Wednesday, January 30, 2008

Concerning Nuclear Triggers

Some people are raising questions about the potential effectiveness of a nuclear warhead trigger replacement design. The old warhead triggers are aging, with the last of them made back in the 80s. Thanks to the nuclear test ban, new designs for the triggers can't be tested empirically. Naturally, this makes some scientists and engineers worry.

Me, I'd be just as happy letting the warheads go bad and the knowledge of how to make them slip away. I can't imagine there is any real possibility that we will ever actually use them (which is a good thing). And I suspect, if we really twisted their arms, the politicians could find other ways of spending the money it takes to keep the nuclear weapons program alive.

If you are not interested in what some uninformed rube on the Internet guesses is happening with aging nuclear warheads, just skip the rest of this post.

As is often the case in the media, the article brushes over the technical details. I'm going to try to explain what I think is going on. Without the true technical details, I can't be sure that I'm accurate, but I think I've got a decent guess. These plutonium "triggers" are themselves essentially fission bombs. Fusion bombs work by using a conventional (chemical) shaped charge to implode the plutonium trigger causing a fission explosion. (This is the "atomic chain reaction" referred to in the article.) The design of the trigger channels the force of the fission explosion into the hydrogen core, generating the heat and pressure needed to trigger a runaway fusion reaction. It's the hydrogen core that is the ultimate source of the problems.

The main reactant in a fission bomb explosion is hydrogen, that's why they are sometimes referred to as hydrogen bombs or H-bombs. But the hydrogen used isn't everyday normal hydrogen, it's a radioactive isotope of hydrogen with a couple extra neutrons known as tritium. But tritium, being radioactive, doesn't stay tritium forever. On average, for any given macroscopic quantity of tritium, about half of it will transform into helium in a little over twelve years. Helium is much harder to get to fuse than tritium, and can cause a fusion bomb to "fizzle" and produce a fraction of its intended explosive yield. Since no new nuclear triggers have been produced since the 1980s, the tritium inside has had plenty of time to decay. Testing to see how far gone the warheads are involves getting inside those precisely machined triggers, which compromises their precision design and thus makes them unusable.

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