Natural uranium (NU, U_nat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% 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% uranium-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% uranium-235, and light water reactors require a concentration of roughly 3% uranium-235. Unenriched natural uranium is appropriate fuel for a heavy-water reactor, like a CANDU reactor.