Application Investigation On Measurement Approach Of Residual Stress On Inner Wall Of Nuclear SG Tubes

The steam generator,as a key equipment among the nuclear power plant,is the barrier of pressure between primary and secondary loop.Connected to the reactor pressure vessel,the steam generator blocks the highly radioactive reaction coolant while heat transfer.It plays an important role to ensure the safe operation of nuclear power plant.The heat transfer tubes are expanded and connected to tube sheet with tensile residual stress on the inner wall of the tubes.With consideration of the radioactive medium flowing in tube-side,stress corrosion cracking may occur with corrosive condition and tensile stress at the same time.In this study,heat transfer surface is taken as the object,however the residual stress was hardly to measure precisely through normal hole-drilling strain-gage method.An inverse drilling method is brought up in order to resolve this issue.The strain-gage is stuck on the inner surface of tube,then the hole is drilled through we drill from the outer surface.The feasibility and accuracy of this method were then proved by analytical and experimental results.The main contents of this paper are as per followings:(1)According to the standards and reference methods,2D and 3D stress distributions for a planar plate with hole have been obtained to establish the relationship of strain and initial stress around the hole boundary,which leading to a series of hole Calibration parameters.Based on the analytical results,the study is divided into two parts,drilling inversely and drilling on non-planar specimen.Besides,geometric analysis is brought to explain the strain release in various models of curved surface.(2)Two models including drilling inversely and drilling on non-planar are both investigated with FEM deactivation elements.FEM results show that factors a,b are influenced by both wall thicknesses and hole diameters while drilling indepth.To reveal this relationship,the numerical values of coefficient a,b were obtained for the hole-drilling method mentioned,and both type A and B rosette mentioned in ASTM E837 were included.The simulation results of concave and convex spherical models are consistent with the above predictions.The calibration constants a and b vary with the curvature of the drilling surface.What's more,a FEM model of SG tube is set up to simulate corresponding coefficients.(3)Two sets of experiments were designed to verify the simulation results based on different conditions.One is residual stress test of Grade 45 carbon steel by both X-ray and drilling method to verify the relation between constants and thickness of specimen obtained by FEM.The other experiment is residual stress test for a same dimension tube by inversely drilling method with both ASTM and FEM constants a,b.From the final stress results compared to stress distribution on inner surface of tube,the improvement of FEM constants is undertaken.(4)Based on the stress-strain release equation of the drilling method,the error analysis is performed with focusing on strain measuring,material constants and a,b constants.Concrete errors are obtained through experiments and numerical simulation.The error at ambient temperature and humidity shall be about 10%,whilst error of strain gauge positioning 1.6%.The error of data transimition line connection joint and measuring circuit is less than 1%.Low speed hole drilling and the eccentricity error cause a lot additional strain,whilst the error of material properties shall not exceed 5%.The operation precautions are put forwarded according to laboratory hole drilling experience and analytical results.