LONDON (Reuters) - The diplomatic thaw between the West and Iran could quickly chill again if the two sides are unable to master the many moving parts of Tehran's disputed nuclear programme under the weight of more than three decades of distrust.
The dispute is not only about the West stopping Iran building a bomb, but also about preventing it expanding its capabilities to the point where it could make a dash for nuclear weapons - known as "breakout" - if it chose to.
Many different conditions need to be met even for an interim agreement to slow Iran's nuclear programme and stop it reaching a point - expected by some nuclear experts by the middle of next year - when the United States and Israel could be drawn into military action to prevent it advancing further.
"The debate is more about breakout," said Shashank Joshi at the Royal United Services Institute (RUSI) in London.
Unlike India and Pakistan, which developed nuclear weapons in secret before publicly testing in 1998, Iran is a member of the Nuclear Non-Proliferation Treaty (NPT), subjecting it to international inspections. As a result, the outside world would know fairly quickly if it made a break for a nuclear bomb.
But Iran is advancing its nuclear capabilities - including the ability to enrich uranium - at such a rate that it has narrowed the time it would need for breakout, meaning it could build a bomb before the West had time to detect and stop it.
"In as much as they have the ability to indigenously develop a nuclear bomb, they already have a nuclear-weapons capability," said Joshi. "Now the issue that is looming is enrichment capacity. By the middle of next year, capacity will be so high that some fear that it would be at that dangerous level of undetectable breakout."
Iran has insisted it is not seeking nuclear weapons - an assertion reiterated last week by President Hassan Rouhani, whose diplomatic overtures to the West have raised hopes of progress in the long-running nuclear dispute.
Supreme Leader Ali Khamenei, who has ultimate authority over the nuclear programme, has issued a fatwa, or religious ruling, against nuclear weapons, saying these are against Islam. Western nuclear experts believe that holds, for now.
But Khamenei's fatwa could change, said Mehdi Khalaji, a trained Shi'ite theologian and Iran scholar at the Washington Institute for Near East Policy, for example if Iranian rulers faced what they believed to be an existential threat.
"In Shia jurisprudence we believe that we don't have access to the truth, so it is in a way very relativistic." Decision-making in Iran, he said, is also driven by "the principle of expediency of the regime ... Therefore, the logic of the decision-making is more pragmatism and survivalism rather than the Islamic legal system."
Moreover, the fatwa covers only nuclear bombs. "He never said that nuclear capability is against Islamic law; he never said that militarising the nuclear programme is against Islamic law. Everything he says is about the actual bomb."
With Iran insisting on its sovereign right to enrichment for peaceful purposes, any resolution of the dispute would have to find some kind of balance between Tehran's desire to maintain its capabilities and the West's determination to restrict them.
Any agreement could be an interim deal in return for a partial lifting of sanctions, a longer-term nuclear settlement, or a "grand bargain" between Washington and Tehran resolving differences dating back to the 1979 Iranian revolution.
Some analysts have suggested a combination of processes, whereby U.S.-Iran talks on non-nuclear security issues, including Syria, would build trust, underpinning progress in nuclear negotiations between the five permanent members of the Security Council plus Germany (the P5 +1) and Iran.
But if managing the moving parts of parallel negotiating tracks would be hard, agreeing even basic conditions for an interim deal could be even harder.
That is partly because the speed of breakout depends on multiple factors, including the size of Iran's stocks of enriched uranium, its technological progress, the extent of international inspections and whether it has any previously undisclosed enrichment sites.
Over the last year, much attention focused on Iran's enrichment of uranium to 20 percent, a level that can quickly be enriched to 90 percent weapons-grade, after Israeli Prime Minister Benjamin Netanyahu said Israel would not allow Tehran to acquire enough enriched uranium to produce one nuclear bomb.
Israel, the only country in the Middle East with a presumed nuclear arsenal, sees Iran as an existential threat.
Experts estimate that enough uranium for one bomb would correspond to 25 kg of weapons-grade uranium, or roughly 250 kg enriched to 20 percent.
Netanyahu's "red line" appears to have partially worked. Iran has kept below the threshold by converting some of its stockpiles of 20 percent enriched uranium into reactor fuel.
According to an August report by the International Atomic Energy Agency (IAEA), its stockpile had increased since May by just 3.8 kg to 185.8 kg, largely due to fuel conversions.
Yet at the same time it has developed advanced centrifuges used to enrich uranium, thereby improving the efficiency and speed of enrichment and narrowing the potential breakout time.
So while suspension of enrichment to 20 percent is seen as essential for any interim deal, probably followed by its eventual cessation in any lasting settlement, that alone is not sufficient to allay Western fears.
In a paper published in July by the Institute for Science and International Security (ISIS), authors David Albright and Christina Walrond argued that centrifuge development meant Iran would reach a "critical capability" - the ability to produce enough weapons-grade uranium from its stocks for a nuclear explosive without being detected - in mid 2014.
"Preventing Iran from reaching critical capability will require a broad set of responses, but the most important is limiting the number and type of centrifuges Iran builds," it said.
Along with curbs on stocks of enriched uranium and limits on centrifuges, western countries are worried about Iran's Fordow enrichment facility. Because its enrichment halls are buried 91 metres underground they are harder to bomb, should the West or Israel ever decide they needed to take military action.
Among the conditions for an interim deal would therefore likely be continuous monitoring at Fordow, including using remotely controlled cameras.
With the primary concern for now being the speed of breakout, the West has also been pushing for greater access by IAEA inspectors so they could detect any shift to a weapons programme before it was too late to stop it.
That would include at a minimum Iran signing the IAEA's Additional Protocol, which gives IAEA inspectors greater access and information and enhances its ability to detect undeclared nuclear activities, including at any so far undiscovered sites.
Even then, concerns remain about other parts of Iran's nuclear programme, including its Arak nuclear reactor, expected to come onstream by mid- to late next year.
While there is nothing in the NPT to prevent Iran building the reactor for electricity, it could theoretically yield plutonium for nuclear bombs. As a result, both the United States and Israel are watching it warily, largely because once it does become operational it would be impossible to bomb it without causing huge environmental damage.
Any interim deal could try to convince Iran to postpone operations at Arak until the nuclear dispute is resolved.
Finally, even if an interim agreement were reached to slow or suspend Iran's nuclear programme, the process would have a long way to go before a final settlement, including clearing up lingering Western anxieties about Iranian weapons research.
Although a 2007 U.S. intelligence assessment said Iran had likely suspended what it said was its weapons programme in 2003, an IAEA report in late 2011 suggested that some weapons-related research activities may have continued after 2003 up until 2009.
Among other details sought by the West is information on past nuclear weapons research, including access to scientists. (Additional reporting by Fredrik Dahl in Geneva and Yeganeh Torbati in Dubai; Editing by Will Waterman)