A Physicist Grumbles About Jericho

As regular readers know, I’ve been watching the CBS show “Jericho,” about the town of Jericho, Kansas and what happens after Denver vanishes in a nuclear conflagration. The drama has been so-so, but in the pilot I was hooked by the eerie and frightening sight of the mushroom cloud rising into the air.

So, into the second episode. We’re ticking along, the fine upstanding citizens of Jericho merrily worrying about the fallout that’s headed their way. Someone says that Denver was probably hit with a hydrogen bomb. “How are they different from nuclear bombs?” someone else asks. And the mysterious-stranger-with-a-secret ominously says, “They literally explode the air.”

He probably said something after that, but I couldn’t hear it over my choked-back cries of rage.

So: a short primer on nuclear weapons. I’m a physicist, so you know I’m right.

Fusion and Fission Weapons

Nuclear weapons fiddle with atoms’ nuclei to release a whomping huge amount of energy. They come in two flavors, depending on whether they use fission or fusion.

Fission bombs were the first kind created. When people talk about atomic bombs, this is usually what they mean. The idea is that you take an atom and, using neutrons, split it into smaller bits, plus neutrons and some leftover energy. If you get enough of the right kind of material smushed together, those extra neutrons from one atom splitting causes other atoms to split, and so on and so on. The result is a cascade of splitting atoms and a huge amount of energy released in a split-second. In other words: big boom. To smush the radioactive material together, you can shoot a bullet of the material into a target made of the same material, or you can surround a core of the material with normal explosives and squeeze it down like play-doh in your fist. Dangerous play-doh.

Fission bombs are okay, and they’re dead easy to make if you’ve got the right uranium or plutonium hanging about, but you run into a couple of problems with them. One, they can explode too early, before you’ve smushed all of your material into a compact-enough mass to make the fission reaction run away. Then you get a small boom and a lot of left-over radioactive material that gets spread around. Hello, dirty bomb! Two, at best you get an explosion that’s equivalent to around 700 to 750 kilotons (kt) of dynamite, because you can only use up so much of your radioactive fuel before it all goes kablooey. 750 kt is a lot, but you’ll want something bigger when your next-door neighbor drives up in his shiny new weapon of mass destruction.

Enter the fusion bomb. Instead of splitting atoms’ nuclei apart, fusion bombs smush nuclei together. You start with something like hydrogen (or, really, a version of it called tritium) and squeeze it together until you have helium and a whole bunch of energy. The hydrogen bit is why they’re sometimes called “hydrogen bombs.” In theory you could squeeze the tritium together with regular explosives. In practice, you do it with a separate fission bomb. Fusion bombs are really effective, using almost all of their nuclear fuel, so you can build bombs that are small enough to be delivered by rocket or whatever but still pack one hell of a wallop. How big? Megatons (Mt). The USSR detonated the world’s largest fusion bomb in 1961. It had a yield of 50 Mt.

Both fission and fusion bombs are nuclear bombs. And, he says, finally getting to the punchline, they do not literally explode the air. I mean, c’mon. The real difference is that hydrogen bombs are much more powerful and efficient.

(ETA: phanatic pointed out that, in effect, nuclear weapons do explode the air. The overpressure and shock wave aren’t caused by the bomb’s plasma ball, like I had thought, but by the gamma rays ionizing the air and causing ozone and other smog-like products, which is then heated by the bomb’s x-rays. The effect is common to both fusion and fission weapons. My only defense is that, as I say in the next paragraph, I thought they were referring to the concern that nuclear bombs would ignite the atmosphere.)

The only thing I can figure is that the writers remembered that the Manhattan Project scientists were afraid that a nuclear explosion would ignite the atmosphere. It doesn’t happen. Atmospheric nitrogen requires much higher temperatures to fuse than you get in the center of the nuclear fireball, which cools surprisingly quickly because the fireball is expanding so fast.

So, to sum up: hydrogen bombs are different because they’re super-powerful, requiring a fission bomb to set them off. They do not literally explode the air. While they sort-of explode the air, so do regular fission weapons.

Hey, that was kind of fun. While I’m on a roll, let’s discuss fallout!

Fallout

In the same episode, everyone’s afraid of the fallout coming from Denver. Run! Hide! Put plastic sheeting over your windows and doors to keep it out and you’ll be okay!

Er, no. Not even close. Here’s the thing: fallout is radioactive. When a nuclear bomb cranks up, it makes all kinds of nasty radioactive byproducts. Many of them tend to be unstable, which is both good and bad. Good: they decay rapidly. Bad: in decaying, they toss off beta and gamma radiation the way college students toss down tequila. All of that plastic sheeting will keep out the fallout particles themselves. The radiation that the particles are emitting? Ha ha, he laughs hollowly.

What you want is as much mass between you and the particles as possible. More mass means more radiation is absorbed by the stuff between you and the fallout and less is absorbed by you directly. The beta radiation isn’t so bad. It’s the gamma rays that’ll get you. They penetrate like crazy. Your best bet is to put a lot of earth between you and the fallout, so your basement is better than your attic. Just over three and a half inches of dirt will cut the gamma radiation flux in half. A reasonable rule of thumb is that you want five to ten times that thickness to give you really good protection, so you’re looking at one and a half feet to three feet of dirt for really good protection. If you’re lucky enough to use concrete instead, you can get by with one to two feet.

What about air? you ask. Won’t it bring in the nasty fallout particles? Yes, but most of the really dangerous stuff has the consistency of sand, so it’s not too hard to filter out.

But do not sit near your house’s entrance and listen to the probably-radioactive rain pattering down.

That’s enough for now. I’ll probably have more grumbles later, like people who think that drinking iodine is a good substitute for potassium iodide pills, and saying that storms travel from Denver to wherever Jericho is supposed to be in Kansas in two hours. What, they have 100-mile-an-hour winds to blow the storms the 200 miles from Denver to the Colorado-Kansas border?

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