Combustion of large quantities of hydrogen may occur inside the nuclear reactor containment following a postulated severe accident condition. This may jeopardize the integrity of the containment building and associated safety systems. Hence it is very essential to have an idea about the damage potential of this kind of combustion events from the perspective of containment safety. In this work, modelling of lean (<12% v/v) hydrogen deflagration in large confined volume has been carried out. For this purpose, hydrogen deflagration experiments performed in 60m³ closed vessel of THAI facility, Germany is selected considering its relevance in reactor safety. Two upward flame propagation experiment, HD-12 and HD-3 (8% and 9% v/v hydrogen in air) have been simulated. Eddy Dissipation model has been used for reaction rate calculation. The CFD simulations have been carried out in 3D domain as well as in 2D axisymmetric domain and the results have been compared against the experimental data. Comparison of 3D and 2D axisymmetric simulations have been performed to understand the consequences in result when reduction in computational requirement is required. The effect of radiation heat transfer is also highlighted in this study. The effect of ignition strength for lean hydrogen deflagration is presented in detail.