How close is Iran to building a nuclear weapon? And what can the US do to stop it?by Mark Fitzpatrick / June 25, 2006 / Leave a comment
Other articles in the Prospect online symposium on the Iranian nuclear crisis:
Philip Gordon explains why the US is unlikely to bomb Iran Michael Rubin argues that diplomacy is not enough Alastair Crooke says that the west are trampling over Iran’s rights Nazenin Ansari suggests that the Iranian state may be susceptible to sanctions Esther Herman on her encounters with everyday Iranians
Anyone seeking to build a nuclear weapon needs two things: 1) enough fissile material for a critical mass (either 20-25kg of highly enriched uranium, the material used in the Hiroshima A-bomb, or 6-8kg of plutonium, as used in Nagasaki) and 2) a “weaponisation” package for a controlled fission reaction. They will also need a delivery vehicle—typically an aircraft or ballistic missile, but a suicide vessel or truck would do.
In the case of Iran, attention has focused on its uranium enrichment programme. Uranium enrichment involves increasing the concentration of fissile U-235 in uranium. What does this mean? The U-235 isotope makes up 0.7 per cent of naturally occurring uranium. U-235 is an isotope that will split, or fission, when struck by a loose neutron, emitting radiation energy and more neutrons that can split other atoms in a chain reaction. (Isotopes are atoms of a given element with the same chemical make-up and the same number of protons but varying numbers of neutrons. The number after the chemical symbol—U in uranium’s case—is the atomic mass, the number of protons and neutrons, and is used to denote different isotopes.) But the bulk of natural uranium is the stable U-238 isotope, which cannot sustain a chain reaction. The point of the process of enrichment is to increase the concentration of U-235.
Uranium is enriched by passing it through a series of centrifuges—1.8cm-high spinning tubes that use centrifugal force to alter the concentration of the different uranium isotopes. Connecting 164 of the centrifuge machines together in a cascade, where the gas is successively enriched in each of several stages, provides a basic module for an enrichment facility. For nuclear fuel for reactors, such as the one Russia is completing at Bushehr, the U-235 content must be enriched to about 3.5 per cent for a controlled nuclear reaction. By contrast, weapons-grade uranium requires enrichment to over 90 per cent. Although that seems to be a far greater leap, once you have reached the 3.5 per cent fuel threshold, half the work…