physics, science

The Moment a Nuke Goes Off

Nuclear weapons give me mixed feelings. On the one hand, I really like explosions and physics and crazy shit. But on the other hand, I don’t like that somebody thought “You know what the world needs? A bomb capable of ruining the shit of everybody in an entire city. And you know what we need? Like fifty thousand of the bastards, all in the hands of angry buggers that all have beef with each other.”

That aside, though, the physics of a nuclear explosion is pretty amazing. Especially when you consider that nuclear bombs were developed at a time when: there was no vaccine for polio, commercial airliners hadn’t been invented, the big brains in Framingham hadn’t even started to work out just what causes heart disease, and a computer needed one room for all the vacuum tubes and another for its air conditioning system.

There’s an absolutely awesome 1977 paper by Glasstone & Dolan that describes, in great detail, and from beginning to end, the things that happen when a nuke goes off. The paper’s also surprisingly readable. Even if you’re a little rusty on your physics, you can still learn a hell of a lot just by skimming it. That’s the mark of a good paper.

To me, the most shocking thing in that paper is just how quickly the actual nuclear explosion happens. But first, a little background. This is what the inside of an implosion-type fission bomb looks like (This is the type that was dropped on Nagasaki, and seems to be the fission device used in modern arsenals. Correct me if I’m wrong.)

Fat_Man_Internal_Components (1)

(Source.)

It looks complicated, but it’s really not. The red thing at the center is the plutonium-239 that actually does the exploding. The dark-gray thing surrounding it is a hollow sphere of uranium-238 (I’ll explain what that’s for in a second). The light-gray thing is an aluminum pusher (I’ll explain that in a second, too). And the peach-colored stuff is the explosive that sets the whole thing off. The yellow things it’s studded with are the detonators.

When the bomb is triggered, the detonators go off. Spherical detonation waves spread through the dark-peach explosives on the outside. When they hit the light-peach cones, the shape of those cones forms the thirty-two separate waves into one smooth, contracting sphere. That spherical implosion wave then passes into the dark-peach charges surrounding the aluminum pusher. So far, the process has taken roughly 30 microseconds.

When the implosion wave hits the pusher, it crushes the aluminum inward, generating remarkable pressures. This takes something like 10 microseconds.  The pusher’s job is to evenly transfer the implosion force to the core.

The imploding pusher then crushes the uranium tamper in roughly 15 microseconds. The tamper serves two purposes: it helps reflect the neutrons generated by the plutonium-238, and, being such a dense, heavy metal, its inertia keeps the core from blowing itself apart too quickly, so more of it can fission.

Speaking of the core, a whole bunch of crazy shit is about to happen in there. Normally, I don’t think of metals as the sort of thing you can compress. But when you’ve got hundreds of kilos of high explosives all pointing inwards, you can compress anything. The core is a whopping 6.4 kilos of plutonium (14 pounds). That’s how much plutonium it takes to wreck an entire city. But just having 6.4 kilos of plutonium lying around isn’t that dangerous. (Relatively speaking.) 6.4 kilos is below plutonium’s critical mass. At least, it is at normal densities. That implosion wave, though, crushes the plutonium down much smaller, until it passes the critical limit by density alone. (There’s also a fancy polonium-210 initiator in the center, to make sure the core goes off when it’s supposed to, but this post is already getting too rambly…)

Once the plutonium passes its critical limit, things happen very quickly. Inevitably, a neutron will be emitted from an atom. That neutron will strike a Pu-238 nucleus and cause it to fission and release a couple more neutrons. Each of these neutrons sets off another Pu-238 nucleus, and bam! We’ve got the right conditions for an exponential chain reaction.

Still, from the outside, it doesn’t look like much has happened. It’s been approximately a hundred microseconds since the detonators detonated, but next to none of the plutonium’s fission energy has been released. Here’s a graph to explain why:

Nuclear Explosion

(Generated using the excellent fooplot.com)

Here, the x-axis represents time in nanoseconds. The y-axis represents the number of neutrons, expressed as a percentage of the number needed to release 21 kilotons-TNT of energy (the amount of energy released by the Fat Man bomb that destroyed Nagasaki). At time-zero, the neutron that initiates the chain reaction is released. And by time 240, all of the energy has been released. But the thing to notice is that it takes all of 50 nanoseconds for the vast, vast majority of the fissions to happen. That is to say, the plutonium core does all the fissioning it’s going to do–releases all of its energy–within 50 nanoseconds.

