In an intent to improve the monitoring of steam generators, a technique based on vibration measurements is developed for the detection of a water leak into sodium. Background noise can mask the leak-induced vibrations. In order to increase the signal-to-noise ratio (SNR), a beamforming technique may be considered. In the purpose of studying the feasibility and the efficiency of this technique for the present configuration, experimental investigations have been performed on a mock-up composed by a straight cylindrical pipe coupled to a hydraulic circuit through two flanges. A sound emitter introduced in the pipe simulates the source to detect, whereas a flow speed controls the background noise vibrations. Beamforming is applied on the signals measured by an array of accelerometers externally mounted on the pipe. Two different kinds of beamforming are considered: the conventional (Bartlett) one and an advanced one based on SNR maximization (MaxSNR). After an analysis of the vibroacoustic behaviour of the mock-up, one studies the efficiency of the two beamforming treatments for narrow bands and broad bands.
This study takes part of the R&D framework on sodium-water steam generators, mainly used in the cooling of Fast Sodium Reactors. The aim is to develop a vibro-acoustic monitoring technique that can detect a leak of water into sodium from a defective tube inside the heat exchanger. In the presence of a significantly high background noise and a rather large bandwidth for the leak signature, threshold-detection methods might be insufficient. A beamforming technique is thus considered in order to enhance the leak’s signature against the nuclear facility’s background noise. The present study is carried out on a laboratory test case composed by a cylindrical shell filled with water which is coupled to a hydraulic circuit with two axisymmetric stiffeners. Two kind of excitations are considered: first is a monopole radiating inside the fluid-filled cylinder, the second is a well-established turbulent boundary layer at the neighborhood of the shell’s walls. Numerical simulations are performed to estimate the shell acceleration field used as input of the beamforming technique. Additionally, an experiment is carried out on a mock-up to study the correlations between the numerical and experimental results associated with each excitation. The theoretical and experimental performances of the beamforming are also compared.
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