Editing Nuclear fuel cycle (section)

===Exploration===
A deposit of uranium, such as [[uraninite]], discovered by geophysical techniques, is evaluated and sampled to determine the amounts of uranium materials that are extractable at specified costs from the deposit. Uranium reserves are the amounts of ore that are estimated to be recoverable at stated costs.

Naturally occurring uranium consists primarily of two isotopes U-238 and U-235, with 99.28% of the metal being U-238 while 0.71% is U-235, and the remaining 0.01% is mostly U-234. The number in such names refers to the [[isotope]]'s atomic [[mass number]], which is the number of [[proton]]s plus the number of [[neutron]]s in the [[atomic nucleus]].

The atomic nucleus of U-235 will nearly always fission when struck by a [[free neutron]], and the isotope is therefore said to be a "[[fissile]]" isotope. The nucleus of a U-238 atom on the other hand, rather than undergoing fission when struck by a free neutron, will nearly always absorb the neutron and yield an atom of the isotope U-239. This isotope then undergoes natural radioactive decay to yield Pu-239, which, like U-235, is a fissile isotope. The atoms of U-238 are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile Pu-239.