Modelling multi-phase flow phenomena in concrete barriers used for geological disposal of radioactive waste

Gas generation and gas transport phenomena occur in geological repositories of radioactive waste. This has been extensively studied over the past ten years, usually within the framework of international projects (PEGASUS, MEGAS, PROGRESS, etc.). These studies indicate that the production of hydrogen by anaerobic corrosion of metals is the most important source for gas generation. Laboratory and in situ experiments carried out at SCK•CEN indicate that, in the presence of Boom Clay (the reference geologic formation for deep disposal studies in Belgium), carbon steel suffers generalised corrosion estimated conservatively at 1 µm y-1. Simulations with the finite difference multi-phase flow code TOUGH2 were carried out in an attempt to quantify the effects of hydrogen gas generation on desaturation of initially saturated concrete components of the disposal gallery and the concomitant expulsion of cementitious pore-water into the surrounding host formation. Several simulation cases were considered and addressed differences in (1) initial concrete saturation degree, (2) hydrogen gas generation rate, and (3) porosity. Several conceptual models have been developed to better understand the phenomena at work in the transport of gas in the cementitious engineered barriers and Boom Clay. Multi-phase flow modelling was found to be helpful to get insight into the phenomenology of coupled water-gas flow in the cementitious engineered barriers. However, discontinuous variation in the conductivity of the clay relative to the gas (creation of preferential pathways) makes it impossible to use conventional models based on the laws of two-phase flow.