Lets see if I can pull some of this together for you.
There are two principal materials used for nuclear bombs Highly enriched Uranium and Plutonium.
The first American bomb at the Trinity test site was Uranium as was Hiroshima. The Nagasaki Bomb was Plutonium.
You don't need a nuclear powerstation to enrich Uranium but you do to make Plutonium.
Nuclear powerstations generally run on uranium enriched to a lower level than you use for a bomb but some older ones use raw, un enriched Uranium. The spent Fuel rods removed from a nuclear reactor contain all sorts of nasty materials amoungst them Plutonium. Plutonium is extracted from such spent fuel rods at Sellafield.
Plutonium is more difficult to make a bomb from but it's a lot easier to manufacture than enriched uranium. To enrich the Uranium it has to be made into a gas Uranium hexafloride and the Uranium 235 which is lighter split off from the non radioactive Uranium 238 (this is depleted Uranium you may have heard of being used in artiullary shells). In WWII this can be done with huge centrifuges, or various diffusion processes. In any case its dangerous and difficult.
The Americans did construct nuclear reactors to create the required plutonium during the war this was done at Oak Ridge and Hanford
http://www.mbe.doe.gov/me70/manhattan/come_thr ough.htm
I think it's pretty clear rthat the main reason for Iran's nuclear program is to keep George Bush out of their country