According to U.S. Nuclear Regulatory Commission (NRC) Generic Letter 2008, the gas accumulation in the nuclear emergency core cooling systems is concerned since it may critically damage pipes, pumps and valves. There is a need to detect the inside gas accumulation including the quantification of gas location and volume. In this paper, we propose a in-situ technique for gas detection in a gas tank by using Lamb waves. Lamb wave propagation in a plate-like structure is affected by the boundary conditions. For structures in air or submerged in liquid, wave propagations are different. When the structure is in contact with liquid such as water, wave energy leaks into it from the solid material. Therefore, the way of gas detection is related to the detection of change in wave propagation characteristics. Experimental tests in a steel water tank were conducted and shown the Lamb wave’s response to the water presence. Theoretical study of Lamb waves propagation on a free plate in air and on a plate with one surface submerged in liquid were then conducted and compared. Further investigation to understand the change in Lamb wave propagation when water is present was conducted with frequency-wavenumber analysis. In the frequency-wavenumber space, it was found that a new plate wave mode, quasi-Scholte wave showed up. A0 Lamb mode showed a decreased propagation while S0 Lamb wave showed no changes. The change in the Lamb wave propagation is found to be frequency dependent.

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