It is fortunate for the world that making a reliable nuclear weapon is not easy. Just initiating a runaway fission reaction under controlled conditions in a friendly test range is tricky enough. To take that laboratory prototype and turn it into a deliverable, secure weapon that will explode when you want it to, and more importantly, will
not go off when you don't want it to, is trickier still. Things like neutron triggers and tampers provide greater reliability that it will detonate as desired, and they mean additional subsystems on the bomb must be separately armed, with separate arming codes and fail-safes. To wrap a compound of hydrogen with half a dozen or more fission bombs, all perfectly timed to within a microsecond, to
compress and heat the hydrogen to fusion temperatures is exponentially harder.
But the most important part of any bomb, whether it's a Little Boy fission device that yields mere kilotons or the massive Tsar hydrogen bomb that pushed 100 megatons, is the heavy nuclear material at the fission core. If that stuff doesn't go super-critical just like its supposed to, the entire bomb fizzles. Uranium 235 is the most common choice for that material (Its not the only material that will work). But in nature, once natural uranium has been refined from ore, the desired 235U, only accounts for about 1% of the refined stuff. The rest of it is mostly 238U, which does not posses the right properties for a super-critical fission chain reaction, i.e., a nuclear explosion.
Keeping the Iranians from getting that pure stuff is really what this nuke deal is all about. Follow below the fold for weapons designed 101 and a question for those in the know.
The process for producing concentrated 235U is called enrichment. To power a civilian nuclear reactor, you need to get about three or four percent pure 235U. To run the kind of military reactors used on a carrier or a submarine, that has to be bumped up to around 10 - 20%. But to build a fission bomb you need 90% or more.
Separating 235U from 238U is is a laborious process, the two isotopes are chemically identical, they only vary in the number of neutrons in the nucleus. That means they only vary slightly in mass. When uranium is combined with another element to make it easier to handle, that tiny mass differential is diluted even further.
There are several ways to concentrate the 235U, the most common method used today starts with uranium hexa-fluoride or "hex,", easily heated to a heavy gas which is then piped through a series of gas centrifuges. These particular centrifuges look a lot like the bottom stage of a rocket without fins, they are narrow, tall cylinders that spin on the vertical axis like a pottery wheel, only very, very fast -- that's why the Bush admin was able to portray components of artillery rockets in Iraq as gas centrifuges producing Weapons of Mass Destruction, the 'smoking gun that would come in the form of a mushroom cloud'.
While the details are classified, it is generally understood that during the Cold War, the US developed small arrays of very large centrifuges which made a lot of weapons grade material to serve our large and growing nuclear arsenal. They were so big the production bottle-neck shifted from spin time to supplying the centrifuges with the large, high precision ball bearings they needed to operate. Or so go unclassified rumors anyway ...
Cascade of gas centrifuges used to produce enriched uranium. U.S. gas centrifuge testbed in Piketon, Ohio, 1984. Each centrifuge is some 40 feet (12 m) tall. (Conventional centrifuges in use today are much smaller, less than 5 metres (16 ft) high.)
But other nations continued to use much, much larger series of much smaller centrifuges. These produced far less material per operational hour, but its a robust, proven technology. If a particular one broke down it could be valved off resulting in a slightly longer time for production in the rest of the series to the desired grade. And, some would argue, smaller centrifuges in larger series made it easier to cheat: if the goal was to convince nosy inspectors or nosy allies that the series was only producing reactor grade material, and not highly enriched uranium (HEU), centrifuges could be removed from premises, or the piping between them changed around.
The takeaway from this is an enrichment facility using small centrifuges is a big operation. It draws lots of power, it takes up lots of space, it requires many technicians and engineers to keep it going. Reports are Iran has their primary enrichment facility under many meters of rock and earth. It's thus essentially bomb proof; only another nuclear bomb dropped dead on would provide the destructive power to knock it out of commission permanently.
It takes dozens to hundreds of large centrifuges, or thousands of smaller ones, all operating in a series, to produce enough HEU for a single bomb in any practical time-frame. Significantly reducing the number of small centrifuges in operation in a large series is thus a highly effective way to slow down weapons grade production while still leaving the facility with the capacity to produce material for a reactor or other uses. And that's what this deal with Iran does. It reduces their centrifuge count:
Iran has agreed to reduce by approximately two-thirds its installed centrifuges. Iran will go from having about 19,000 installed today to 6,104 installed under the deal, with only 5,060 of these enriching uranium for 10 years. All 6,104 centrifuges will be IR-1s, Iran’s first-generation centrifuge.
Having unfettered access to the facility is important for verification purposes. But in the end, the only people who can
guarantee that facility lacks the number of centrifuges to make enough HEU for bombs within an arbitrary time-frame are the Iranians who run it.
My question for crowd-sourcing and the reason for posting this as a diary, how easy would it be to cheat on that deal? Is hex portable, could it be moved from one facility to another at some intermediate grade, how easy would thousands of extra centrifuges be to conceal or remove when inspectors are due?