Analytical Modelling of Effect of Steam Dilution on Hydrogen Combustion and Application to a Typical Nuclear Reactor Containment

During postulated accident sequences in water-cooled nuclear reactors, steam and hydrogen may be released from the core and form a flammable mixture in the surrounding containment structure. Combustion of such mixtures and the subsequent pressure rise are an imminent threat for reactor containment integrity. Methods for evaluating combustion pressure rise are important for determining the design safety margins in such scenarios. Typically, combustion calculations are based on NS equations and CFD modelling, which are complex and time-consuming. A simpler and much faster approach is to use thermodynamic analysis to compute the final state after combustion for a given initial state. In the present work, thermodynamic modelling based on free energy minimization is presented. Predictions from the thermodynamic model have first been validated with published experimental data for binary hydrogen–air mixture. Then, parametric studies have been carried to compute combustion pressure rise in ternary mixture of hydrogen–steam–air as it represents more realistic mixture during accident. Finally, the model has been applied to a typical nuclear reactor containment to determine design safety margin as well as margin with respect to functional and structural failure of the containment.