[en] A series of flow visualization tests was performed in order to qualitatively investigate chugging phenomena for the Mark III pressure suppression containment system. The tests were conducted in a small scale test apparatus consisting of a steam generator, steam conditioning valve, vent system, and condensation tank serving as a suppression pool. The two vent systems used represented ¹/₁₀ linear scale models of the Mark III full size and the Mark III 1/√3 scale Pressure Suppression Test Facility (PSTF) vent systems. Tests were run with two vent length-to-diameter ratios, two vent spacings, and two vent diameters for three different values of vent steam flux and bulk pool temperature. High-speed films were taken to determine the fluid interface motion in the annulus section and the horizontal vents, and the steam bubble formation and collapse location during vent chugging