Quick ‘Chemistry 101’ Refresher
Naturally occurring Uranium (U) is composed of isotopes , all have the same Atomic Number (92) but different Atomic Weights. These isotopes have similar chemical properties but different nuclear properties.

U-238 The most plentiful/abundant; over 99% of nature uranium is U-238.
U-235 The only fissile uranium isotope; approximately 0.72% of natural uranium is U-235.
U-234 Makes up only 0.0055% (55 parts per million) of the raw uranium, although enriched uranium contains more U-234 than natural uranium.