Liquid–liquid interface stability in accelerating and constant-velocity tube flows

This paper presents new experimental data on interface stability when one liquid displaces another in a small-diameter tube. Interface stability theory suggests that the interface is stable when the displacement is directed from a dense and viscous liquid to a less dense and less viscous liquid. This theoretical result has important implications in the evaluation of a fast breeder reactor (FBR) core disruptive accident in that it rules out coolant-boiling-driven compaction of the reactor-core fuel, and has been classified by Fauske (Nucl. Safety, 17 (1976) 550) as one of several general behavior principles (GBPs) which may be used to argue against the occurrence of energetic events that could threaten the reactor vessel. The experimental data agreed well with the GBP for accelerated liquid/liquid systems with liquid density ratios and interfacial tensions similar in magnitude to those of the FBR mixed oxide fuel/sodium coolant pair. The data agreed well with the GBP when the displacement occurred in the laminar regime at constant speed and the viscosity of one of the liquid components was destabilizing. The investigation also uncovered behavior that was not in concert with the GBP, although this behavior appears to be limited to small density-difference, low-interfacial-tension liquid/liquid systems and is probably not relevant to the FBR application.