Following a safety-oriented, stepwise procedure for geological repository siting, the Nuclear Waste Management Organization of Japan (NUMO) requested in 2021 that the NEA facilitate an independent international review on NUMO’s pre-siting Site Descriptive Model-based safety case. NUMO is responsible for the long-term management of high-level radioactive waste (HLW) and long-lived, low heat-generating radioactive waste (i.e. transuranic or “TRU” wastes) in Japan. This review was conducted to inform stakeholders in a transparent, impartial and open manner; evaluate the maturity and readiness of the generic safety case at a pre-siting stage; and confirm the technical feasibility of deep geological disposal in Japan. The review also develops a framing structure applicable to site selection based on existing national and international legislation and the best practices of other national programmes, and identifies key future challenges.