Gas generation and migration in the near field of a supercontainer-based disposal system for vitrified high-level radioactive waste

The Belgian Radioactive Waste Agency recently introduced the Supercontainer as the reference design for the geological disposal of vitrified HLW in Boom Clay. A key argument in favour of this design is that corrosion rates of the steel components are supposed to be very low in an alkaline environment. This should ensure that the watertightness of the overpack will last well over the duration of the thermal transient. A low corrosion rate will also mitigate hydrogen generation, hence reduce the adverse consequences of a high gas pressure build-up. SCK•CEN performed a series of scoping calculations to assess the evolution of the near field (gas) pressures. With best estimate values for the corrosion rate of carbon steel in a cementitious environment, calculations show that a free gas phase is likely to develop. Therefore, a detailed TOUGH2 two-phase-flow model was created. This model computed a maximum near field gas pressure which is significantly lower than the local lithostatic pressure. Sensitivity studies showed that, apart from the corrosion rate, the porosity of the buffer and the unsaturated hydraulic characteristics of the near field are the main influential parameters.