It must be noted, nature has already proven that geological isolation of radioactive material is possible through several natural examples (or ‘analogues’). One of the most important case is the natural nuclear reactor in Oklo. The world’s first nuclear reactor operated about two billion years ago (at that time the concentration of U-235 in all natural uranium was about 3%). The natural nuclear reactor formed at Oklo in Gabon, Africa, when a uranium-rich mineral deposit became flooded with groundwater that acted as a neutron moderator, and a nuclear chain reaction started. These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported. This was confirmed by existence of isotopes of the fission-product gas xenon and by different ratio of U235/U238 (enrichment of natural uranium).
During this spontaneous fission, all the radionuclides found in HLW were also produced, including over 5 tonnes of fission products and 1.5 tonnes of plutonium, all of which remained at the site and eventually decayed into non-radioactive elements. The study of such natural phenomena is important for any assessment of geologic repositories, and is the subject of several international research projects.