Air fountains in the erosion of gaseous stratifications

In 1979, the Three Mile Island accident demonstrated that, in a nuclear reactor undergoing to a severe accident, a loss of core cooling may enhance the oxidation of the clads, and this may turn in the production of a safety-relevant hydrogen amount. Since then, major efforts have been devoting to hydrogen safety R&D in the nuclear field. Owing to buoyancy forces, hydrogen may accumulate in the higher regions of the containment. Therefore, we are now experiencing a renewed interest on stratified enclosures. In this frame, relevant experimental data are required to validate and improve the computer codes that are aimed to predict the post-accident thermal-hydraulic conditions of the containment. The goal of this work is to investigate, on a small scale, the break-up of a gaseous stratification by the injection of vertical fountains. A helium layer is created on the top of an air-filled containment of a total volume of 1 m(3). A vertical round air fountain is then injected upward from the containment bottom plate. The Particle Image Velocimetry technique and a series of thermal conductivity gauges are used to measure the flow velocity and the species evolution. To describe the stratification break-up phenomenon, a review of a nondimensional approach is presented and applied successfully to the test cases. In order to point out some leading features in the mixing, the influence of molecular diffusion is also taken into account. Finally, during the experimental campaign, a diffusion-, a gravity-, and an inertia-dominated regime are observed. (c) 2010 Elsevier Inc. All rights reserved.