The Times: One formula stands out in the documents obtained by The Times: UD3, or uranium deuteride. Independent experts have confirmed that the only possible use for UD3 is as a neutron source, the trigger to the chain reaction for a nuclear explosion. The Times

One formula stands out in the documents obtained by The Times: UD3, or uranium deuteride. Independent experts have confirmed that the only possible use for UD3 is as a neutron source, the trigger to the chain reaction for a nuclear explosion.

Critically, while other neutron sources have possible civilian uses, UD3 has only one application — to be the metaphorical match that lights a nuclear bomb.

UD3, when used in a neutron initiator, emits a stream of neutrons that ignite the core of a bomb, either weapons-grade uranium or plutonium. The stream of neutrons is released using high explosives to compress a core of solid UD3, creating fusion.

Foreign intelligence agencies are closely monitoring all of Iran’s weapons activities and a test explosion would not go unnoticed, which poses a problem to the covert development of a nuclear weapon.

An explosion could be explained as conventional weapons testing, but not if traces of uranium were left behind from the use of UD3. Such a discovery would be regarded as proof of weapons testing, and as a statement of Iran’s nuclear intent.

In the documents obtained by The Times, Iranian military scientists suggest a way around the problem: by running surrogate tests that substitute titanium deuteride for the uranium compound. They suggest “continuing the work of replacement materials such as TiD2 [titanium deuteride] in order to avoid U [Uranium] pollution in the production of UD3”.

Titanium deuteride would behave in the same way, producing a measurable flow of neutrons, but without leaving the traces of uranium. The explosion would also be smaller.

“They want to test the match without burning it,” a Western intelligence source said. “Then they can take it to the Supreme Leader and say, ‘We know it all works now.’ Then he has everything he needs for the moment, if it comes, that he gives the order to build a bomb.”

That order, according to the source, does not appear to have been made, and there is no evidence that it will. But even without it Iran is edging closer to nuclear latency — the immediate potential to assemble a bomb — effectively giving it a nuclear deterrent.

The last ingredient Iran requires is enough fissile material to arm a weapon. Estimates vary, but at present Iran’s known stockpile of low-enriched uranium is only just enough to arm one warhead — if that material were enriched further to weapons-grade. That stockpile of low-enriched uranium is under International Atomic Energy Agency (IAEA) supervision at Iran’s enrichment plant at Natanz.

However, the recent discovery of a nuclear facility near Qom, which was due to come online next year as a back-up enrichment plant, has renewed fears that Iran may have other secret sites where it is enriching or preparing to enrich uranium.

A proposal backed by the United Nations to remove Iran’s stockpile failed last week when President Ahmadinejad said that Iran would begin enriching its uranium to a higher grade, bringing it a step closer to producing the 90 per cent weapons-grade fuel needed for a bomb.