The safety case for a GDF is built on the principle of multiple barriers, each providing independent containment:
| Barrier | Function | Example |
|---|---|---|
| Waste form | Immobilise radionuclides in a stable matrix | Vitrified glass or ceramic UO₂ spent fuel |
| Canister / Container | Provide physical containment for hundreds to thousands of years | Stainless steel, copper, or titanium canisters |
| Buffer / Backfill | Swell to seal gaps, limit water flow, absorb radionuclides | Bentonite clay |
| Host rock (geosphere) | Isolate waste from biosphere, retard radionuclide migration, long-term stability | Granite, clay, salt, volcanic tuff |
Key Point: No single barrier is relied upon. The multi-barrier approach means that even if one barrier fails, the remaining barriers continue to provide containment. Over the far future, all engineered barriers will eventually degrade, but the waste radioactivity will also have decayed significantly.