Chris Bellamy: An enigma that only the Israelis can fully explain

The initial tests on samples taken from the site of the Israeli strike on Khiam present an enigma which will only be solved when the people who produced and deployed the weapon explain themselves. Speculation that the device was some form of "dirty bomb" or micro-yield nuclear weapon can probably be dismissed. The radiation levels and the amount of Uranium-235 in the sample clearly indicate that it was not a nuclear fission weapon.

Uranium has been widely used in conventional weapons - and on the battlefield - for the past 30 years, for three reasons. Firstly, uranium is very dense - 70 per cent denser than lead. Therefore, a smaller projectile delivers more kinetic energy, making it ideal for armour-piercing shot. Secondly, it is pyrophoric, which means that when slammed into a target at high speed it liquefies and ignites spontaneously. Thirdly, the type of uranium most widely used in weapons, depleted uranium (DU), is plentiful. It is a by-product of uranium enrichment, which produces the fuel for nuclear power stations and nuclear weapons. Because there is so much of it about, it makes sense for those who have it to turn DU into armour-piercing munitions.

The only logical military reason for the presence of traces of uranium, of any kind, would be the use of that element to make a hard, dense penetrator for an armour-piercing or "bunker-busting" device. Natural uranium consists of three isotopes - Uranium-238 (99.27 per cent), U-235 - the crucial component of fissionable material (0.72 per cent) and U-234 (0.0054 per cent). To make the fuel for a nuclear reactor this needs to be enriched to three or four per cent U-235, and the resulting waste product, with only 0.25 per cent U-235 and 99.8 per cent U-238, is DU. To make a bomb you would need up to 90 per cent U-235 - hence the concern about Iran's uranium enrichment programme.

The Khiam sample, with 108 parts U-238 to one of U-235 - just under one per cent - is clearly enriched - but not much. So, in the absence of any palpable military advantage, in terms of its mass and its ability to generate heat and fire compared with DU or natural uranium, why was this enigmatic material used? There are several possibilities. The first is that there was a simple mistake - that uranium with an elevated U-235 content was used instead of DU or natural uranium. The Khiam sample was very small - 25 grams. Contamination with soil could easily obscure a higher degree of enrichment. Spent nuclear fuel - after the power has been generated - typically contains 2.5 per cent U-235, but it can be as low as 1.5 per cent - close to the Khiam sample level. So the uranium in the Khiam projectile could just have been spent nuclear fuel.

One way to dispose of enriched uranium safely is to blend it with natural uranium, in such a way that the U-235 is extremely difficult to re-extract. That might well produce a substance with just under one per cent U-235, which was a component of the Israeli Khiam bomb.

It is also uncertain whether the munition was made in the US or by the Israelis themselves. If the Israelis or the Americans want to avoid accusations, at the very least, of a cavalier attitude to the use of nuclear waste products, they need to explain what was in that bomb and why it was there.

Chris Bellamy is professor of military science and doctrine at Cranfield University