Thermohydraulic analysis of gas generation in a disposal facility for vitrified high-level radioactive waste in Boom Clay

Assessment of gas generation and transport is inevitable for evaluation of the safety of nuclear waste disposal in deep geological formations. The reference disposal concept in Belgium consists of a concrete-based repository situated in Boom Clay, which is a low-permeability plastic clay. The mobility of gas and liquid within these barriers is very small and may lead, in combination with increased temperatures due to decay heat of the waste, to pressure buildup and the potential structural failure of barriers. The main gas production mechanism is anaerobic corrosion of metal barriers, generating H-2 gas. The corrosion process itself and therefore the intensity of the gas source is temperature dependent. Furthermore, the heat source is time dependent due to the decaying nature of the radioactive material. Results demonstrate that the peak pressures for the isothermal and non isothermal cases do not differ considerably in the case of high-permeability buffer material. On the contrary, the peak pressures differ considerably for low-permeability material, which hinders the flow of water induced by thermal expansion of water with temperature increase. This near-field analysis showed that the effect of pressure increase remains relatively localized and should not affect the structural integrity of the host formation.