Processes similar to those in a backfilled geological repository also occur in nature.
Some two billion years ago, nuclear reactors were created naturally in a uranium ore deposit at Oklo (Gabon) in West Africa.
Uranium-235 (at the time still around 3% of the natural content of the uranium ore) triggered a self-sustaining chain reaction that was moderated by water in the fissures in the rock. Several tons of high-level waste (fission products) and plutonium were produced and confined in the rock.
At Oklo, nature thus created both a nuclear power plant and a «geological repository» for high-level waste. Since that time, the radioactive substances have barely moved from where they were produced, even without the optimised geological conditions and engineered safety barriers that would prevent migration of fission products and plutonium and that will be part of engineered geological repositories. At Oklo, nature has given us a glimpse into a radioactive waste repository after two billion years.