Wednesday, September 06, 2006

On nuclear weapons

There is a loud group of folk who say Iran has, or at worst is on the brink of having, nuclear weapons. When confronted with the enriched uranium production capacity of Iran, they fall back to one of two positions - imports, and gun-type nukes. I'd like to discuss the latter here.

The general argument is that a gun-type fission bomb is still a nuke, and it's a lot easier to build than a fusion bomb, and that takes a lot lower quality uranium to be effective which, they claim, Iran has. Let's look at these briefly.

A gun-type fission bomb is still a nuke. Yes. It's approximately 15KT (using the Hiroshima bomb as an example). Which means pretty much all damage occurs at under 3 miles from the blast point - all damage meaning burns are first degree or greater, and civil (non-reinforced) buildings suffer at least moderate damage (roof and windows, no collapsed walls), and the radiation dosage is nonfatal. Basically, as long as you weren't looking at the blast you're shaken but that's about it if you are more than three miles from the blast. If you're inside a typical WalMart, you're fine if you're more than 2 miles from the blast. If you're in a concrete building, you could be as close as a mile from the blast and be 'just shaken' - though the flying glass will be dangerous. 3KT is big and nasty and a lot of people get hurt and die. It's far from devastating for ANY modern nation. If we were to make an analogy between the US capacity and this, we're seeing a set of brass knuckles facing a .50 cal machine gun supplemented by perrsonal side arms.

A lot easier to build. Yep. Easier doesn't mean easy. Look, the principle is well known. You shove two pieces of uranium, each not quite large enough to attain critical mass, together. They achieve critical mass and you have an explosion. Except that's not quite true. Let's start by looking at another explosive - black powder. If I've got a pile of black powder and I set it on fire, it burns really fast. Things get hot, and there's a flash, but ... "where's the kaboom?" To get the explosion the stuff's got to be contained - the expansion has to be trapped. The critical mass explosion is kind of similar. You can't just have a pile, you've got to have it come together so the burning caused by the critical mass is trapped. This means the stuff has to come together in a near-perfect form and REALLY fast to go "boom" instead of just fizzling. You can supposedly compensate by having more stuff, but it still matters.

One of the interesting things that's publicly known about the bomb used in Hiroshima is that the actual bomb part wasn't the majority of the "bomb". It was basically a big pipe bomb, less than a foot in diameter and only about nine feet long. All the rest, though, was either to ensure it didn't go off early or to ensure it DID go off when it WAS supposed to. Just as one example ... picture the pipe being just a little bit warped when the smaller slug is fired toward the bigger. Oops, it stuck.

Again, "easier" isn't "easy".

Final point - Iran has enough uranium of that strength to make a gun-type nuclear bomb. Maybe. Actually, assuming they ARE trying to reinforce for more effective nuclear weapons, I'll go so far as to say probably - enough for one, maybe two. But if they build a gun nuke with what they've got, well, then they ate their seed corn for more powerful weapons and it puts them WAY behind on that process.

All in all, it makes me doubtful that Iran has a home-made nuclear weapon at this time.


Post a Comment

<< Home