A projectile weapon which uses the density and instability of post-powerplant uranium for armour piercing and general destruction. It consists of a large explosive charge to propel a long, thin bolt of uranium towards the target (usually a tank).
This apparently then punches a very small hole through the armour due to sheer impact, and disintegrates and catches fire inside, causing an explosion.
It is however not a nuclear weapon.

Depleted uranium is used generally for making anti-tank shells. As uranium is one of the heaviest substances around, it impacts with great momentum, much to the detriment of the target.
One of the best ways to protect against these shells is to have explosive panels on the tank that explode outwards, much diminishing the momentum of the shell. Not surprisingly these shells are not good for the environment as they result in sprays of still somewhat radioactive uranium all over the battle zone.

The kinetic energy in a round fired from a gun is well known, 0.5*m*v2

I was wondering, since the energy goes with the square of the velocity, why doesn't the military just jack up the projectile speed to get the desired kill energy instead of resorting to these exotic, dense materials such as depleted uranium?

Well, I discovered that this is because there is an upper limit of muzzle velocity using the conventional "gun" technique, using explosive chemicals to accelerate projectiles.

From http://www.islandone.org/APC/Catapults/01.html :

"The lowest practical peak acceleration level achievable with these guns is about 10 kgee. Because the final projectile velocity is a function of the speed of sound in the driver gas, the relatively high molecular weight of the combustion gasses (and the finite capability of materials to contain high pressures) used in cannons constrains the upper limit on muzzle velocity for existing guns to about 3 km/s"

There is also another limitation, this being the resistance of the air through which the projectile has to travel. At higher velocities, the projectile may reach such high temperatures due to air friction that it can be destroyed.

So, you quickly reach a limit to the velocity you can impart to your projectile. The only way to deliver any more kill energy to the target is to increase the projectile mass by using something heavier, like tungsten or the famous depleted uranium shell.

Depleted uranium is among the heaviest known materials. It has a density of about 19000 kg per cubic meter. This is almost twice that of lead, which has a density of around 11300 kg per cubic meter 1.

This heavy weight may be desirable in ballistics. To understand why, we have to review two important concepts in mechanics: kinetic energy and momentum.

Kinetic energy is determined by the charge that fires the bullet. It is given by 0.5*m*v2, with m the projectile massand v the velocity. Momentum is, roughly, what kills stuff. The physical reason for this is that the collision between target and bullet is inelastic. In an inelastic collision, momentum is conserved, but energy is dissipated as rather harmless heat. Hence, the momentum rather than the energy determines the killing power. Momentum is given by mv.

So, we want to maximize our momentum with a minimum of kinetic energy. This means we want as large a mass as possible, given that momentum goes with the velocity and kinetic energy goes with the velocity squared.

Of course, using slow, large bullets has disadvantages of its own, i.e. lower armor penetration, reduced accuracy due to a longer flight time and a greater area, which increases drag and the effect of wind.

So, basically, what you want is a small, heavy bullet, hence, a dense material such as depleted uranium.

Q: But isn't uranium dangerous?

A: It's a bullet, it's meant to be dangerous!

Q: Yeah, but isn't it radioactive and stuff?

A: Not really. Depleted uranium constains almost no U-235, which is the most radioactive component in normal Uranium. I imagine the U-235 is used for making nukes or running power plants, and the depleted uranium is waste. However, uranium is very toxic, and will accumulate in the bones and kidneys 2,3. The toxicity is chemical, not radioactive. However, most metals are toxic.

There are about half a dozen metals which are about as heavy or heavier than uranium, but none are as readily available or suitable for use as bullets.

Interesting bit of trivia: Uranium was first isolated from pitchblende in 1789 by a guy named... Klaproth4.


Sources

  1. wwww.webelements.com
  2. http://www.epa.gov/radiation/radionuclides/uranium.htm
  3. http://www.atsdr.cdc.gov/toxprofiles/phs150.html
  4. http://education.jlab.org/itselemental/ele092.html

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