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