, near the root. The analysis results are consistent with the facts.In normal condition, the heat transfer tubes work under high temperature and high pressure environment. High temperature and high pressure may result in a greater tensile stress in heat transfer tube. For the analysis of the main environmental factors of heat transfer tubes to generate tensile stress, thermal analysis and structural analysis for the heat transfer tube are carried out by finite element software AYSYS. The calculations result show that tensile stress value of heat transfer tube produced under conditions of high temperature is two orders of magnitude larger than one under the condition of high pressure. It is concluded that high temperature is more important factor for the heat transfer tube stress corrosion damage.The vibration of heat transfer tube will be caused by the excitation of the fluid in the SG’s secondary side. The vibration of heat transfer tube can lead to collision wear between the tube and the tube support plate or anti-vibration bar. Generally the vibration amplitude is smaller caused by longitudinal flow. Transverse flow are the main reasons causing vibration. Heat transfer tube vibration force is approximately harmonic response with periodic load. Through harmonic response analysis can clarify the characteristics of the dynamic response of heat transfer tubes under the action of fluid action. Using the ANSYS finite element software to carry out the modal analysis of the heat transfer tube, and the first three order natural frequencies and vibration mode of the heat transfer tube have been determined. On this basis, the harmonic response analysis of heat transfer tubes also been completed. When the harmonic load frequencies close to the natural frequency of the heat transfer tube, the tube will occur resonance. The heat transfer tube internal stress distribution in the case of resonance is presented.Finally, ANSYS/FLOTRAN finite element software is used to carry out the fluid-solid two-way coupling analysis for steam generator tube of the nuclear power plant. The analysis shows that periodic vortex shedding often appears at the tail of the heat transfer tube. The detached vortex leaves the tube surface and makes the tube surface to produce periodic change of the drag and lift, resulting in tube vibration. This will accelerate the failure of the heat transfer tube. The analysis shows that great press against the bottom of the heat transfer tube is produced by the force of fluid.