It would be useful at this point to talk a bit more about Uranium and its isotopes.

Uranium naturally occurs as two isotopes details of which are shown below.

Of the two isotopes only Uranium-235 is fissile. However some of the Uranium-238 is converted to Plutonium-239 via neutron capture in a nuclear reactor.

It occurs as an oxide mainly U₃O₈ or UO₂ although other oxides also occur. The oxides are chemically quite stable.

Uranium average concentration in the earths crust is about 3 parts per million. In sea water it is about 3 parts per billion. With a few rare exceptions it rarely occurs in high concetrations. A good ore for mining may contain 0.2% U₃O₈ although ores with as low as 0.02% U₃O₈ are currently mined.

Since both isotopes of Uranium undergo decay they always exist with their daughter products (see decay chain).

Enrichment

It is possible to use natural uranium as a fuel if heavy water or graphite is used as a moderator. Most modern reactors, however, use fuel with a greater percentage of Uranium 235 than the 0.7% that occurs naturally.

This process is called enrichment. The uranium is enriched so that it contains 3-5% Uranium 235 for most reactors. If it is enriched further it can be used to make nuclear weapons.

Since these are two isotopes of the same element they are chemically identical and so they must be seperated by they physical properties – i.e. their mass. Since their masses differ by just over 1% this is a very difficult task

 WIkipedia