During an unprotected loss-of-flow accident in a liquid-metal fast breeder reactor, the mode of fuel disruption in sodium-voided channels and the subsequent fuel and clad motion are important issues that determine the further accident sequence. To study these phenomena, a series of in-pile fuel disruption experiments, FD2/4, were performed, and the fuel behavior was recorded by high-speed cinematography. Power transients typical of a heterogeneous (e.g., Clinch River Breeder Reactor) and a homogeneous (e.g., SNR-300, Federal Republic of Germany) core design were employed. In the first case, large-scale liquid swelling was observed, whereas in the second case, disruption by solid-state breakup occurred. Both observations provided direct experimental confirmation of the assumption usually made in the accident analysis, and thus removed still existing modeling uncertainties concerning the disruption behavior.