Depleted Uranium

What is depleted uranium and why is it used in various kinds of munitions?

Depleted Uranium is former nuclear reactor fuel. It has been part of a forced chain reaction which has “used up” part of the uranium.

It is used in munitions because it is very dense; more so than lead. This lets it punch through things (like armor) more effectively.

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Dennis Matheson —
Hike, Dive, Ski, Climb —

I believe depleted uranium is just what’s left over after the fissionable portion has been separated out.
Not all uranium can be used for bombs and reactors and such; I think only one isotope (U-235?) can sustain a fission chain reaction. Some of the rest is also radioactive, though non-fissionable; but the majority (U-238?) is basically stable. The stable stuff, by itself, is called “depleted” because it’s no longer radioactive.

Depleted uranium is used in weapons because it’s so dense-- much heavier than an equal volume of lead. Denser bullets are less affected by air resistance, carry more momentum to their target, and generally do more damage to what they hit. (Think of the difference between a baseball, and an iron sphere of the same size. Which would you rather have someone throw at your head?)

No, this is not true at all. Depleted uranium is pure U-238, with all of the U-235 (the “good stuff”) removed. It has never been inside a reactor- it has merely undergone a process whereby the lighter isotope is removed via a complicated process. The isotopic purification is done by attacking the uranium with an excess of fluorine. This is not something you want to monkey around with at home. The result is uranium hexafluoride, or UF6 - an extremely heavy gas which sounds poisonous as hell to boot. With the uranium in gaseous form, extraction of the lighter isotope becomes possible, although it is still a tremendous amount of work and involves thousands of repeated steps.

Actually depleted uranium is still quite radioactive. U-238 has an extremely long half-life of 4.5 billion years which is comparable to the age of the earth, and certainly dwarfs that of U-235 which decays much more rapidly, but it still emits radioactivity. Depleted uranium is fully 60% as radioactive as uranium which has not had its U-235 removed. The radiation is alpha, which is “mostly harmless”; outside the body, alpha particles are stopped before they even penetrate dead skin. But woe to those who inhale the dust created by a DU projectile when it hits steel and conflagarates!

The military became interested in using DU in weapons systems in the Sixties because it is cheaply available in large quantities and outperforms tungsten, the heavy metal they had been using prior to discovering DU. Today, depleted uranium is used by the military in tank armor and ammunition fired against tanks by M-1 tanks and A-10 Thunderbolt aircraft.
Depleted uranium is effective as an armor penetrator due to its high density (18.9 g/cc, compared to 11.3 for lead) and pyrophoric nature. (Uranium likes to ignite spontaneously.) A DU projectile has a higher velocity and greater range than non-DU projectiles, and rather than explode, a DU penetrator will fragment and burn on impact with steel armor, melting the metal surface (and producing a smoke cloud with high concentrations of uranium particulates).

Despite what people tend to think, depleted uranium is quite fissionable. It simply can’t sustain a chain fission reaction the way its lighter isotope can. U-235 will fission when a stray neutron strikes the nucleus, and it blows into 2 or 3 smaller nuclei, along with a few neutrons (the average is something like 2.2 neutrons per fission). Those neutrons then strike other nuclei and cause them to fission as well, so that you get a chain reaction. If more neutrons are produced than can escape through the surface of the fissile material, boom. While U-238 also fissions when hit by neutrons, it doesn’t liberate nearly as many free neutrons when it does, so you can’t set off an explosion with it.

You can, however, use it passively to intensify an explosion. In the Fifties, a standard A-bomb consisted of wedges of a fissile material like U-235 (Pu-239, actually), set up with TNT shape charges, with a neutron source such as a small piece of beryllium-radium alloy at dead center, and chunks of a neutron moderator like graphite (you can even use wax if you have trouble finding graphite). A common way of getting more bang for the buck was to surround all this crap with a shell of depleted uranium. When the bomb exploded, an abundance of neutrons would escape the core. When they hit the U-238 shell, they would cause some extra fissioning, and essentially for free (depleted uranium is cheap, about a few dollars per pound- while U-235 and Pu-239 are more expensive than gold). Bomb designers could increase the yield of a warhead by about one-third this way.
(A variation on this idea in the Sixties was the “cobalt bomb” where they would put cobalt in the shell. The cobalt would absorb neutrons and become cobalt-60 which goes all over the place and creates a vast radioactive wasteland. I forget whose bright idea this was, ours or the Rooskies.)
Aside from that, depleted uranium is not as “depleted” as its name might make you think. It is the raw material used within breeder reactors. The idea there is to hit the U-238 with neutrons again, but these are reactor neutrons with less energy and they don’t cause fissions. They are merely absorbed by the U-238, which becomes U-239. A series of beta decays produces neptunium 239 and finally plutonium 239 after a few days. With plutonium 239 you can do all kinds of stuff.
For a gung ho military view of how depleted uranium munitions are the best thing since sliced bread, and general guidelines for soldiers to follow when encountering DU dust created after they explode, see Depleted Uranium Without the Rocket Science.

Lexicographal note: at the same time this was going on, “cobalt bomb” was also being used jocularly to mean a cobalt-60 charged radiation “gun” used for cancer treatment, etc.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams