Expanding the boundaries of the explosion risk assessment for H$_2$/O$_2$/N$_2$ mixtures in conditions relevant to radioactive materials transportation

Abstract : The aim of the paper is to present the major results of a three years study which goal was to assess the risk of explosion due to hydrogen accumulation in closed containments used for the transportation of radioactive materials. To do so, an experimental work on the flammability domain of multiple ternary H$_2$/O$_2$/N$_2$ mixtures has been conducted at the CNRS-ICARE laboratory in collaboration with the CEA. The flammability limits of H$_2$/O$_2$/N$_2$ mixtures were assessed using two spherical bombs equipped with a central ignition system consisting of two tungsten electrodes linked to a controlled high voltage discharge device. The combustion is monitored using two different diagnostics: pressure measurements during the combustion test and the recording of the flame with a high-speed camera. A mixture is considered as flammable when both the imaging and the pressure indicate a successful ignition followed by a flame propagation. The first part of the study has been focused on the precise determination of lower flammability limits of H$_2$/O$_2$/N$_2$ mixtures at a wide range of pressure (from 0.3 bar up to 4 bar) and for different initial temperatures ranging between 25°C and 100°C. The limit of the total inerting of a H$_2$/O$_2$ mixture by N$_2$ has also been determined. In addition, the lower flammability limits of H$_2$/Air/H$_2$O$_{vapor}$ mixtures has also been studied from 0.3 bar to 2 bar and from 25°C up to 200°C. The study has then been extended to the flammable domain close to the lower flammability limit and up to hydrogen concentration of 13%. The visualization of the flame coupled with the pressure monitoring has allowed the observation and characterization of three sub-domains, each one with very specific behaviour in terms of combustion parameters (maximum overpressure, explosion index). The first sub-domain close to the lower flammability limit allows the introduction of a new limit for the study of hydrogen risk in nuclear transport safety studies: the explosion severity low limit. Indeed, in this domain beginning after the low flammability limit, the overpressure stays very limited and opens a new way of considering safety studies related to hydrogen risk of explosion.