[en] One concern in the safety analysis of liquid metal fast breeder reactor (LMFBRs) is the release of radioactive core materials to the environment following a hypothetical core distruptive accident (HCDA). The release depends on the transport properties of the expanding, two-phase, fuel-coolant bubble produced during the HCDA and the energy available in the bubble to damage the reactor containment system. Most predictions of the HCDA bubble pressure-volume change relationship assume an isentropic expansion of the bubble material from some known initial state. This work describes the development and characterization of an energy source that models the assumed initial conditions and the subsequent expansion of an HCDA bubble in a simple 1/30-scale model of the Clinch River Breeder Reactor (CRBR). The source is used in a study of the phenomena associated with expanding, rising HCDA bubbles, the transport of radioactive core materials to the cover gas region, and the loading resulting from the impact of the coolant slug with the vessel cover. The results imply that gaswork and slug kinetic energy predictions based on an equilibrium expansion of a superheated liquid may be too high by a factor of two. (Auth.)