Bentonite barrier materials and the control of microbial processes: Safety case implications for the geological disposal of radioactive waste

Higher activity radioactive wastes represent a significant long-term human and ecological hazard. There is an international consensus that geological disposal of these materials is the most responsible approach to their long-term management, in order to safely contain and isolate them from people and ecosystems. Some higher radioactivity disposal concepts use a bentonite barrier to surround metallic waste containers. For bentonite to satisfy its function as a barrier material, it is required to protect waste containers from corroding agents, limit the release of radionuclides, provide stability against rock displacements, and ensure excess gas pressure build-up does not occur. Microbial processes within bentonites, if they develop, have the potential to alter the properties of the material and associated pore waters. This review therefore focuses on the microbial colonization of bentonite buffers, and in particular examines the roles of (i) sulphide-producing bacteria (SPB) and (ii) iron(III)-reducing bacteria (IRB). These groups are significant, since sulphide production is implicated in container corrosion and longevity and, the reduction of structural iron(III) in bentonite could affect its geo-chemical/physical properties. Conversely, microbial activity may have positive safety case-related functions, reducing radionuclide transport by transforming radionuclides into insoluble forms and reducing gas build-up by consuming hydrogen, for example. On balance the review indicates that preventing microbial activity within bentonite buffers, to avert any potential deleterious effects, is a higher priority than harnessing any potential benefits which may arise. In order to do this, bentonites used in a geological disposal context, e.g., in barrier systems, should be engineered so as to be able to attain a swelling pressure, on re-saturation, to an extent proven to disrupt microbial activity. Some uncertainty remains, however, in very long-term evolution of the bentonite, where degradation and loss of swelling pressure may occur in localized areas. Further research should consider the rates of microbial growth and metabolism under repository relevant conditions, through experimentation, study of natural analogues and numerical modelling.