Western strategists have long debated the spectre of Iran "breaking out" - suddenly showing the ability to explode an atom bomb. But some see a "sneak-out" less visible to UN inspectors as a possibly bigger risk and world powers have calibrated their demands in negotiations with Iran to forestall any such outcome.
Under a "sneak-out" scenario, Western officials and experts say, Iran could build a uranium enrichment plant in secret to make bomb material unbeknownst to the UN nuclear watchdog, now empowered to visit only Tehran's declared nuclear sites.
To counter this risk, they say, any breakthrough diplomatic settlement with Iran must grant the International Atomic Energy Agency (IAEA) broader surveillance powers in this vast country crisscrossed by remote, often inaccessible mountains and desert.
"Under current circumstances, I believe that a 'sneak-out' from an undeclared enrichment facility is more a likely threat than a 'break-out' from a declared facility," said Gary Samore, until last year the top nuclear proliferation expert on US President Barack Obama's national security staff.
A US National Intelligence Estimate in 2007 offered a similar view, saying the Islamic Republic "probably would use covert facilities ... for the production of highly enriched uranium for a weapon", if it were to pursue nuclear arms.
Iran, with which six big powers resumed talks on the fringes of the U.N. General Assembly last week, says it is refining uranium only to lower levels for a future network of nuclear power stations. It rejects Western suspicions that its ultimate, underlying goal is high-enriched uranium for nuclear bombs.
A nuclear "break-out" is usually defined as amassing sufficient weapons-grade uranium for one bomb. Western nuclear analysts believe Iran can now make enough high-enriched uranium for one bomb in a few months' time.
Any apparent Iranian dash for a nuclear bomb at a known site could well trigger military action by the United States and Israel - and risk a wider Middle East war - to try to prevent Tehran from irreversibly acquiring weapons of mass destruction.
But if Iran were to make fissile material for a weapon at a clandestine site, it could more easily escape detection before it was too late. Although Iran would need more time to compress high-enriched uranium into a missile cone for a workable weapon, this activity would be even harder to discover and prevent.
"If Iran successfully produced enough WGU (weapons-grade uranium) for a nuclear weapon, the ensuing weaponization process might not be detectable until Iran tested its nuclear device underground or otherwise revealed its acquisition of nuclear weapons," the Institute for Science and International Security (ISIS), a Washington think-tank that has long tracked the Iranian nuclear programme, said in a report.
Extent of inspection powers at issue
India tested its first nuclear weapon in 1974, stunning the world and triggering punitive international sanctions, and arch-rival Pakistan followed suit in 1998. But unlike Iran, neither country is a signatory to the nuclear Non-Proliferation Treaty, which bans such doomsday arms. North Korea had quit the NPT by the time it conducted its first nuclear test in 2006.
If Iran also were to opt for weaponizing the enrichment process, "I think it would test in order to demonstrate its nuclear capability to the world and therefore deter military attacks," Samore said in an email.
He said Iran is not now believed to have any covert site for refining uranium. But the IAEA cannot rule out the possibility of undeclared nuclear activity, unnerving Iran's U.S.-backed regional Arab rivals and arch-foe Israel, widely believed to have the Middle East's only nuclear arsenal.
The main stumbling block in the negotiations between Iran and the United States, Russia, China, Britain, France and Germany remains the permissible future capacity of Iran's enrichment programme, with Tehran continuing to resist demands for its scope to be scaled back sharply.
After a two-month hiatus following the failure to reach a diplomatic settlement by a self-imposed July 20 deadline, the talks resumed in New York in mid-September with the aim of reaching a comprehensive, permanent solution by late November.
Iranian President Hassan Rouhani, a conciliation-minded pragmatist whose election last year revived diplomacy with the West after years of increasing sabre-rattling, said in a speech to the U.N. General Assembly on Thursday the negotiations had been marked by "seriousness and optimism on both sides" and he hoped for a deal in the "short amount of time left".
But the Islamic Republic, he emphasised, remained committed to its current enrichment programme.
World powers, however, will insist on "long-term significant constraints on Iran's declared enrichment facilities and additional monitoring and inspection measures to help detect covert activities", Samore said.
Alarm bells after secret sites exposed
Iran built its two known enrichment plants at Natanz and Fordow in secret, acknowledging their existence only after they were uncovered in 2002 and 2009, respectively, either by exiled dissidents or foreign intelligence services.
Such concealment of sensitive activity from the IAEA raised international alarm bells about Iran's nuclear ambitions.
Tehran has long maintained that it is legally obliged to inform the Vienna-based IAEA about a nuclear facility six months before it starts operating it, not when it is being built.
But Iran needs to be more transparent to reassure the world about its nuclear intentions, said Kelsey Davenport of the Arms Control Association, a U.S.-based research and advocacy group.
"A comprehensive agreement must allow inspectors greater access to all aspects of Iran's nuclear fuel cycle to ensure that Iran cannot siphon off materials, such as centrifuges and natural uranium feed, for a covert programme," she said.
That would mean a bigger workload for the IAEA, which has inspectors on the ground regularly checking Iranian nuclear installations - but has frequently appealed for increased funding to fulfil its non-proliferation mandate effectively.
Although the IAEA has daily access to Natanz and Fordow, underground sites where the Islamic Republic has more than 9,000 enrichment centrifuges running, inspectors may not roam beyond plants the country has reported to the UN watchdog.
That is why any deal envisaged by Western leaders to verifiably limit Iran's nuclear capacity in exchange for an end to sanctions almost certainly would require Tehran to grant the IAEA broader authority under a so-called Additional Protocol to Tehran's nuclear Safeguards Agreement with the agency.
By enabling the IAEA to conduct short-notice inspections beyond a country's declared facilities and expanding the amount of information it must provide, the Protocol is seen as vital to providing assurances that nuclear activity is wholly peaceful.
Iran has hinted a readiness to accept the Additional Protocol under any final settlement with the powers, although its parliament would have the final say and it has long been dominated by security-minded conservatives.
Greater IAEA oversight could be contentious for anti-Western hardliners influential in Tehran, which two years ago alleged that "terrorists" had infiltrated the ranks of IAEA inspectors to sabotage the country's enrichment plants.
Tehran would probably also have to comply with another IAEA transparency measure - known as the modified Code 3.1 - under which it would have to notify the agency about plans for a new nuclear facility as soon as it has decided to construct one.
The global powers seek a deal that would extend Iran's potential "break-out" timeline to a year from about 2-3 months now, a senior Western diplomat said. "At the moment, the Iranians are not ready to give us this time guarantee," he said, speaking before the negotiations in New York got under way.
Another Western diplomat agreed the potential of a "sneak-out" was worrying although he said this would not necessarily be easy to pull off without transferring and using material from known enrichment sites that are under regular IAEA scrutiny.
"The only true sneak-out scenario would be to have a completely covert, (nuclear) fuel cycle starting at mining or importing uranium and including conversion and enrichment and this is much more difficult to hide," said the diplomat, who is not from any of the six major powers.