Hydrogen gathering at the top of the containment or reactor building may occur due to an interaction of molten corium and water followed by a severe accident of a nuclear reactor (like TMI, Chernobyl, Fukushima accidents). It accumulates usually in a containment of nuclear reactor as a stratified semi-confined layer of hydrogen-air mixture. Depending on the local concentration and/or presence and activation of mitigation devices, hydrogen may burn following different combustion regimes. Thus, there is a need to estimate the severity of a combustion process under given geometrical configuration, scale, ignition position, and uniformity and composition of combustible mixture. A series of experiments performed at Karlsruhe Institute of Technology (KIT) in the framework of the LACOMECO European project is devoted to flame propagation in an obstructed large scale cylinder vessel with vertical hydrogen concentration gradients. A 33 m3 volume divided in two sub-compartments, lower (21.6 m3) and upper (11.2 m3), separated by round duplex door of 1 m diameter with a metal structure with 6 layers of wooden plates reproduce real geometry of obstructions with a blockage ratio of 0.33–0.43. A sampling probes method is used to control real distribution of hydrogen. Practically linear positive and negative vertical concentration gradients in the range from 4 % to 13 % are created prior to ignition. Pressure sensors (PCB type), ion probes and thermocouples are used in the experiments to monitor dynamics of hydrogen combustion. The process of flame propagation is investigated depending on hydrogen concentration gradient and ignition positions. Critical conditions for flame acceleration-deceleration and quenching of the flame due to hydrogen concentration gradient are the main scopes of the work. The experimental data obtained during the project will be used for safety analysis, risk assessment and as benchmark experiments for CFD numerical codes validation.

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