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Natural uranium (NU) refers to uranium with the same isotopic ratio as found in nature. It contains 0.7% uranium-235, 99.3% uranium-238, and a trace of uranium-234 by weight (0.0055%). In terms of the amount of radioactivity, approximately 2.2% comes from uranium-235, 48.6% uranium-238, and 49.2% uranium-234.
Natural uranium can be used to fuel both low- and high-power reactors. Historically, graphite moderated reactors and heavy water moderated reactors have been fueled with natural uranium in the pure metal (U) or uranium dioxide (UO2) ceramic forms, however experimental fuelings with uranium trioxide (UO3) and triuranium octaoxide, (U3O8) have shown promise.
The 0.72% U-235 is not sufficient to produce a self-sustaining critical chain reaction in light water reactors or nuclear weapons; these applications must use enriched uranium. Nuclear weapons take a concentration of 90% U-235 and light water reactors require a concentration of roughly 3% U-235. Unenriched natural uranium is appropriate fuel for a heavy-water reactor, like a CANDU reactor
In rare occasions, earlier in geologic history when U-235 was more abundant, uranium ore was found to have naturally engaged in fission, forming natural nuclear fission reactors. Uranium-235 decays at a much slower rate (half-life 700 million years) rather then uranium-238 which decays extremely slowly (half-life 4.8 billion years) Therefore a billion years ago there was much more then double the U-235 that there is now.
During the Manhattan Project, the name Tuballoy was used to refer to natural uranium in the refined condition; this term is still in occasional use. Uranium was also called codenamed "X-Metal", during World War II.
Natural uranium, contains some U-235, unlike depleted uranium.
See also 
- Depleted uranium
- Enriched uranium (Oralloy)
- Nuclear engineering
- Nuclear fuel cycle
- Nuclear physics
- Nuclear reactor
- Nuclear weapon
- Natural nuclear fission reactor
- Oklo reactor
- List of uranium mines
- Design Parameters for a Natural Uranium Fueled Nuclear Reactor, C. M. Hopper et al., ORNL/TM-2002/240, November 2002.
- Loveland, W.; Morrissey, D.J.; Seaborg, G.T. (2006). "Chapter 16 Nuclear Reactor Chemistry". Modern Nuclear Chemistry.