NUMO has developed a safety case to confirm previous generic demonstration of feasibility of geological disposal of high-level radioactive waste (HLW) and low and intermediate level waste generated from reproccessing and MOX fabrication (named TRU waste) in Japan and extend this to consider the geological settings that may result from the volunteering approach to siting. The reference groundwater at repository depth and associated radionuclide migration parameters in the engineered barriers, i.e. solubilities, distribution coefficients (Kds) and effective diffusion coefficients (Des), were derived to allow safety analysis for a range of scenarios. In the performance assessment, parameters assessed as the most realistic were used to assess a “likely” scenario. Variations of parameters to account for uncertainties were included in “less-likely” scenarios.