However, you DO need breeder technology to treat all the 238U as a fuel. With burner reactors, the 239Pu produced is less than the fissionable material consumed, and most of the 238U ends up not being usable.

I'll add that with current technology, you can't reprocess MOX fuel another time (too many higher isotopes of Pu and transuranics beyond Pu are produced), so any Pu in it after it's consumed must be disposed of.