Natural fission reactors (NFR) are high-grade uranium deposits in which self-sustained fission chain reactions took place approximately 2 billion years ago. Found only in southeast Gabon, Africa, they are unique physical phenomena in the Earth's crust and the only place where some minerals are composed of elements with nonprimordial isotopic abundance. Despite great efforts, evidence for large-scale nuclear reactions has not been found anywhere else. Natural fission reactors have recently been studied as “natural analogs” for the disposal of high-level radioactive waste in the lithosphere, because they provide the exceptional opportunity to study geochemical behavior of natural fission products that do not normally occur in any significant quantities.