[Redcloak]

As far as I know (And I'll leave more definitive responses to our resident Glow-boys), a nuclear reactor can run on mixed fuel; i.e. rods that contain both isotopes. I don't think that you need to pull out the U₂₃₅ to make the reactor work. The only real reason would be to obtain the isotope for weapons.

[far sider]

Uranium facts:

U-235 is fissionable. U-238 is not.

Enriched Uranium has a higher-than-natural level of U-235. The more highly enriched the Uranium is, the more U-235 it contains, and the more potentially dangerous it is. Moderately enriched Uranium is generally used for domestic power generation. Highly enriched Uranium is needed for nuclear weapons (and nuclear fuel for subs, carriers, etc.)

Enrichment of uranium is done by diffusion (old technology, still used in the US and France), centrifuge (prototype only in the US), and laser (high tech).

Depleted Uranium (99+% U-238) is about as dangerous as Lead but heavier.