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Natural uranium (NU orUnat[1]) isuranium with the sameisotopic ratio as found in nature. It contains 0.711%uranium-235, 99.284%uranium-238, and a trace ofuranium-234 by weight (0.0055%). Approximately 2.2% of itsradioactivity comes from uranium-235, 48.6% from uranium-238, and 49.2% from uranium-234.
Natural uranium can be used to fuel both low- and high-powernuclear reactors. Historically,graphite-moderated reactors andheavy water-moderated reactors have been fueled with natural uranium in the pure metal (U) oruranium dioxide (UO2)ceramic forms. However, experimental fuelings withuranium trioxide (UO3) andtriuranium octaoxide (U3O8) have shown promise.[2]
The 0.72% uranium-235 is not sufficient to produce a self-sustaining criticalchain reaction inlight water reactors ornuclear weapons; these applications must useenriched uranium. Nuclear weapons take a concentration of 90% uranium-235, and light water reactors require a concentration of roughly 3% uranium-235.[3] Unenriched natural uranium is appropriate fuel for aheavy-water reactor, like aCANDU reactor.
On rare occasions, earlier in geologic history when uranium-235 was more abundant, uranium ore was found to have naturally engaged in fission, formingnatural nuclear fission reactors. Uranium-235 decays at a faster rate (half-life of 700 million years) compared to uranium-238, which decays extremely slowly (half-life of 4.5 billion years). Therefore, a billion years ago, there was more than double the uranium-235 compared to now.
During theManhattan Project, the nameTuballoy was used to refer to natural uranium in the refined condition; this term is still in occasional use. Uranium was also codenamed "X-Metal" during World War II. Similarly, enriched uranium was referred to asOralloy (Oak Ridge alloy), anddepleted uranium was referred to asDepletalloy (depleted alloy).