NAGRA (National Cooperative for the Disposal of Radioactive Waste) has been investigating the feasibility for siting a low- and intermediate-level radioactive waste (L/ILW) repository in subsurface geologic environments. The design of the L/ILW repository consists of a horizontally accessed cavern system located beneath Wellenberg in the low-permeability unit of Valanginian marl. One of the concerns for the safety analysis is the effect of gas generation (primarily hydrogen) from anaerobic corrosion and degradation of waste material. The development of a free gas phase and the concomitant pressure buildup can result in the displacement of contaminated pore waters from the repository. Moreover, excess buildup of pressures in the repository can impact the structural integrity of the engineered barrier and of the surrounding host rock. For the investigation of the gas-related phenomena in a proposed L/ILW repository, a process model was developed to simulate the different performance periods of the repository which include: (1) construction/operation period of the repository resulting in desaturation of the host rock caused by pressure decline and ventilation in the cavern, and (2) post-closure period after backfilling and sealing of the caverns resulting in resaturation of the host rock and of the cavern, and in the release of waste-generated gas. Period (1) takes into account degassing of dissolved gas (methane) in the formation water due to the pressure decline around the cavern, and period (2) considers dissolution of free gas caused by the pressure increase associated with the resaturation and with the gas production in the cavern.