Structural health monitoring of pipelines in nuclear power plants using optical fiber guided laser ultrasound

Abstract

This study aims to develop a laser-based nuclear power plant (NPP) monitoring system and techniques. The proposed system transmits laser beams for generation and sensing of ultrasound from the laser sources to any target inspection points. This system is designed to be applied even under the harsh environ-mental conditions of operating NPPs. This study also proposes several techniques that can measure guided waves and mechanical impedance for instantaneous damage detection of pipes in NPPs. First, this study de-velops an optical fiber guided laser ultrasound measurement system specifically designed for NPP pipelines exposed to high temperature and radiation. Ultrasound waves are generated using an Nd:Yag laser beam, which generates a localized thermal expansion on the pipe surface. The corresponding ultrasound responses are measured using a laser vibrometer, which measures the out-of-plane velocity of the surface. The laser beams are transmitted through optical fibers to target excitation and sensing points. There are two types of ultrasound responses of main interests in this study. The first one is laser-based guided waves (LGW), which is a transient response of the generated laser ultrasound. This response can be extracted within the several wave components from first to several reflection arrivals. It can also detect the signal changes from the direct ultrasound path. The other type of ultrasound response is laser-based mechanical impedance (LMI). The LMI is a steady-state response of the generated laser ultrasound, so the generated transient responses are converged into structural vibration responses. It can detect the changes of structural vibration induced by the existence of structural damage. Both the LGW and LMI responses are well observed through numerical simulation and experimental tests. Finally, an autonomous damage detection method is developed based on the axi-symmetry nature of pipe structures. Due to the axisymmetric nature, there are severa...