Iran most likely will develop a nuclear capability far sooner than the NIE timeline of around 2015 . They will do it because they likely already have the technology and they are in a hurry to close the window of vulnerability from an attack by the West. All they need is sufficient weapons grade uranium.
Iran will probably take the South African approach, where 25 years ago they developed a lightweight gun type weapon for the similar reasons. South Africa needed a weapon soon, not later because of a perceived threat from Soviet backed Cuban forces in nearby Angola. Much of the analysis both classified and from think tanks, mistakenly presupposes Iran will only pursue an implosion device. That’s what most nations would do and are doing. But most nations have different aims; deterrence and massive retaliation if attacked. Numbers of weapons are important. Iran on the other hand is more into terror and nuclear blackmail. For Iran numbers are less important than a credible threat.
There are two roads to nuclear explosives; one, highly enriched uranium (U-235), the other - plutonium. Uranium is the most difficult road, requiring massive centrifuge plants that use large amounts of electricity to concentrate the fissionable isotope U-235 to weapons grade (80-93%). The plants are difficult to hide from prying eyes.
Plutonium on the other hand is naturally produced in power and research reactors and can be extracted easily from spent fuel by simple chemical precipitation. Covert extraction of plutonium is far easier to hide than enriching uranium. Iran is pursuing the more difficult uranium road. North Korea is going for plutonium.
And there are also two roads for weapons design. One is an implosion device (used at Nagasaki) or a gun type (Hiroshima) where two subcritical masses are literally shot into each other to form a critical mass and a nuclear explosion. The former is more efficient and produces a bigger bang. For these reasons, most in the nuclear club have developed only implosion devices. On the other hand, the less efficient gun type device can be developed faster and is easier to design with a very high assurance of success even without testing (the Hiroshima device was never tested before it was used). A limiting factor for gun type weapons is they can not use more easily obtained plutonium, only uranium. But this limitation doesn’t affect Iran, which has begun enriching uranium on a massive scale.
The South African Experience
In the mid 1970’s feeling threatened by Soviet/Cuban intervention in Africa, the South African government started down the road to becoming a nuclear power. Similar to Iran today, they settled on the production of uranium as the fuel and developed a gun type of weapon. Between 1982 and 1989 they completed 6 lightweight (750 kg/1,650 lb) weapons with an estimated yield of 10-18 kt. (Hiroshima was 15 kt.). A seventh was partially completed when the South African government voluntarily made the decision to give up its nuclear aspirations. The weapons were deliverable by either their Buccaneer jet fighter-bombers or locally produced ballistic missiles.
There are serious parallels between Iranian and South African programs. Both use(d) uranium, giving the ability to make either implosion or gun devices. Iran sees time as the enemy, prolonging its window of vulnerability. The question is not whether they will develop the simpler gun type weapon, but why wouldn’t they? They can have their cake and eat it too. While it takes about three times as much uranium for a gun weapon, around 5 years down the road when they have an operational implosion system, they can use the fissile fuel from the earlier weapons for three of the new. It’s win, win.
In addition you can’t ignore the knowledge sharing going on between Iran and North Korea in both nuclear and missile technology. Iranian scientists were present at both N. Korean nuclear tests and most of their missile technology is N. Korean. The Koreans are doing the heavy lifting on implosion devices, but they are stumbling. Their first test was essentially a fizzle (less than 1 kt) and the second most likely only around 4 kt. That shouldn’t give the Iranians a lot of confidence.
No, they will pursue the low tech gun approach for an interim weapon and surprise us.