lol, all these talk about nulcear weapon, but I thinks most of you have no clue on how you can make one.
First of all, we are talking about Uranium-based Nuclear Weapon, not Plutonium-based, you enrich Plutonium cake differently than Uranium, so I am not going to talk about Plutonium base enrichment here, if you want to know the different, go read up this
en.wikipedia.org
The natural existence of Uranium is U238, U235 is the fissile isotrop that exist in 0.7% of Uranium naturally, and a U238 ore will make you sick but it will not be able to sustain nuclear fission because there are not enough unstable electron in it, and you need to keep splitting the Uranium atom to self-substain a fission cycle, that's the main reason why nuclear energy exist, you split a U238 with a electron from U-235 and break it down, energy release when that atom break down, and if you have enough U-235 in an fuel cake, you can sustain that cycle, 0.7% will get you no where.
NUlear Power Plant uses 3 to 5 % enriched Uranium, Enriching Uranium is basically you put more U235 on a fuel, above the original 0.7%, that way you can sustain a longer chain reaction, you do this by putting Uranium into a Centrifuge, both 235 and 238 have the same chemical property, and the only thing different is the atomic mass, and 3 is not a very big mass, a centrifuge will spin the Uranium and separate the tiny mass by driving it into a centrifical force, the "heavier" 238 will stay outside of the cylinder, while the lighter 235 will concentrate in the middle, that's how you separate U235 and U238, however, you can't do this on one go, and it takes helluva power to do that, so you have to constantly spin the same fuel and taken out a chunk of U238 with each spin, which mean there are a lot of calibration you will need to do during the entire process, and for a Weapon Grade Uranium, you need to enrich that Uranium beyond 85% (the lowest of the require mass to self sustain "indefinitely". Most Centrifuge works from stage between 0.7% to 1% then from 1% to 3% then from 3 to 10, 10-20, 20-50, 50-60 (the most critical, as the U235 mass is then stable enough to not swing out with U238 atom) then 60-85+. Once you have the cake at 85%, you have your HEU (Highly Enriched Uranium) that is good for nuclear weapon.
Conventional (not experimental power plant) can only take a 5% cake, otherwise you are going to melt the reactor down, so when a country having an Uranium cake beyond 20%, that mean you have the intention on making it into Nuclear Weapon, the last report IAEA finding is Iran have a 22% cake (That's about 10 years ago IIRC, but don't quote me on that) so Iran is without doubt they are trying to make nuclear weapon..