Elastic-plastic Fracture Failure Analysis Of A Reactor Pressure Vessel Subjected To Pressurized Thermal Shock

In the safety assessment of a nuclear power plant,cracks are generally postulated in the reactor’s core region.Whereas high stress can also be produced in the wall of reactor pressure vessel(RPV).Pressurized thermal shock(PTS)poses a great challenge on the integrity of RPV,especially in the beltline region around the inlet nozzles.Once crack initiation occurs on there,the stress concentration around the crack tips is likely to cause material yield,crack propagation,and finally lead to a catastrophic disaster.So the fracture analysis of RPV has been an important topic in the nuclear industry,and it is necessary to study the influence of PTS on the ultimate bearing capacity of the RPV with defects.The current PTS analysis methods are mostly based on the assumption of linear elasticity or small range yield,and there is little research on the crack growth behavior and the ultimate bearing capacity of the RPV structure.Especially when the transient temperatures are obviously higher than the nil-ductility transition temperature,the nonlinear elastic-plastic material properties should be taken into account to simulate the thermo-mechanical coupling field of the RPV.In this paper,the stress-strain relationship for the temperature-related materials is introduced into the Ramberg–Osgood numerical model,so as to fit the stress-strain curves of the base and cladding materials.Since the test and safety inspection for RPV with defects under PTS loads are quite difficult and dangerous,the three-dimensional model of RPV with surface crack is established by using ABAQUS finite element software,and the loading history of thermo-mechanical loads is also taken into account.For higher and lower nil-ductility transition temperatures,elastic and elastic-plastic frameworks are used for mechanical analysis respectively.The main work and achievements are shown as follows:1.The three-dimensional finite element models of the RPV cylinder area and nozzle area are established.The general form of the full discretized equation for the thermo-mechanical coupling field is obtained by discretization of displacement vector,temperature and time variables.With the help of ABAQUS finite element software,the transient coupled stress and strain fields are achieved using the iterative Newton–Raphson method.According to the French FIS prediction formula and the American RG 1.99 Rev.2 formula,combined with the10 CFR 50.61 regulations,the different nil-ductility transition temperatures are calculated and used as the reference temperatures for brittle and ductile fracture.2.To test the accuracy of the model,the results of thermal analysis and stress analysis for the cylinder area without defect are compared with those obtained by the multinational research institutions,and they show a good consistency.The different thermo-mechanical coupling methods are applied to analyze the transient stress components of the cylinder area and nozzle area of the RPV.The results show that the numerical results obtained by the direct and indirect coupling methods are in good agreement.It is also revealed that I type crack is most likely to be produced,especially on the nozzle close to the inner surface of the vessel.At the initial temperature,the stress values of the crack tip region obtained by the finite element method are compared with the analytical solutions of the theoretical part,and the results coincide with the J-Q-T_z three parameter solutions.3.Under the PTS loads,the interaction integral method is used to extract the transient stress intensity factors at the crack tips,which are compared with the calculated fracture toughness curve.For brittle and ductile fracture,different damage evolution models are set up respectively.The modified extended finite element method(XFEM)is used to simulate the process of crack propagation,so as to avoid the phenomenon of mesh locking.In view of the weak link of the nozzle area,the critical crack sizes corresponding to different wall thicknesses are obtained under the effect of PTS peak stress.Through mesh refinement,the results obtained from the refined element sizes are compared with those obtained from the original element sizes to verify the accuracy of the calculation.4.The complete RPV finite element model containing the cylinder area and the nozzle area is established.The propagation law of the cracks in different positions,sizes and shapes,as well as their influence on the ultimate bearing capacity of the structure are analyzed quantitatively and qualitatively.With reference to different nil-ductility transition temperatures,the influence range of nozzle area on structural bearing capacity obtained by ductile fracture analysis is obviously less than the results of brittle fracture analysis.In this paper,elastic-plastic fracture analysis is mainly carried out for the RPV structure.Due to the lack of experimental data on fracture failure of RPV under PTS at home and abroad,the original work in this study can provide an important reference for the safety and reliability assessment of the cracked RPV in the thermo-mechanical coupling field.