Based on the state-of-the-art, IAEA (2012) and NEA (2014, 2016) provided guidance and gave recommendations for preparing a safety case for radioactive waste repositories. For safety demonstration, it is necessary to predict the expected evolution of the overall system, which consists of the repository and the surrounding geosphere and biosphere. Due to limited knowledge on the system and the suitability of the modelling tools applied, a significant amount of uncertainties is related to the prediction of the complex system’s evolution. Therefore, a basic requirement for safety case methodology is to reduce and handle these uncertainties. A FEP list (compiled in a FEP catalogue) and the scenario development are two measures to meet this objective. The NEA FEP catalogue compiles the FEP lists of many international repository projects and provides a comprehensive generic description of the repository system (features) and identifies processes and events that will occur in the future (NEA 2019). Based on the information in the FEP catalogue, scenarios will describe possible future evolutions of the system, which correspond to a combination of events and processes together with their characteristics and their chronological sequence. The occurrence of FEP as well as their properties (for features) or their intensities (for processes/events) may significantly vary, depending on location and time. Based on the extrapolation of geosphere/biosphere evolutions in the past on future evolution (actualism principle) as well as expert knowledge on the repository's impact on system evolution, possible (or expected) properties/intensities of processes/events can be identified and combined in probable/expected scenarios (also called: normal evolution scenarios, reference scenarios). Probabilities/intensities with lower probabilities will be described in alternative scenarios. The set of all analysed scenarios is supposed to cover the significant uncertainties regarding the future repository evolution (NEA 2016).