Liquid entrainment by an expanding core disruptive accident bubble - a Kelvin/Helmholtz phenomenon

The final stage of a postulated energetic core disruptive accident (CDA) in a liquid metal fast breeder reactor is believed to involve the expansion o f a high-pressure core-material bubble against the overlying pool of sodium . Some of the sodium will be entrained by the CDA bubble which may influenc e the mechanical energy available for damage to the reactor vessel. The fol lowing considerations of liquid surface instability indicate that the Kelvi n-Helmholtz (K-H) mechanism is primarily responsible for liquid entrainment by the expanding CDA bubble. First, an instability analysis is presented w hich shows that the K-H mechanism is faster than the Taylor acceleration me chanism of entrainment at the high fluid velocities expected within the int erior of the expanding CDA bubble. Secondly, a new model of liquid entrainm ent by the CDA bubble is introduced which is based on spherical-core-vortex motion and entrainment via the K-H instability along the bubble surface. T he model is in agreement with new experimental results presented here on th e reduction of nitrogen-gas-simulant CDA bubble work potential. Finally, a one-dimensional air-over-water parallel flow experiment was undertaken whic h demonstrates that the K-H instability results in sufficiently rapid and f ine liquid atomization to account for observed CDA gas-bubble work reductio ns. An important byproduct of the theoretical and experimental work is that the liquid entrainment rate is well described by the Ricou-Spalding entrai nment law. (C) 2001 Elsevier Science B.V. All rights reserved.