Nuclear environmental effects on piezoelectric wafer active sensors based acousto-ultrasonic sensing system

The increasing number, size, and complexity of nuclear dry cask storage system (DCSS) put an increasing need of developing innovative sensing materials and methodologies for monitoring these important to safety structures (ITS). Technologies for the diagnosis and prognosis of DCSS integrity and performance can monitor the structural degradation, improve verification of the health status of the structure, and eventually reduce the likelihood of inadvertently failure. The past two decades have witnessed an extensive sensor technology development using permanently installed piezoelectric wafer active sensors (PWAS) for structural health monitoring (SHM). PWAS have emerged as one of the major SHM technologies developed particularly for generating and receiving acousto-ultrasonic waves for the purpose of continuous monitoring and diagnosis. The primary goal of continuous sensing of structural components is to reliably interrogate large areas and detect structural anomalies. The PWAS acousto-ultrasonic technologies, however, when applied to nuclear environment, even the relatively less harsh situation in DCSS, face the challenges caused by prolonged exposure to nuclear radiation and elevated operational temperature which may introduce significantly affect the performance of the sensors as well as their sensing capabilities. This paper presents a combined modeling and experimental study on assessing nuclear effects on the PWAS acousto-ultrasonic system. Our aim is to identify and quantify the possible influences of the nuclear environment typical of DCSS (temperature and radiation) to the PWAS based sensor and sensing system, and to develop adequate solutions and guidelines accordingly. We studied the effect of the temperature on the guided wave propagation through analytical modeling. In the governing ultrasonic dispersion equations, the ultrasonic waves depend on both density ρ and modulus E. We found that the temperature effect is principally due to the modification of the elastic modulus, E, rather than the geometrical expansion contraction. We also examined a short term irradiation effects. During the study, PWAS were exposed to high energy Co-60 gamma radiation maintained their piezoelectricity in a series of 2-h, 4-h, 8-h, 24-h tests. The visual inspection indicates degradation in the electrodes after irradiation. A statistical study in the PWAS capacitance change was performed. The electromechanical impedance spectrum (EMIS) measurement showed that majority of the irradiation effect in the PWAS was developed during the first 8 hours in both free and wired PWAS groups. This preliminary exploratory work has shown that the irradiation decreased electrical capacitance, decreases the Curie point, and increased the thickness mode resonant frequencies. The paper ends with conclusions and suggestions for further work.