What does Iran need to make a nuclear bomb?
Making a nuclear bomb is a complex process. The first challenge is getting hold of the radioactive raw material required.
Uranium is one option. Commercially available, it also occurs naturally. However, it is made up of two isotopes: 99.3 percent is uranium-238 and 0.7 percent is uranium-235. Only the latter is used to produce nuclear energy.
To separate the two, centrifuges are used.
The heavier U-238 is discarded and the lighter U-235 is re-injected into the next centrifuge. Many centrifuges are needed to obtain a significant amount of useable uranium-235.
To generate power uranium is enriched only to five percent. To make a nuclear bomb it must be enriched to at least 90 percent. Plutonium-239 is a second option. It is produced with uranium in nuclear power reactors. The uranium is separated off and the plutonium extracted.
It takes some 25kg of Uranium-235 or eight kilograms of Plutonium-239 to make a nuclear bomb.
The final challenge is actually building a device with a detonating system and explosives capable of triggering the required nuclear fission chain reaction. Source