Ultra Deep Disposal: Evaluating Fault Behavior for Long-Term Nuclear Waste Containment at 5000 m depth

With the growing global demand for energy and the transition toward low-carbon sources, nuclear power is expected to play a key role in the foreseeable future. However, nuclear power generation produces high-level radioactive waste (HLW) that requires safe, long-term isolation from the biosphere. Currently, HLW is stored in surface facilities, while several countries (e.g., Finland, USA, Sweden, France, Switzerland) are developing mined geological repositories at depths of 500–1000 m.

This study is related to Ultra Deep Disposal in a borehole at kilometers depth. TNO is investigating a concept for large-diameter borehole disposal of HLW at depths of ~5000 m. Ultra-deep disposal offers several potential advantages over mined repositories: enhanced isolation, reduced migration risk, and lower costs. At these depths, waste is placed at great distance to the biosphere well below fresh groundwater resources, relying on both the thickness of overlying strata and the sealing properties of host rocks as natural barriers,

A robust geological safety case for ultra-deep disposal requires evaluation of multiple criteria, including barrier integrity, mechanical and geological stability, and subsurface usage. A key safety factor is the prevention of degradation of the engineered barriers and radionuclide migration —the primary mechanism for containment failure. At ultra-deep levels, stagnant formations significantly reduce fluid transport potential. Without fluid transport, engineered barriers, canister degradation and radionuclide transport are negligible, ensuring long-term safety.

Faults represent potential migration pathways; however, many faults act as sealed fluid traps due to juxtaposition mechanisms, as extensively studied in hydrocarbon systems. Importantly, fault sealing behavior differs fundamentally between crystalline and sedimentary environments. In crystalline rocks, faults often remain open because deformation is dominated by brittle fracturing, making them potential conduits. In contrast, plastic deformation in sedimentary environments results in sealing of faults through clay smear, shale gouge, and cementation, enabling faults to act as effective barriers.

This research focuses on assessing fault connectivity within the Namurian (Upper Mississippian–Lower Pennsylvanian) as one of the potential host formation for ultra-deep disposal. Preliminary work involves analyzing fault geometry and structural characteristics to identify zones of potential connectivity and sealing capacity. These insights will inform future modeling of fault behavior and its role in long-term containment for Ultra Deep Disposal.

Results indicate that faults in the study area, under the conditions examined, are not necessarily migration pathways. They can function as additional barriers, enhancing the geological containment system for ultra-deep disposal.