A contact in the Pentagon asked for a cheat sheet on nuclear technology. Putting something together that is technically correct and concise, while introducing all the policy-relevant terminology is a challenge. Matthias, John, any suggestions/corrections?

There are two “pathways” to the bomb: uranium enrichment and plutonium.

Diagram from: http://www.isisnucleariran.org/sites/weapons-fuel-cycle/

1. Uranium enrichment
Natural uranium is plentiful in nature, but to be weaponized it must be converted into a form that can be used to sustain a nuclear chain reaction, the physical process that releases energy. This 15 minute video from the 1950’s is my favorite explanation of nuclear fission: A is for Atom

Natural uranium is made up almost entirely of two isotopes, one of which is the slightest bit heavier than the other. Only the lighter isotope, U-235, is useful for sustaining a chain reaction. Fortunately, at least from a nonproliferation perspective, natural uranium is 99.3% U-238, so in order to be weapons usable it must be “enriched” to separate out the desirable U-235 from the undesirable U-238–or fed into a Heavy Water Reactor, which I will come back to when I explain the plutonium pathway.

The process of enrichment is mechanical. Natural uranium in its gaseous form (UF6) is fed into a centrifuge:

Image from: http://fissilematerials.org/library/ipfmreport06.pdf

Because U-235 is lighter than U-238, when you spin UF6 the heavier U-238 flies toward the outside wall and collects in the bottom of the centrifuge. The “depleted” stream of U-238 can then be funneled out. The enriched uranium, U-235 along with the remaining U-238, is siphoned off and into another centrifuge. The process is repeated thousands and thousands of times until the desired level of enrichment is achieved.

At first the enrichment process goes very slowly. Getting from the .7% U-235 found in natural uranium to 3-4.5%–the minimum amount necessary to fuel a light water reactor–requires 70% of the time and effort it would take to produce weapons-grade material (90% U-235). By the time uranium is enriched to 20% U-235, you are already 85-90% of the way there. This is why the cut-off for what counts as low enriched uranium (LEU) is set just below 20%. There are no reactors that require more than 20% enrichment and if you go any higher you basically already have what you need to create a bomb. (Jeffrey Lewis has a nice post about this on Arms Control Wonk.)

So, to summarize:
Natural Uranium = .7% U-235
Low Enriched Uranium = < 20% U-235
High Enriched Uranium = 20-90% U-235 (90% of the way to weapons grade)
Weapons Grade Uranium = >90% U-235

2. Plutonium
Plutonium, in theory, could appear in nature. However, in practice, it must be generated through a nuclear chain reaction. All nuclear reactors produce plutonium, but there are many kinds of reactors and some are better for plutonium production than others. If a country wants to build a nuclear explosive device without having to enrich uranium, it can use a heavy water reactor. Unlike light water reactors, which are cooled with regular old H2O, heavy water reactors are cooled with water that has an extra isotope of hydrogen (D20), which enables natural uranium to sustain a nuclear chain reaction. The plutonium necessary for a bomb can then be separated out from the spent fuel. This is what North Korea did.

Iran has uranium enrichment facilities, a heavy water research reactor and another under construction, and light water reactors for training purposes and energy production. For more information than you will possibly need on Iran’s nuclear sites visit ISIS’s page on Nuclear Iran.