Uranium is a naturally occurring radioactive element found everywhere on Earth including rocks and soil, rivers and oceans, and even the human body. Uranium ranks 48th among the most abundant elements found in natural crystal rock and is 40 times more naturally abundant than silver.

Natural uranium is primarily made up of two isotopes, uranium 235 ("U235") and uranium 238 ("U238"). The relative proportions of each isotope found in natural occurring uranium is 99.28% U238 and 0.71% U235.

The important characteristic of uranium for nuclear power, is its ability to fission, or split into two lighter fragments when bombarded with neutrons releasing energy in the process. Of the naturally occurring uranium isotopes, only U235 can sustain a chain reaction, a reaction in which each fission produces enough neutrons to trigger another, such that the fission process is maintained without any external source of neutrons.