Why Does Iran Want a Heavy Water Reactor?
The mullahs plan to build a heavy water reactor near Arak, Iran. But what the hell is a heavy water reactor? Jane's has a summary here. In short, a nation with a sufficiently powerful heavy water reactor can use it to turn uranium into bomb-usable plutonium without requiring enrichment facilities.
A heavy water reactor uses natural uranium which the Iranians can mine themselves. The kind of reactor the Iranians have planned could produce 10kg of high-quality plutonium per year. Plutonium is a key ingredient when building an implosion-trigger fission bomb or worse, fusion bomb. Fat Man, an implosion-triggered fission bomb, was dropped on Nagasaki in 1945. It had a 23-kiloton yield and killed 45,000 instantly, about 25% of the population. No one has yet used the more powerful fusion type bomb in war. These types of bombs produce yields in the megaton range, thousands of times more powerful than fission bombs, equal to the explosive power of millions of tons of TNT.
Could Iran build a Fat Man? In theory, if they are allowed to build the reactor and the other necessary facilities, why not? They mine their own uranium in Iran. They can use the reactor in Arak to produce plutonium. The other required ingredients are either common elements, such as beryllium or are found in uranium ore, such as polonium.
The US built Fat Man using the technology and resources available in 1945 and pursuing a theory that no one knew would even work. With all the technology and scientific knowledge available in 2004, it is certainly believable that the Iranians can build their own Fat Man. The real question is: Will the world let them?