CFD calculations of stratification build-up tests of light gas in a closed vessel under controlled boundary conditions

Abstract

During the course of a severe accident in a pressurized water reactor, pressurization of the containment occurs due to steam release from the break in the primary circuit and hydrogen generation by the reactor core oxidation. A mixture of steam, hydrogen and air is thus distributed in the containment according to natural, mixed and forced convection flows induced by wall condensation, passive catalytic recombiners and spray systems. An increase of CFD calculations is observed since the last decade, despite the very challenging task of computational calculations at such a large scale.The present paper focuses mainly on nine stratification build-up tests of helium or steam injections under various gaseous conditions (dry air, air–steam mixtures, cold, hot tests, different mass flow-rates, with light gas injected above or below a denser gas, without steam condensation on walls). A validation step of the numerical tool is presented in the first part of the paper to get confidence to the analysis of simulations performed on these tests. The second part of the paper concerns an analysis of the stratifications obtained in the different cases: it is shown that in the case of injection of a light gas above a denser gas, the stratification is controlled by the filling of the upper part: the position of the upper limit of the stratification layer is correlated to the injection velocity, whereas the width of the stratification interface depends on the diffusion process.