The use of depleted uranium weapons is once more 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 US and the Uk continue to use a waste product of the nuclear industry in weapons?  some commentators allege that it is a conspiracy between the military and the nuclear industry to dispose of dangerous waste in hostile territories.  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 remainder is depleted uranium.  It is a poisonous heavy metal like lead or mercury but only slightly radioactive.  You could hold a piece of DU for days without serious exposure to radiation, but swallowing or inhaling it is more dangerous.  Experts are still arguing about how long uranium stays in the body; the longer it remains, the greater the chance that the radioactivity will cause cancer.

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 strange things happen.  Hardened steel shatters like glass.  Metal flows like putty, or simply vapourises.  A faster shell does not necessarily go through more armour, but like a pebble thrown into the water, 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 resemble giant darts.  The denser the projectile, the harder the impact for any 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 a vapour so that the projectile stays sharp.  This results in what the military describes as "behind armour effect".  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 so that it forms a single large metallic crystal rather than the usual chaotic mass of tiny crystals.  This mono crystalline structure is very strong and produces an improvement like the difference between a brittle pencil lead and a carbon-fibre tennis racket.  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 conflict.  Thirteen crew members were killed, but 113 others - almost 90% survived.  The survivors are being monitored for long-term adverse effects and the arguments continue over the dust produced when a DU weapon strikes a target.  Estimates of the risk range from "negligible" to a 7% chance of developing cancer for anyone living in an area contaminated by DU.  There is also the question of whether uranium dust plays a part in Gulf War Syndrome.

These arguments are likely to continue indefinitely.  But it is likely that DU will be phased out, not for health reasons but for military ones.  It was introduced as an engineering solution to the problem of breaking through heavy armour.  As with other areas, heavy machinery is being superceded by computer technology.  Tank armour is concentrated mainly at the front, facing the main threat; it is thinner of the sides and thinner still on top.  If the entire vehicle was 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 and several false starts, 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 shelling the target area, where it ejects two submunitions which descend by parachute.  As they fall, Sadarm scans the ground with radar and infra-red sensors.  targets are identified and the most important selected - a Scud launcher in preference to a tank, a tank rather than a truck.  Sadarm attacks by firing 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, the explosively formed projectile can break through the top armour of any tank.

Engagements between tanks are fought face to face, at a maximum distance of about four kilometres.  Munitions like Sadarm can be lobbed at an enemy 20 kilometres away.  Missiles carrying brilliant munitions can range out to 100 kilometres or more; the only limitation is the ability to locate enemy tank formations.  DU weapons will be used as long as they are useful; indications are that this will not be for many more years.

Sadarm and other 'brilliant' weapons do not use radioactive uranium.  They use tantalum, an exotic heavy metal for which very little data is available.  It appears to be highly toxic, especially when vapourised.  We will probably discover the full effects only after another Gulf War.