The aim of this paper is to characterize two-phase phenomena occurring for a rapid energy discharge in the fluid leading to explosive vapour expansion. This study was motivated by the lack of macro-scale experiments characterizing those transient phenomena at high reduced pressure. For that purpose, a complete test section was designed based on the Joule effect to deliver the energy discharge. CO2 was chosen as working fluid, allowing to work at saturation and under saturation conditions. Equipped with pressure sensors and a high-speed camera, the complete process is recorded during few seconds. The thermal shock in the carbon dioxide creates transient pressure peaks and sudden vapour production. The first observed pressure wave is well described as acoustics. This first wave is followed by a quick (60 ms) generation of vapour.The maximum volume of vapour produced is extracted from pressure fluctuations and matches the theoretical value. Following their creation, the bubbles flow upward in the test section as bubbly flow. Visual observation allows the characterization of the shape and the velocity of pertinent bubbles as part of a wobbling flow. This project, motivated by the so-called Fuel Coolant Interaction (FCI) nuclear safety related problematic, brings consistent data allowing to better characterize the small scale processes for such transient vaporization phenomena. This paper focuses on a single test performed under saturated conditions.