Study On Strain Monitoring Sensing System Of Vibration Fatigue Of Steam Generator Tube In Pressurized Water Reactor

The safety of nuclear power plants has be addressed as one of the most significant issues in the developments of nuclear power.Because the steam generator is one of key equipments in pressurized water reactor(PWR),the structural integrity of its tubes is an important factor about affecting the reliability and safety of steam generator.The fretting damage and vibration fatigue due to the fluid motion and thermal energy transmission between the steam generator tube(SGT)and the tube support plate(TSP)is the most important cause of fatigue cracks in the SGT.Therefore,the flow-induced vibration experiment which provides the information of SGT structural condition has vital function on analyzing the failure mechanism of SGT,predicting the service life of SGT and optimizing the structural design.Meanwhile,the fatigue damage due to the fluid motion is the main reason of the failure in the SGT root.Therefore,the strain of SGT root is urgently needed to analyze the vibration fatigue damages.Steam generator tubes are arranged closely in the boundary between primary circuit(350 ?,15MPa)and secondary circuit(250 ?,7~8MPa).This working environment is a great challenge for the sensing system.Due to the volume,the working temperature,waterproofness and other problems,the existing electric sensors could not meet the measuring requirements.The fiber Fabry-Perot(F-P)sensors not only have outstanding advantages in heat resistance,pressure resistance and anti-electromagnetic,but also can realize the measurement of mechanical parameters in different direction.So the fiber F-P sensors is suitable in the measurement of SGT mechanical parameters.In this paper,a fiber F-P strain sensing system consists of strain sensor and high accuracy demodulation system was presented for analyzing the vibration fatigue damages in the SGT root by monitoring the strain of SGT root.An optical model of fiber F-P strain sensor based on non-scanning correlation demodulation method was built.The optical characteristic of the strain sensor was analyzed to calculate its measurement range.The strain transferring model of F-P strain sensor was built and used to analyze the strain transferring characteristic.The characteristic of relation between the resolution of F-P strain sensor and the resolution of demodulation system was modeled.The influence of light spectrum noise caused by mode-mode interference in the tail fiber on the demodulation light source to the demodulation precision and stability was analyzed.The method of eliminating the mode-mode interference noise by using the random mode phase demodulation was proposed.The high-precision demodulation system based on non-scanning correlation demodulation method was designed.Finally,the fabricated strain sensors and demodulation system were tested for their performance.The results show that the designed fiber F-P strain sensing system can work successfully on the high-temperature,high-pressure and humid environment.The measurement range of F-P strain sensor is 60?? with the resolution of 1.5?? and demodulation speed of 5KHz.The fiber F-P strain sensing system could satisfy the monitoring of SGT root's strain,which provide an effective method to the monitoring of SGT structural conditions.