21 kilotons-TNT released over 50 nanoseconds is equivalent to a power of 1.757e21 Watts. That’s ten thousand times more power than the Earth receives from the sun. That’s roughly 5 millionths of a solar luminosity, which sounds small, until you realize that, for those 50 nanoseconds, a 14-pound lump of gray metal is producing 0.0005% as much power as an entire star.

The nuclear explosion happens so fast, in fact, that by the time it’s finished, the x-ray light released just as the chain reaction took off has only traveled 15 meters (about 49 feet). Everything happens so rapidly that the bomb’s components might as well be stationary. The casing might be starting to bulge outward from the detonation of the implosion device, and the bomb, while still bomb-shaped, is rapidly evaporating into plasma as hot as the core of the fucking sun. But even at those temperatures, the atoms in the bomb haven’t had time to move more than a couple centimeters. So, by the time the nuclear detonation has finished, the bomb and the surrounding air look something like this:

Fat Man End of Detonation.png

But perhaps the wildest thing of all is that we’re not limited to hypothetical renderings here. We actually know, thanks to the incomparable Harold Edgerton, exactly what those first moments of a nuclear explosion look like. Doc Edgerton developed the rapatronic camera, whose clever magneto-optic shutter is capable of opening and closing with an exposure time of as little as 10 nanoseconds. The results of Mr. Edgerton’s work speak for themselves:

Glowing Shot Cab

The thing above is the “shot cab” for a nuclear test. It’s a little shack on top of a tower, with a nuclear bomb inside. In this picture, the bomb has already gone off. Those white rectangles are actually the cab’s wall panels, being made to glow brightly by the scream of X-rays bombarding them. And those ominous-looking mushroom-shaped puffs are where the X-rays have just started to escape into the air and make a nuclear fireball. A moment (probably measured in nanoseconds) later, the fireball looks like this:

Very Early Fireball

I take my hat off to Mr. Edgerton for having the guts to say “Oh? You need a photograph of the first microsecond of a nuclear explosion? Yeah. I can probably make that happen.” (Incidentally, both those photos are taken from the paper “Photography of Early Stages of Nuclear Explosions”, by Edgerton himself, which is, regrettably, behind a fucking paywall. Grumble grumble.)

And, thanks to sonicbomb.com, we can see the evolution of one of these nightmare fireballs:

Hardtack_II

Progressing from left to right and top to bottom, we can see the shot cab glowing a little. Then glowing a lot. Then erupting in x-ray hellfire. And after that, just sort of turning into plasma, which things that close to a nuclear explosion tend to do.

Soon enough, this baby fireball evolves into a nightmarish jellyfish from the deepest pit in Hell:

Tumbler_Snapper_rope_tricks.jpg

(Source.)

The horrifying spikes emerging from the bottom of the fireball are caused by the so-called “rope-trick effect”: they’re the guy wires supporting the shot tower vaporizing and exploding under the onslaught of radiation from the explosion.

And soon enough (after about 16 milliseconds), the fireball swells into a monster like this:

Trinity_Test_Fireball_16ms.jpg

(Source. Note, this is the fireball from the Trinity test, humanity’s first-ever nuclear explosion.)

It’s worth noting that, at this point, 16 milliseconds after the bomb goes off, your retinas have barely had time to respond to the flash. In the roughly 75 to 100 milliseconds it takes the retinal signal to travel down the optic nerves and reach your brain, you are already being exposed to maximum thermal radiation. And after a typical human reaction time (something like 150 to 250 milliseconds), about the time it takes to consciously register something, you’re probably already on fire.

So nuclear explosions are cool, and they’re awe-inspiring, but I must pose the question once again: who the hell saw the plans for these hell-bombs and thought “Yeah. That’s a thing that needs to exist. We need to have that nightmare hanging over humanity’s head forever! Let’s build one!”

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2 thoughts on “The Moment a Nuke Goes Off

  1. I think Einstein made some comment along the lines that nuclear weapons had changed everything… except the way we think. Damn. But setting that aside, the rapatronic camera was a simply awesome device for capturing images of anything happening at such inhumanly tiny time-scales. It’s incredible to think that although the principle behind the shutter had been known since the 1840s, it was possible to build such a thing with the technologies of the 1940s. (Of course, there was quite a bit of technology back then that we don’t usually imagine existed, though if it hadn’t, we wouldn’t have the gear we have today – I mean, the first polyphonic synthesiser was built in 1938, not that anybody particularly remembers that today).

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