Cooling methods utilising the phase change of coolant are popular in domestic and industrial applications, including nuclear reactor systems owing to their high heat transfer. During the phase change or boiling of the coolant, it absorbs latent heat and provides more cooling to the hot surface. The orientation of the heated surface plays a crucial role during the phase change of coolant. Boiling on a downward-facing heated surface is more complex as compared to upward-facing pool boiling, as the growth of the bubble is inhibited due to the heater surface acting against buoyancy. The bubble residence time and interaction with surrounding bubbles are significantly affected under such boiling conditions. Experiments were conducted on a downward-facing flat surface simulating the bottom of the calandria vessel (CV) of PHWRs capturing the bubble dynamics at different heat fluxes. This condition arises during In-Vessel Retention (IVR) of corium during severe accidents. At the bottom surface, the boiling process is dominated by various bubble dynamics such as iteratively growing, coalescing and condensing at a particular heat flux. The bubble iteration phenomena have been captured using high-speed photography. Furthermore, heat flux variation and its effect on bubble iteration at particular heat flux are studied. This research provides new insights into nucleate boiling and may contribute to the development of mathematical models for accurately predicting heat transfer in downward-facing nucleate boiling scenarios.