This paper assesses the structural dynamics of the deck structure and the resulting loading on the deck supports of a pool-type liquid-metal fast breeder reactor during a hypothetical core disruptive accident. During this hypothetical accident. the sodium above the core is considered to be propelled upward until it impacts against the deck structure. This hydrodynamic loading could produce structural damage and sodium leak paths. The deck is a three-dimensional composite structure of beams, plates, and concrete fill. It is supported at its outer periphery by the surrounding concrete foundation. The deck provides top closure of the main tank and support for the intermediate heat exchangers, primary pumps, rotating plugs, and primary and secondary tanks. A finite element model is used to study the deck dynamics during slug impact. The structural stiffening effect of the concrete fill is assessed, and results are given for deck displacement and velocity histories and deck support column reaction forces.