Study On Flow Accelerated Corrosion Of Main Pipeline In Secondary Circuit Of Nuclear Power Plant

The pipeline in the secondary circuit system of the nuclear power plant is generally made of carbon steel or chromium-containing carbon steel and is subjected to flow Accelerated corrosion(hereinafter referred to as FAC),and the thickness of the pipe wall is reduced to the safety of the pipe Running is a serious potential threat.This paper aims to carry out CAP1400 secondary circuit pipeline material FAC research.In this experiment,a single-phase FAC facility was established.The pressure in the tube was 3.0MPa,the fluid temperature was 150?,the fluid flow rate was 3.0m/s,the pH of the fluid was 9.5,the oxygen content in the fluid was less than 2.0ug/L Under test conditions.The corrosion thinning phenomena and the reduction rate of the elbow,the reducer and the tee test section of different pipe materials(A335P11,20 # carbon steel and A106 B carbon steel)were studied.The corrosion reduction rate at the position where FAC was most likely to be obtained in these three test sections was obtained,and the surface morphology of the surface of the three test sections after the test was obtained.The experimental results show that:(1)the most serious site of FAC is the arc of the elbow(maximum reduction rate of 0.071mm/y),the reducing pipe section of the reducer(maximum reduction rate 0.055mm/y)and(The maximum reduction rate of 0.067 mm/y)for the tee test section.The overall thinning rate of the reducer pipe section is the smallest,and the reduction rate of the elbow section and the three section is relatively fast.(2)Comparison of the results of these three materials can be found in 20 # carbon steel and A106 B carbon steel corrosion rate was significantly faster than A335P11 alloy steel.Because the content of Cr in A335P11 alloy steel is obviously higher,and the content of Cr in the alloy can improve the anti-FAC performance of the alloy effectively.The fluid flow in the test section is simulated by ANSYS FLUENT.The distribution of the flow field and the distribution of the shear stress in the test section are obtained.The mass transfer coefficient in the test section is calculated.The simulation results show that:(1)After the fluid enters the pipeline,the velocity and pressure stratification of the fluid in the curved section of the elbow.The shear stress of the inner wall surface increases first and then decreases along the direction of the fluid flow.At the same time,the shear stress of the outer wall surface tends to decrease and then increase in the direction of fluid flow.The shear stress of the inner wall is generally larger than that of the outer wall.At the same time in the bend in the mass transfer coefficient is also the largest place in the curved section of the arc.(2)In the gradual expansion section of the reducer,the velocity of the fluid gradually decreases and the pressure increases gradually.In the reducing section,the velocity of the fluid increases and the pressure decreases.In the same way,the flow direction,shear stress and mass transfer coefficient of the variable pipe follow the trend of decreasing first and then increasing.(3)The tee is at greater pressure at the bifurcation of the two outlets;the fluid velocity at the bifurcation of the two outlets is also faster.This indicates that the shear stress and mass transfer coefficient at the bifurcation are larger and the erosion is more severe.And the shear stress and mass transfer coefficient at the bifurcation of the tee are the largest in the whole test section.The flow chart of the simulation results shows that the fluid flows through the bifurcation after a part of the fluid flows through the bifurcation to form a local reflow,resulting in a whirlpool,while forming a certain corrosion erosion on the inner wall of the tube.At the auxiliary outlet of the tee,the flow field is very unstable due to the partial flow of the fluid,with strong turbulence and the location of the FAC.(4)Mass Transfer Coefficient on Inner Wall of Pipeline is influenced by test conditions parameters.The mass transfer coefficient increases with the increase of the flow rate.The mass transfer coefficient is not affected by the system pressure.With the increase of the temperature,the mass transfer coefficient of the wall increases with the increase of the temperature.