Friday, October 20, 2006

Nukes for Beginners

What with all the news about North Korea and Iran, I figured it might be worthwhile to go over some of the basics of how these things work. There's a lot of misinformation out there. And while there's a lot to be worried about, there's a lot going on that you really don't have to be so upset about.

Now, we all know about radioactivity, and how uranium and plutonium can be used in nuclear reactors to generate electrical power. Just about anything that can give off heat slowly can be goaded into doing it faster. But the devil's in the details.

To make uranium or plutonium go boom, you have to achieve critical mass. There are two ways of doing this.

The easy way is just to get enough highly purified metal in the same place at the same time. Collect enough highly enriched uranium in one lump, and it will explode all by itself, without any outside encouragement. That's how the Little Boy bomb worked. They had two parts of a sphere of uranium, and at the appropriate time, all they had to do was slam them together. The reason this approach isn't taken anymore is that it's all to easy for such a bomb to go off before you really want it to. It doesn't matter if the parts all come together on purpose or by accident, when they're united, it's flame on.

The hard way is to compress the metal so that, locally, it's dense enough to constitute a critical mass. This is somewhat tricky. It requires the use of specially-shaped explosives to squeeze down a sphere of already fairly dense metal in exactly the right way to produce the desired result. Do it wrong, and all you have is a very expensive mess.

What got me thinking about all this, is that NK claims to have designed and built a four-kiloton warhead.

It's freaking hard to build a small-yield weapon. Hard enough that, today, the US doesn't think it's even worthwhile. We've had a few, but only a few. Some, like the AIR-2 Genie nuclear air-to-air missile (Mk. 25 Mod. 1), were made obsolete by guided missiles. Others, like the Davy Crockett nuclear bazooka, were just plain stupid ideas to begin with. (What kind of idiot designs a bazooka with a blast radius that exceeds the range?) But by the late '60s, we'd given the whole lot up as a bad idea. The results don't justify the expense.

But ... if your missiles aren't all that big, you need a small payload if you're going to air-mail it to anybody. We had a similar problem in the late '50s, but we also had Edward Teller.

The easiest uranium bomb to build would be something along the lines of Little Boy: weighing in at something like 4 to 5 tons. The upside: you know it's gonna work, first time, every time. The down side: there's no good way to get it to anybody you think needs blowing up. Dragging a five-ton parcel across somebody's border is bound to attract all kinds of unwanted attention.

So: you have to think small. But that takes a lot of sophistication. It takes very meticulous scientific work, and extraordinarily precise machining. And you have to have done your preparation work right. If your metal isn't of the highest purity, it's all wasted effort. Reactor-grade won't cut it, not even a little bit.

Steven Den Beste had an article up a few days ago, referring to this issue. He was wondering if the North Koreans' plutonium hadn't been refined too hastily. Apparently, if you rush the job, you don't get the right mix of isotopes, and it won't catch.

And if their plutonium is, in fact, just plain bad ... well, they can test all they want. The bomb might make a big, awful mess, but it won't go "boom".

Now, setting one of these off in downtown Seoul would be horrible enough for any ordinary purpose. But it's not something that will inspire fear so much as it will inspire all-consuming homicidal rage. Somehow, I don't think that's the effect Mr. Kim was looking for.

So. Now, we wait to see if they're going to have another go at lighting it off. But can Mr. Kim afford another fizzle? Can his credibility afford it, what's left of it? Can he afford yet another spectacle that enrages what few friends he does have left in the world?

Be afraid, Mr. Kim, be very afraid: that next knock at the door won't be us, it'll be the Chinese secret service. And they won't be happy with you. No, not even a little bit.

UPDATE: Mr. Kim takes the hint, and sings a few verses of the Very Sorry Song to China. (Page down for the lyrics.)

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