The spreading behavior of the molten core is an essential phenomenon to consider for understanding the initial conditions of molten core-concreate interaction and the coolability of the core. In this study, experiments using molten tin were performed to obtain the necessary data to analyze the spreading behavior of the molten core under wet conditions. The experimental parameters used were the nozzle diameter, water depth, and molten metal temperature. The pressure waves occurred due to the molten metal-water interaction in the water pool, especially under high molten metal temperature conditions. They were observed during the floor collision process and the spreading process after the collision. Our results indicated that the spreading area and the shape of the solidified metals strongly depended on the pressure waves due to the thermal interaction between the molten tin and water. When the interface temperature between the molten tin and water was lower than the homogeneous nucleation temperature, the pressure wave rarely occurred. The effect of the pressure wave on the spreading areas increased with the superheat of the molten tin.