Stabilization methods for the inverse problem in ultrasonic measurement of microbubble distributions in liquids

Kagi, Eiji*; Sugimoto, Katsumi*; Abe, Yuta ; Aizawa, Kosuke ; Murakawa, Hideki*

In sodium-cooled fast reactors (SFRs), non-condensable gases in the primary sodium coolant may be transported as small bubbles through the cooling circuit, resulting in disturbances in core reactivity. Therefore, it is necessary to establish numerical codes that can accurately predict the behavior of bubbles in liquid sodium. To validate the analytical results, an experimental evaluation of the void fraction and number of bubbles is crucial. However, the measurement technique is limited to opaque liquid sodium. In this study, an acoustic bubble spectrometer (ABS) was employed to measure the bubble size distribution using ultrasound and acoustic emission (AE) sensors for the application to liquid sodium. For the ABS analysis, physical constraints such as the non-negativity of bubble size distributions and upper limits on the void fraction were applied. The ABS system was tested to measure the distribution of microbubbles in water. The results indicate that the ABS system can detect bubble size distributions on the order of several tens of micrometers, demonstrating the potential of applying the system under liquid-sodium conditions.