Experimental Investigation On SCC Behaviour Of Steam Generator Tubes

The steam generator is an important equipment in nuclear power plants. The internal partwhich connected with steam generator heat exchange tubes is in primary coolling system. Theoutside part is secondary circuit system. The liquid in the primary coolling system is deadlyradioactive, but secondary circuit is a comentional power system. The steam generator bothproduces steam and separates primary coolant with secondary circuit system. Duringoperation, circuit system was not allowed to leak,or else nuclear accident would happen.It would leave residual tension stress on wall of heat exchange tube after tube expansion.The residual tension stress together with possible corrosion in primary coolling system whichmaybe lead to SCC,so it is very important to investigate SCC resistance force of Inconel690tube. C-rings, expanded tube and tubesheet test coupons are prepared in the this research.There are also stress measurement, stress calibration and corrosion tests conducted.The tension stress of C-rings’ external wall are calibrated by static strain testing instrument.Based on the test analysis, the author found the main cause for stress decay is thatPTFE（polytetrafluoroethylene） insulation spacer is pressed thicker after creep in hightemperature corrosion which resulted in larger stress decay. For expanded tube, the authorused blind hole method and cutting-slice method to measure tension stress of inside tube wall.There are some factors such as the stick quality of strain gauge, the eccentricity when drillhole and additional that would lead to measurement errors. In addition the stress in tubesurface is measured with X-Ray stress analysis and special testing equipment.The corrosion test contains C-rings corrosion test, expanded tube corrosion test and tubesheet corrosion test. The test coupon was fully immersed in45.5%MgCl₂boilingsolution（boiling point is155℃） according to ASTM standards.Every12h or24h SCC wasinspected regularity and the tendency of pitting corrosion. The longest total corrosion time is456h in these test coupons, but none of test coupon found SCC. Compared to304,316austenitic stainless steel which ever used in steam generator heat exchange tube in totalcorrosion100h to300h were found SCC. Inconel690has higher SCC resistance that in thesame condition. Inconel690pitting corrosion severity has no relation with its tensile stressvalue. Inconel690pitting corrosion severity is connected with the total corrosion time.Inconel690average corrosion rate decreased within the same amount of the total corrosiontime increased and tended to be stable at last.