Neutron radiography was used to observe the behavior of molten lead-bismuth alloy injected into a thin (10-mm) semicircular vessel, which was empty or contained saturated water. The fluid velocity distribution for the melt injected into the empty vessel was successfully measured using particle image velocimetry (PIV). The numerical analysis for this case using the FLOW-3D code revealed that the wall heat transfer attenuates the rotational speed of swirls that were formed during the melt injection. The melt-coolant interaction experiment demonstrated several features of molten fuel and coolant interactions in the reactor pressure vessel lower head. The violent vapor expansion occurred at the initial melt impact on the saturated pool water. A one-dimensional model predicted the observed behavior well by assuming the adiabatic expansion immediately after an instantaneous heat transfer during the first melt coolant contact. A crust was formed between the melt and coolant by the heterogeneous distribution of the coolant and pool melt. The convective velocity distribution in the pool melt was measured well by the PIV.