actually i didn't look, i asked a friend of mine, an astro physicist. as you can tell, i surround myself with nerds, i consider myself one, sort of....
uranium is weapons grade when it's "active" enough. this means that it only needs a little bit of of a push to set off a reaction.
the problems are not only producing it... that's enough of a pain, like you said...
the problem is maintaining it at safe, manageable levels, where the rockin and rolling of a launch won't cause it to melt it's reactor down, destroying the rocket, before any sort of payload can be delivered.
also designing a warhead that can fit on the tip of a multi-stage rocket is also very VERY difficult. hell, building the multi-stage rocket itself is hard enough....
hell, i don't believe Iran has any ICBM's that can reach the U.S.... hmmm.. starting to wonder why the nuclear weapons is such a big deal....