What they're doing, and I should be clear that this is the prevailing opinion of analysts, not stated doctrine, is that they are intentionally enriching uranium to 60% and stockpiling it, to maintain enough deniability to say "teehee, it isn't even weapons grade, chud! We are truly running a peaceful nuclear program!" The obvious problem is that 60% isn't used for anything except to refine it further to weapons grade, but it's just far enough away that a country like the US might not blow your shit up for making nukes. The other thing is that warheads are useless unless you have a delivery system, which is the other part of the strategy, they refine a bunch of uranium up to 60%, stockpile it, then work on implosion tests, miniturization, and delivery vehicles, and then once you have warhead designs you are fairly certain work, and missiles or planes to deliver them, then you reprocess all that 60% into weapons grade, and virtually overnight you have a small arsenal of nuclear weapons.