Study of heavy gas dispersion in ventilated rooms
Titre du congrès :AIHce - VENT 2006
Ville du congrès :Chicago
Date du congrès :13/05/2006
Within the framework of Internal Emergency Plans analysis of nuclear facilities, IRSN has to assess accidental scenarios, as well as their consequences in environment. Some of these plants involve the risk of accidental release of uranium hexafluoride (UF6), heavy gas (density equal to 12), whose behavior is badly known. Assessment of such a gas release consequences first requires the knowledge of the space-time gas concentration in the ventilated room where the release occurs. An experimental and numerical study has thus been carried out at IRSN in order to characterize heavy gas dispersion in ventilated rooms. This study relies on vertical injection experiments of an heavy tracer gas (SF6, density equal to 6) inside two ventilated rooms of 36 m3 and 1,500 m3 volume. About 20 experimental configurations have been performed while varying the injection conditions (diameter, speed and time) and the room ventilation conditions (air exchange rate, continuous/discontinuous mode). The time evolution of SF6 concentrations has been measured in various locations inside each room. A stratification is highlighted in most cases, due to an accumulation of SF6 in the lower part of the room. These experiments have been simulated thereafter using the computational fluid dynamics (CFD) code CFX-5, in order to contribute to the interpretation of experimental results and to qualify this code for future simulations of UF6 dispersion. We propose to present a summary of the numerical results, compared with the experimental ones, by illustrating the influence of the various studied parameters. The levels of concentrations are overall in good agreement with experience, especially during the injection phase. This comparison between simulation and experience is all the more satisfactory as flow velocities are high in each ventilated room. However, complementary studies remain to be carried out in order to explain some phenomena, such as sudden concentration decreases.