Intrinsic transport and combustion issues of steam-air-hydrogen mixtures in nuclear containments

Abstract Understanding local transport behaviour of steam-air-hydrogen mixture inside the containment and associated hydrogen combustion issues are essential to ensure integrity of the nuclear reactor containment in the event of a severe accident. During a severe accident, various mechanisms occurring inside and outside of reactor pressure vessel may lead to generation of steam, and subsequently hydrogen, which eventually gets released in the containment space. Hydrogen may deflagrate in the presence of an ignition source/hot spot depending on local mixture composition of steam-air-hydrogen; this may further lead to flame acceleration, deflagration to detonation transition, and finally to detonation depending upon local conditions and geometric factors, further increasing the internal pressure and temperature of the containment. The fraction of non-condensable gases may also rise due to simultaneous on-going steam condensation, which not only elevates the possibility of hydrogen combustion, but also in turn, affects the eventual local and average steam condensation rates. In this background, this paper reviews the coupled issues between steam condensation, hydrogen transport, hydrogen combustion criteria, location of its sources, and stratification inside reactor containment under plausible severe accident scenarios. Several experiments in the context of containment thermal-hydraulics and hydrogen combustion are elaborated. Looking into the complexity of the problem, necessity to adopt simulation approach is highlighted. Two types of codes, i.e., lumped-parameter (LP) and computational fluid dynamic (CFD), are scrutinized based on their specific applications and limitations. It is inferred that the containment thermal-hydraulics and ensuing safety strategies must address the issue of steam condensation and hydrogen management simultaneously, through a comprehensive and integrated approach.