Actually, I think you guys have it backwards. U-238 is fairly stable uranium, and relatively common. Bombarding it with neutrons will not generate a nuclear reaction.
U-235 is unstable, and extremely rare in nature. In fact, the only way to obtain (as far as I know) is to separate it from sources that mostly contain U-238. But because it is unstable, it can be used to generate nuclear reactions.
If I can find a source to confirm this, I'll post it.
I should know better than to make such a dumb mistake, but sometimes I type faster and better than I think.
I don't mind being wrong, I just hate to do it publicly.