Basically, natural Uranium is made up mostly of the U-238 isotope. The isotope responsible for Fission is isotope U-235. Enrichment increases the amount of the fissile U-235 isotope. Reactor grade Uranium contains about 4% U-235 where Natural Uranium contains < 1% U-235. The article below has some basic explanations of enrichment techniques.
Just hop over the border into Pakistan they understand all the science. U235 fissions more readily. Be careful here because nuclear info is dangerous. Bombs and all that goes with them. Always think of who else may read your posting, not all are friendly.
Uranium Enrichment?
Re: Uranium Enrichment?
