Indian PHWRs have calandria vessels (Figure 1) which contain all the fuel assemblies enclosed inside the coolant channels encompassing the reactor core. In the event of severe core damage accidents, the calandria vessel becomes a container for holding the hot damaged core debris. This is called In-vessel retention (IVR) of corium during a severe accident. The calandria vessel wall becomes a boundary having high-temperature debris on one side (or melted corium) and the other side is the cold water. The main challenge is to hold the debris which is at very high temperatures located in a thin stainless steel vessel. The safety of the calandria vessel wall is limited by heat transfer from the bottom-most part of the vessel as shown in Figure 2. The diameter of the vessel is 7.8 m and the length is 6 m. Its bottom position act as a flat plate of 100 mm width and 6 m length and inside this hot corium is resting as shown in Figure 3. The structural integrity of the vessel wall at a high temperature is ensured by cooling using water in which it is submerged. In this work, an experimental investigation has been carried out to obtain the heat transfer coefficient, limiting Critical heat flux (CHF) at the bottom-most location of the vessel and the bubble dynamics using a high-speed camera.