David HamblingWhy deadly depleted uranium is the tank buster's weapon of choice
Thursday May 18, 2000
The use of depleted uranium weapons is again causing concern. The people of Kosovo have been alarmed to discover that the conflict there has left radioactive contamination, just as it did in Kuwait nine years ago.
Why do the United States and Britain continue to use a waste product of the nuclear industry in their weapons? Some commentators allege that it is a conspiracy between the military and the nuclear industry to dispose of dangerous waste in hostile countries. The real reasons are more complex.
Metallic uranium occurs naturally in tiny quantities. In its native state it is a mixture of highly radioactive uranium-235 and less active U-238. U-235 is used in reactors and atomic weapons; once it is extracted, the rest is depleted uranium (DU). It is a poisonous heavy metal like lead or mercury, but only slightly radioactive.
To understand why DU makes a good anti-tank weapon you have to enter the Alice In Wonderland world of high-energy collisions. When metal meets metal at five times the speed of sound, hardened steel shatters like glass. Metal flows like putty, or simply vaporises. A faster shell does not necessarily go through more armour, but, like a pebble thrown into a pond, it makes a bigger splash.
Armour penetration is increased by concentrating the force of a shell into as small an area as possible, so the projectiles tend to look like giant darts. The denser the projectile, the harder the impact for a given size. DU is almost twice as dense as lead, making it highly suitable. The other metal used for anti-tank rounds is tungsten, which is also very hard and dense. When a tungsten rod strikes armour, it deforms and mushrooms, making it progressively blunter. Uranium is "pyrophoric": at the point of impact it burns away into vapour, so the projectile stays sharp. When it breaks through, the burning DU turns the inside of a vehicle into an inferno of white-hot gas and sparks.
Normal uranium is not as hard as tungsten. But a classified technique allows it to be hardened. This is believed to involve alloying it with titanium and cooling it so that it forms a single large metallic crystal rather than a chaotic mass of tiny crystals. This structure is very strong and produces an improvement similar to the difference between a brittle pencil lead and a carbon-fibre tennis racquet. The final advantage of uranium is cost. Machined tungsten is expensive, but governments supply DU more or less free.
As with most weapons, depleted uranium is not as deadly as its proponents - or its critics - claim. One tank was hit four times with no casualties. Twenty US vehicles took penetrating hits from DU weapons during the Gulf war. Thirteen crew members were killed, but 113 others - almost 90% - survived. The long-term health effects are not known.
It is likely that DU will be phased out eventually, not for health reasons but for military ones. It was introduced to solve the problem of breaking through heavy armour. But tank armour is concentrated mainly at the front, facing the main threat; it is thinner on the sides, and thinner still on top. If the entire vehicle were clad in thick armour it would be too heavy to move. Instead of brute force, the clever approach would be to attack the weakest point.
After decades of development a new generation of anti-armour weapons is being fielded. These "brilliant" weapons find their own targets, unlike mere smart bombs, which have to be directed. One example is Sadarm (Seek And Destroy Armour). It is fired like a normal artillery shell into the target area, where it ejects two submunitions that descend by parachute. As they fall, Sadarm scans the ground with radar and infrared sensors. Targets are identified, and the most important are selected - a Scud launcher in preference to a tank, a tank rather than a truck.
Sadarm fires a slug of molten metal at the selected target. The slug takes on an aerodynamic shape as it travels through the air, ideal for piercing armour. Though less powerful than a DU shell, it can break through the top armour of any tank.
Engagements between tanks are fought face-to-face, at a maximum distance of about 4km. Sadarm can be lobbed at an enemy 20km away. Missiles carrying brilliant munitions can range out to 100km or more.
Sadarm and other brilliant weapons use tantalum, an exotic heavy metal for which little data is available. But it appears to be highly toxic, especially when vaporised. We will probably discover its full effects only after the next hi-tech war.