A previous single-bubble model describing the coolant motion within an oxide fuel subassembly of a liquid-metal fast breeder reactor due to rapid gas release from multiple pin failure has been extended to include the description of coolant motion following the release of fission gas into the exit coolant plenum. The present model supplements the previous model in that it follows the motion of the lower gas-liquid interface by allowing for the expansion of gas in the exit plenum in the form of a spherical bubble, and by allowing it to detach and form another bubble in its place. The model assumes that the motion of the liquid surrounding the expanding bubble can be described by potential flow theory and that the motion of lower liquid slug in the subassembly can be described by one-dimensional continuity and momentum equations for incompressible flow model. The model also considers the translation of the center of the plenum bubble during its expansion. It is demonstrated that the behavior of the first bubble (i.e., when the difference between bubble pressure and the pressure of the surroundings is large) is analogous to that of the high-pressure bubble formed under large depths of water,and the behavior of those bubbles formed subsequently resembles that of the bubbles due to orifice bubbling above a gas chamber of finite volume. The sample calculations for a Fast Flux Test Facility reactor subassembly indicate that the recovery of coolant flow, even with a nearly simultaneous breach of all 217 pins in the sub-assembly, is very rapid, and the total transient time is not long enough to cause any significant overheating of the coolant and the cladding.