To make bunker busters that can go even deeper, designers have three choices:
- They can make the weapon heavier. More weight gives the bomb more kinetic energy when it hits the target.
- They can make the weapon smaller in diameter. The smaller cross-sectional area means that the bomb has to move less material (earth or concrete) "out of the way" as it penetrates.
- They can make the bomb faster to increase its kinetic energy. The only practical way to do this is to add some sort of large rocket engine that fires right before impact.
One way to make a bunker buster heavier while maintaining a narrow cross-sectional area is to use a metal that is heavier than steel. Lead is heavier, but it is so soft that it is useless in a penetrator -- lead would deform or disintegrate when the bomb hits the target.
One material that is both extremely strong and extremely dense is depleted uranium. DU is the material of choice for penetrating weapons because of these properties. For example, the M829 is an armor-piercing "dart" fired from the cannon of an M1 tank. These 10-pound (4.5-kg) darts are 2 feet (61 cm) long, approximately 1 inch (2.5 cm) in diameter and leave the barrel of the tank's cannon traveling at over 1 mile (1.6 km) per second. The dart has so much kinetic energy and is so strong that it is able to pierce the strongest armor plating.
Depleted uranium is a by-product of the nuclear power industry. Natural uranium from a mine contains two isotopes: U-235 and U-238. The U-235 is what is needed to produce nuclear power (see How Nuclear Power Plants Work for details), so the uranium is refined to extract the U-235 and create "enriched uranium." The U-238 that is left over is known as "depleted uranium."
U-238 is a radioactive metal that produces alpha and beta particles. In its solid form, it is not particularly dangerous because its half-life is 4.5 billion years, meaning that the atomic decay is very slow. Depleted uranium is used, for example, in boats and airplanes as ballast. The three properties that make depleted uranium useful in penetrating weapons are its:
- Density - Depleted uranium is 1.7 times heavier than lead, and 2.4 times heavier than steel.
- Hardness - If you look at a Web site like WebElements.com, you can see that the Brinell hardness of U-238 is 2,400, which is just shy of tungsten at 2,570. Iron is 490. Depleted uranium alloyed with a small amount of titanium is even harder.
- Incendiary properties - Depleted uranium burns. It is something like magnesium in this regard. If you heat uranium up in an oxygen environment (normal air), it will ignite and burn with an extremely intense flame. Once inside the target, burning uranium is another part of the bomb's destructive power.
These three properties make depleted uranium an obvious choice when creating advanced bunker-busting bombs. With depleted uranium, it is possible to create extremely heavy, strong and narrow bombs that have tremendous penetrating force.
But there are problems with using depleted uranium.