Study On Creep Characteristics At Crack Tip And Quantitative Predictive Model Of Environmentally Assisted Cracking Of Structure Materials In Nuclear Power Plants

The safety of nuclear power plants is threatened by environmentally assistedcracking(EAC) of austenitic stainless steels and nickel-based alloys in high temperature waterenvironments. And the accurately prediction of EAC crack growth rate(CGR) has become oneof the key issues in safety assessment of nuclear power plants. By adopting theoreticalanalysis and finite element method, the creep characteristics of crack tip was studied, and thequantitative predictive model of EAC in structure materials of nuclear power plants was alsobuilt based on passivation film rupture induced by the creep at the crack tip. The main worksin this dissertation are as follow:(1) The quantitative predictive model of EAC crack growth rate was built according thethe idea that the rupture of crack tip passivation film in high temperature water environmentsis dominant by the creep of metal based on slip dissolution theory and Ford-Andresen model.The mechanism of the crack tip passivation film ruptures and EAC crack growth process werereinterpreted.(2) The relationship between CGR and electrochemical parameters at the crack tip werestudied by theory and numerical calculation, the results indicates that the CGR is positivelycorrelated with duration of constant current density and oxidization current density, and isnegatively correlated with exponent of current decay curve. The mass transport andelectrochemical behavior in the crack were studied according to the electrochemicalenvironment modeling built, and the effects of anode potential, pH and crack size on localelectrochemical parameters at crack tip were achieved.(3) The effects of the creep on the mechanical characteristics at the crack tip werestudied with a compact tension specimen and finite element method, and the creep strain ratein front of the crack tip was selected to use in the EAC rate prediction model. The effects of material mechanical properties and stress status of crack tip on mechanical characteristics ofcreep crack tip such as creep strain rates, Mises stress and equivalent plastic strain underdifferent electrochemical parameters were obtained.(4) The comparisons under different model parameters among predictive CGR of thenew model proposed in this dissertation, FRI model, crack tip strain gradient model andexperimental data were taken, the results indicate that it is practicable to predicate the EACrate in high temperature water by the model based on crack tip creep induced passivation filmrupture.(5) The inhomogeneous of material caused by weld are common in key structures ofnuclear power plants. The effects of creep on mechanical characteristics of weld joints werestudied by finite element model of simplified weld joints, the results indicate that thedifferences of stress distribution caused by weld inhomogeneous would be reduceddramatically by creep. The EAC rates in the weld joint were also achieved by applying theCGR prediction model to the weld joints.The proposed EAC quantitative predictive model, which is based on the crack tip creepinduced passivation film rupture, provides new method to evaluate the safety and predict thelife of structural materials in nuclear power plants.