Iran's nuclear program is spread throughout a variety of experimental laboratories, hardened enrichment facilities, heavy water manufacturing plants, and two plutonium reactors currently under development (Bushehr could come online within a few months). That far exceeds what's needed to turn on the lights, but it's also beyond what's needed for a basic nuclear weapons program.
Consider North Korea, which manufactured two limited yield nuclear weapons using only a plutonium reactor, a plutonium reprocessing facility, and -- presumably -- some sort of weapons laboratory. Why is Iran pumping billions more into building and protecting triple the number of facilities required to build a basic nuclear weapon, akin to the Fat Man or Little Boy bombs detonated in 1945?
The answer could be that Tehran is skipping basic weapons construction and moving towards an advanced thermonuclear design. Consider that they've already experimented with advanced weapons designs like two-point implosion, nuclear triggers, and have built their own facility at Arak that could be used to produce both tritium, which is a suspected boosting agent in hydrogen bomb designs, as well as weapons-grade plutonium. They've spent billions building, hardening, and protecting uranium enrichment, which could be used along with plutonium in a staged nuclear device. All this at an astronomical cost and effort compared to the similar North Korean nuclear program.
And what of Ahmadinejad's recent visit to South America, where he showed keen interest in Bolivia's massive lithium reserves? Lithium is commonly used in batteries and electric cars, but it can also be engineered into lithium-6, which is reportedly used as fusion fuel in two-stage thermonuclear devices.
Iran may be skipping basic, cumbersome nuclear designs and moving straight to a fully deliverable hydrogen style weapon akin to what's employed by the United States and Russia. Such a powerful weapon would compensate for Iran's inaccurate missile fleet and allow them to hold vast swaths of allied and American territory at risk.