Break Out Capacity
"The answer I propose, which explains all the anomalies elegantly and concisely, is that Iran is seeking nuclear latency. Latency is the possession of a nuclear energy program and of reactors, which would allow the production of an atomic bomb on short notice if an extreme danger to national autonomy reared its ugly head. Nuclear latency is sometimes called the 'Japan option,' because given its sophisticated scientific establishment and enormous economy, Japan could clearly produce a nuclear weapon on short notice if its government decided to mount a crash program.
"The reason for the construction of the Qom facility, in this reading, would be that the Natanz facility is too easily bombed or struck with missiles. Moreover, the Israelis and some Americans have repeatedly threatened to strike it. A nuclear enrichment program such as that at Natanz, which is subjec to being wiped out by a military strike, cannot truly provide nuclear latency. The Qom facility was necessary in the regime's eyes if the latency strategy was to be preserved.
"The regime has every reason to maintain latency and no reasons to go further and construct a nuclear device. The latter step would attract severe international sanctions."