Hydrogen bubble characteristics, the parameters of a defect in a steam generator heat-transfer tube, resonant frequencies and initial bubble radii can all be experimentally calculated using micro-leak rates resulting from a water/ steam-into-liquid-sodium injection. The conditions of this experiment were sodium temperatures of 300, 400, and 500 ℃ and water injection rates of 0.005, 0.03, 0.08, and 0.183 g/sec. The acoustic spectra and hydrogen bubble frequency for a leak rate of 0.005 g/s were measured. Thereafter, the initial radius of hydrogen bubbles was estimated from the acoustic noise spectra and the frequency of the hydrogen bubbles in sodium. Since the lifetime of hydrogen bubbles with a radius of about 3 mm was not more than 3 sec, the acoustic detection of a water/ steam-into-sodium leak accident in a KALIMER (Korea Advanced Liquid Metal Reactor) steam generator can satisfy the safety requirements.