Radioactive waste is among the most dangerous anthropogenic waste and its safe disposal is a challenging multi-generational task. Many nations have sought for the peaceful application of nuclear fission for the reliable production of electric energy to power their societies. The increasing volumes of radioactive waste generated are, however, proving an increasingly urgent problem to tackle. Deep geological disposal is deemed our best option for the safe long-term disposal of radioactive waste. Different host rocks are being considered for deep geological disposal, depending on the nation's geology. Switzerland favours the Opalinus Clay Formation due to its very low hydraulic conductivity and good radionuclide retention capabilities. Microorganisms in the deep subsurface are active and must be considered as their activity will influence a deep geological repository for radioactive waste. On one hand, the microorganism's activity represents a safety concern due to the possibility of enhanced corrosion, when considering the metallic waste canister encapsulating spent fuel from nuclear power plants. In other cases, however, they can contribute to the safety of a deep geological repository, when considering the gas balance of a Swiss low- and intermediate-level repository. Anoxic corrosion of metallic radioactive waste poses risks to the structural integrity of the host rock. Harnessing the activity of naturally occurring microorganisms at a safe distance from the waste itself can result in the net reduction of gases and thus their activity can be viewed as beneficial and worth exploring. In this thesis, both the positive and negative aspects of the microbial presence are explored. Wyoming bentonite is a back-fill material and an important engineered barrier system for spent nuclear fuel and high-level waste. Within the framework of an in-situ corrosion experiment, the presence and growth of microorganisms in Wyoming bentonite are investigated. The results show that the activity of corrosion enhancing sulfate-reducing bacteria in Wyoming bentonite is inhibited during the critical bentonite saturation phase but other microorganisms were able to grow. The second in-situ research project explores the potential implementation of a microbial gas sink, a natural biofilm composed of hydrogen-oxidizing sulfate-reducing bacteria and methanogenic archaea. Indeed, under the right conditions, a microbial gas sink can fulfil its intended design purposes and consume all hydrogen-gas evolving from the anoxic corrosion of the waste. This thesis contributes to our understanding of the near-field processes of a Swiss radioactive waste repository within the Opalinus Clay Formation through the contribution of experimental insight obtained under in-situ conditions.