Prediction of Light Gas Distribution in Containment Experimental Facilities Using Cfx4 Code: Jozef Stefan Institute Experience

Two and three-dimensional simulations of experiments on atmosphere mixing and stratification in a nuclear power plant containment were performed with the code CFX4.4, with the inclusion of simple models for steam condensation. The purpose was to assess the applicability of the approach to simulate the behaviour of light gases in containments at accident conditions. The comparisons of experimental and simulated results show that, despite a tendency to simulate more intensive mixing, the proposed approach may replicate the non-homogeneous structure of the atmosphere reasonably well. Introduction One of the nuclear reactor safety issues that have lately been considered using Computational Fluid Dynamics (CFD) codes is the problem of predicting the eventual non-homogeneous concentration of light flammable gas (hydrogen) in the containment of a nuclear power plant (NPP) at accident conditions. During a hypothetical severe accident in a Pressurized Water Reactor NPP, hydrogen could be generated due to Zircaloy oxidation in the reactor core. Eventual high concentrations of hydrogen in some parts of the containment could cause hydrogen ignition and combustion, which could threaten the containment integrity. The purpose of theoretical investigations is to predict hydrogen behaviour at accident conditions prior to combustion. In the past few years, many investigations about the possible application of CFD codes for this purpose have been started [1-5]. CFD codes solve the transport mass, momentum and energy equations when a fluid system is modelled using local instantaneous description. Some codes, which also use local instantaneous description, have been developed specifically for nuclear applications [68]. Although many CFD codes are multi-purpose, some of them still lack some models, which are necessary for adequate simulations of containment phenomena. In particular, the modelling of steam condensation often has to be incorporated in the codes by the users. These theoretical investigations are complemented by adequate experiments. Recently, the following novel integral experimental facilities have been set up in Europe: TOSQAN [9,10], at the Institut de Radioprotection et de Sureté Nucléaire (IRSN) in Saclay (France), MISTRA [9,11], at the