Study of flame propagation in a gaseous atmosphere representative of the late phase of a severe accident in a PWR

France currently has 58 nuclear reactors, which provide approximately three quarters of the energy needed to run the daily lives of its 67 million inhabitants. Nevertheless, this source of energy is not immune to incidents and accidents, the severity of which is classified on the International Nuclear Event Scale (INES). In the context of the study of severe accidents in water-cooled reactors, the term 'hydrogen risk' is defined as the possibility of a loss of containment integrity in the reactor or its safety systems as a result of hydrogen combustion. Hydrogen is produced primarily by the oxidation of zirconium in fuel element cladding and structures during the core degradation phase, and by the oxidation of metals in the corium bath (molten core) or in the raft during the interaction of corium and concrete (late phase of a severe accident). This interaction also generates another combustible gas: carbon monoxide. The hydrogen and carbon monoxide produced in this manner are conveyed and subsequently transported into the containment via convection loops, which are primarily caused by the condensation of steam emitted via the primary breach or during the corium/concrete interaction. The distribution of hydrogen and carbon monoxide within the containment is either more or less homogeneous, depending on the mixing of the containment atmosphere. In the event of strong heterogeneity, the combustible gases can reach significant local concentrations that exceed the flammability threshold of the gas mixture. This study is concerned with the experimental characterisation of the ignition conditions of gas mixtures representative of the atmosphere in the containment during the late phase of a severe accident. The impact of oxygen deficiency, pressure and temperature are examined. In order to achieve this objective, two experimental devices were employed: a spherical bomb was utilized to ascertain the impact of a number of initial parameters on the flammability limits of H₂/CO/Air mixtures, while a flame acceleration tube was employed to observe the behavior of these flames in the presence of obstacles within a closed tube, which represented the interior of a PWR (Pressurized Water Reactor). (author)