Deep disposal in a mined repository is now the preferred option for HLW final disposal in most countries. The Waste Isolation Pilot Plant (WIPP) deep geological waste repository operates in the US to dispose of transuranic waste – long-lived ILW from military sources contaminated with plutonium.
A deep geological repository entails a combination of waste form, waste package, engineered seals, and geology suited to provide a high level of long-term isolation and containment without future maintenance. Crystalline rock (granite, welded tuff, and basalt), salt, and clay are the most suitable formations for geological disposal. For example, the once-through cycle considers the spent nuclear fuel to be high-level waste (HLW) and, consequently, it is directly disposed of in a storage facility without being put through any chemical processes, where it will be safely stored for millions of years until its radiotoxicity reaches natural uranium levels or another safe reference level.
This strategy is favored by several countries: the United States, Canada, Sweden, Finland, Spain, and South Africa. Some countries, notably Finland, Sweden, and Canada, have designed repositories to permit future material recovery should the need arise. In contrast, others plan for permanent sequestration in a geological repository like the Yucca Mountain nuclear waste repository in the United States.
Natural precedents for geological disposal – Oklo Reactor
It must be noted that nature has already proven that geological isolation of radioactive material is possible through several natural examples (or ‘analogs’). One of the most important cases 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 the existence of isotopes of the fission-product gas xenon and by different ratios 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. Studying such natural phenomena is important for assessing geologic repositories and is the subject of several international research projects.