Visualization of simulated molten-fuel behavior in a pressure vessel lower head using high-frame-rate neutron radiography

High-frame-rate neutron radiography is used to observe the behavior of a high-temperature (≤773 K) molten Pb-Bi alloy dropped into a vessel that contains water. Experiments are also performed with an empty vessel. Using high-speed cameras combined with image intensifiers and a high-flux neutron source, the interactions among the molten and solidified alloy with water and steam are visualized at imaging rates of 500 and 125 frames/s. The behavior of the melt and steam bubbles is observed clearly in contrast to water. Observation of AuCd3 tracer particles in the molten metal dropped into the vessel that contains water is also successful. The velocity distribution in the melt is measured successfully by means of particle image velocimetry (PIV) using tracer particles. This visualization technique proves to be a promising tool to observe and measure the rapid and complex phenomena of a metal-gas-liquid mixture.