A method is proposed to determine the initiation of bulk boiling in a sodium-cooled fast reactor. The technique could also be used to study two-phase single-component fluid behavior. The method consists of introducing a standing acoustic wave in a coolant channel of the core. This changes the coolant density, fission rate, and gamma-ray production by fission. The gamma rays leaking out of that region of the core are monitored with and without the acoustic waves. It is shown that this ratio is strongly coupled to the acoustic velocity, and this depends sensitively on the average void fraction in the channel. A drastic reduction in the acoustic velocity (by a factor of the order of one hundredth for sodium at 1830 °F) with the formation of the first sodium voids makes this ratio very sensitive to the initiation of bulk boiling.