Significant quantities of hydrogen can be produced by the corrosion of metal components. It is necessary to forecast gas migration and pressure buildup in the context of deep geological radioactive waste disposal. One of the major problems in representing gas migration in a radioactive waste repository is that of simultaneously modeling all gas sources and complex transfer pathways constituted by the network of underground drifts and the surrounding low-permeability rock. In 2006, the French National Agency for Radioactive Waste Management launched an international multiphase flow simulation benchmark exercise for modeling such a two-phase (gas and liquid) flow system. The exercise was designed to compare the performance of the numerical methods being used to resolve the designed problems. This paper presents the results of test case 2 of the exercise completed by the authors. The three-dimensional model represents a fraction of a repository for long-lived radioactive waste in a clay rock. The model simulates ambient pressure and flow conditions (considering influence of site evacuation on the flow system) after placement of wastes, with full consideration of two-phase initial and boundary conditions. Isothermal conditions are assumed. Time-dependent gas sources are applied to the model. Since the natural environment is unable to evacuate the entire amount of hydrogen in a dissolved state, a free gas phase is formed within the disposal structures. The model is used to study the dissipation of those gases to determine their influence on the transient phases throughout the lifetime of the repository, and to investigate possible pressure buildup, which may introduce a risk of damage to the host rock. We use the model to investigate how the presence of gas in the repository influences the nature of water flow around the disposal structures and the resaturation (process of saturation increasing) transient processes after closure of the repository. The TOUGH2-MP code, a parallel multiphase flow simulator, has been adopted for this study.