Geological disposal is the preferred option for the long term management of British intermediate level radioactive waste. The disposal site is currently being identified, with possible geological environments including fractured crystalline rocks and low permeability rocks such as clay. The selection of the host rock will have an impact on the design of the waste repository. This thesis investigates the ways the behaviour of repository borne gas can be affected by the repository design and the selection of the host rock. Commercially available TOUGH2 package is used to model the resaturation of the disposal facility, along with gas migration out of the repository and towards the ground surface in a generic geology.A facility located in fractured rock is estimated to resaturate within 6.5 years of its closure. The resaturation time is found to be strongly dependent on the presence and properties of a low permeability liner around the disposal vaults. The inflowing water starts gas generation processes within the repository; gas initially accumulates within the facility, but it is estimated to find its way into the host rock approximately 450 years after the facility has been closed. A maximum outflow rate is reached after approximately 1,000 years. The flow of gas migrating through the host rock is strongly affected by site-specific features. In the case of a uniform crystalline rock, gas is found to break through at the surface after 29,000 years.For a disposal site with a very slow groundwater flow rate, the resaturation phase may take several decades and gas outflow will occur much later. It is estimated that, in very low permeability environments, gas breakthrough may not occur before 100,000 years.