Study On Effect Of Mechanical Field At Tip Of Small Crack On Stress Corrosion Cracking In Key Structural Material Of Nuclear Power Plant

Stress corrosion cracking (SCC) is one of the main failure mechanisms which affects theservice life and safety of key structures in nuclear power plants. Austenitic stainless steel andnickel-based alloy have been widely used in structural materials of nuclear power plant, dueto their good corrosion resistance and mechanical properties. However, the stress corrosioncracking (SCC) in these materials in high temperature water environments of nuclear pressurevessels and piping is a key issue of safety and life in nuclear power plant. The SCC processcould be divided into two stages: crack initiation and crack propagation. The crackpropagation of SCC could be subdivided into small crack propagation and crack propagationbecause of the different characteristics in the “small crack effect”. So far, the SCC research ismainly focused on the crack initiation and long crack growth, and SCC in the small crack israrely involved. Because SCC in the small crack has important influence on the decay processin service life of the key structures in nuclear power plants, the SCC in small crack isinvesitigated under reactor pressure vessel (RPV) condition in this dissertation. The mainresearch contents are described as follows:(1) The mechanical field at the tip of small crack is significantly different with themechanical field at the tip of long crack for the elastic-plastic material. By using theelastic-plastic finite element method (EPFEM) and the analysis and comparison approach, thenumerical simulation method of the mechanical field at the tip of the stationary small crackwas established, and corresponding elastic plastic mechanical parameters was studied, whichestablish a foundation for the research of SCC in the small crack.(2) The characteristic at the tip of small crack of austenitic stainless steel in hightemperature water environment was analyzed by comprising of the mechanical field at thecrack tip between normal temperature and high temperature. The differences of plastic zonesize at the tip of the small crack were discussed that represented by different mechanism parameters. And the effect of the material mechanical properties of austenitic stainless steelon the plastic zone at the tip of small crack was determined.(3) The effect of the mechanical property on the stress and strain field at the tip of smallcrack in high temperature water environment was discussed, and a suitable dominantparameter represented the mechanical property at the tip of small crack of austenitic stainlesssteel in high temperature water environment was obtained, which could be used to determinethe SCC growth rate at the tip of small crack in the key structural material of nuclear powerplants.(4) Because the inner surface of pipe is one of the origin of the stress corrosion crackingin the nuclear pressure vessel, in the view of practical engineering, the differences of SCCgrowth rate along the crack fronts of the inner crack of the pipe specimen under the shallowcrack and the deep crack were respectively discussed, which establishes the foundation forquantitatively estimating the small crack growth rate of practical component crack area inSCC enviorment.(5) Based on the analysis of the stress-strain field at the tip of the small crack of the keymaterials in high temperature water environment of nuclear power plants and researchachievement in the corresponding research field, a predict method and model of SCC growthrate at the tip of small crack of the austenitic stainless steel in high temperature environmentwere proposed.The results provide a new idea and the basis for quantitatively estimating service life andsetting right maintenance scheduleof the key components with small cracks in SCCenviorment.