The main objectives of PhD work concern the characterisation of hydrogen flame propagation in air-steam mixture representative of reactor containment atmosphere in severe accident situations. Laminar and turbulent flame regimes were investigated with: (1) spherical bomb is used to perform laminar flame tests. Different parameters were identified: the laminar flame velocity, SL°, and flame thickness, d, the integral length scale, LT, and intensity of turbulence, the Lewis and Zeldovich numbers, Le, b, the expansion ratio, s, the product speed of sound Csp. (2) ENACCEF facility dedicated to flame acceleration. This facility is highly instrumented (16 optical windows with PMT, 9 pressure transducers) to follow the flame propagation. The lower part of ENACCEF has 6 gas sampling locations. The mixture is ignited by a spark discharge at the bottom-end using electrodes. Tests performed on ENACCEF show the effect of blockage ratio, obstacles shape and gas composition on flame velocity. Some tests performed on the previous facilities were numerically simulated with TONUS CFD code.