The performance of the cementitious materials within a radioactive waste repository as a physical barrier to the migration of radionuclides depends on the maintenance of the integrity of the barrier. One mechanism which can potentially compromise the behaviour of a repository is physical damage to the barrier caused by pressurisation as gas is generated. The maintenance of chemical homogeneity within the material used for backfilling the repository may also be compromised as a consequence of gas pressurisation through the formation of additional cracks and the reaction of cementitious materials with gases such as carbon dioxide. Consequently, the migration of gas within repository construction materials may be a significant parameter in both the design of a repository and the provision of a safety case for disposal. The migration of hydrogen, helium, methane, argon, and carbon dioxide have been studied for materials selected to be typical of repository structural concretes and grouts that are being considered for both repository backfilling and waste encapsulation and solidification. The apparent permeability of these materials to gas has been shown to be dependent on gas type and average pressure in the structural concrete due to the effects of Knudsen flow at pressures of the order of 100 kPa. This is not observed in the grouts due to the significantly greater pore size. The permeability coefficients for the grouts are several orders of magnitude greater than those of the concrete. Gas migration is strongly influenced by the degree of water saturation of the materials. The presence of interfaces within the materials results in an increase in permeability at higher degrees of water saturation. A simple model has been developed to simulate the effects of gas pressurisation. The tangential hoop stress at the surface of a void is calculated and comparison with the expected tensile strength of the materials is used to assess the potential for cracking. The backfill grouts seem to have sufficient permeability to disperse gas without crack formation.
- concrete barriers
- waste management