LONDON: Attacks on buried targets are likely to be a feature of the next Gulf War. Key Iraqi assets are concealed under layers of concrete. The US aims to take out these targets with bunker-busting bombs, and the concern is that massive amounts of depleted uranium will be used in the process.

Depleted uranium has been used in anti-tank weapons for decades, and it has been suggested it could add weight to bunker-busting weapons. Although they maintain that depleted uranium is not an environmental hazard, the military is reluctant to discuss specific uses. So can we identify depleted uranium in bombs from the available information?

It’s all a matter of density, like a modern version of the problem put to Archimedes by King Hiero. The King had given some gold to a goldsmith to fashion a crown. The crown was the right weight, but Hiero suspected that some of the gold had been replaced with silver. Archimedes had to find out if the goldsmith was a crook.

Archimedes knew gold was denser than silver. If he could calculate the density of the crown he could determine its purity. His “Eureka!” moment came when he realized that immersing it in water would displace a volume of water equal to the volume of the crown. Given both the weight and the volume, he could calculate the crown’s density.

Archimedes proved the crown had been alloyed with silver. A triumph for physics, it was a disaster for the goldsmith, who was put to death.

Density is the key for a bunker-busting bomb. Its forward momentum is determined by weight and speed. When travelling through earth or concrete, the resistance is proportional to its cross-sectional area. Ideally, it should concentrate as much weight as possible into a long, slender shape. Depleted uranium is more than twice as dense as steel, making it an obvious choice for ballast.

We know the density of explosives, steel alloys and depleted uranium, so we can make some calculations from the weights and dimensions of bombs.

In the Gulf War, the BLU-109 was ineffective against some well-protected targets, and a programme was instituted to build something better. The end product was the BLU-113, twice the weight and with three times the penetrating power. Was depleted uranium added for extra punch? Again, these calculations yield a density that closely matches steel.

The latest development is the Advanced Unitary Penetrator. This has the same weight and external dimensions as the 109, but the exterior is a thin shell that breaks away on impact. Inside is a smaller, denser “subcalibre penetrator” capable of piercing 11 feet of concrete.

The internal dimensions of the AUP have not been disclosed, nor the explosive payload; this makes density calculations more difficult. However, a picture of the AUP going through a target shows a ruler marked in feet, so we can estimate its size fairly accurately.

Explosive is comparatively light; the more explosive the AUP contains, the more likely it is to have depleted uranium ballast. If it contains only 100 kgs of explosive, the casing must be far denser than steel. Even if it has half that — a very modest amount for this size bomb — it is still too dense. Taking the lower figure implies that the AUP has around a quarter of a ton of dense metal ballast.

On its first test, the BLU-113 ended up more than 100 feet below the New Mexico desert floor. A depleted uranium weapon doing the same would present environmental problems, because if it did not detonate, it would soon break down into uranium oxide and get into groundwater.

The small depleted uranium antitank rounds fired from aircraft weigh about 90gms and penetrate a few feet into the ground. The depleted uranium from a bunker-busting bomb would end up far below the water table, where it would remain a threat to the environment for thousands of years.—Dawn/The Guardian News Service.