Evaluation of Flammability Limits of H2/O2/N2 Mixtures in Conditions Relevant to Nuclear Waste Transportation

The aim of the present work is to assess the risk of explosion in closed containments used for the transportation of nuclear materials or nuclear waste. Indeed, it is very well-known that hydrogen can be produced due to (i) the radiolysis of different materials within the containment, (ii) the thermal decomposition of mainly the organic part in the containment. Since hydrogen has a very low ignition energy and a very wide flammability domain, it is important to determine the risk of ignition of the subsequent mixture produced by the aforementioned mechanisms. The quantity of the hydrogen that can be produced can vary depending on the containment type and on the state of the material/waste being transported. It is then mandatory to have a very good knowledge not only of the flammability domain of hydrogen in air but also for different N2/O2 ratios. To do so, an experimental work on theflammability domain of multiple ternary mixtures containing H2/O2/N2 is being conducted at the CNRS-ICARE laboratory within collaboration with the CEA.The flammability limits of H2/O2/N2 will be determined in a spherical bomb equipped with a central ignition at an initial pressure of 1 bar and for different initial temperatures between 60 and 200°C. Two tungsten electrodes are located along a diameter of the sphere. They are linked to a controlled high voltage discharge device. The voltage and intensity discharge are measured via a high voltage and a current probes. The adjustable gap between the electrodes is usually fixed around 1 mm and can be larger near the flammability limits. The combustion is monitored using 2 different diagnostics: pressure measurements during the combustion test using a piezo-electric pressure transducer (Kistler) and the recording of the flame. The visualization of the flame is obtained via a Schlieren diagnostic previously described [1]. A mixture will be considered as flammable when both the imaging and the pressure indicate a successful ignition followed by flame propagation. On the contrary, the mixture will be considered non-flammable when no flame propagation is observed and no increase of pressure inside the vessel is recorded.