Dimensionless parameters controlling the spreading behaviors of free-falling molten metal on dry surface

Understanding the spreading behavior of a molten core is important for predicting the progression of severe accidents and ensuring the smooth decommissioning of severe accident-experienced nuclear power plants. In the current study, molten-metal drop experiments were performed using tin and copper. These experiments were conducted to expand the fundamental knowledge on spreading and deposition behaviors to verify the analysis model. In the case of a nozzle diameter of 4 mm, the deposited metal was uniformly relocated; however, the center was extremely thin and Sn demonstrated a circular shape for a nozzle diameter of 10 mm, while Cu spread non-uniformly. The unevenness ratios for Cu were much higher than that of the Sn independent of the dropped height and the nozzle diameters. The effects of hydraulic and thermal dimensionless numbers on the deposition and spreading behaviors were analyzed. A correlation between the spreading and deposition behaviors and the dimensionless numbers of Re, We, Pe, Oh, and Bo was observed. In particular, the Oh and Bo numbers had significantly high correlation coefficients with the dimensionless spreading area, where effects of inertial force on the spreading process were relatively low. The experimental correlation of the dimensionless spreading area was proposed using the Re and Bo numbers. The prediction capability was improved, in which the predicted values were within 28% for the current experiments.