Iran's nuclear reactor being built at Arak figured highly in recent failed talks between Tehran and six world powers in Geneva, with France in particular wanting work there stopped.
The so-called "heavy water" reactor is of concern because, in theory, it could provide the Islamic republic with plutonium -- an alternative to highly enriched uranium used for a nuclear bomb.
Once completed, Iran could extract from Arak's spent fuel between five and 10 kilogrammes (10-20 pounds) of weapons-grade plutonium a year, enough for one nuclear weapon, experts estimate.
Iran denies wanting to do any such thing, saying the reactor in western Iran, known as the IR-40, will be used to produce medical isotopes and for research.
Israel, widely believed to have nuclear weapons itself, has refused to rule out bombing Arak, as it is assumed to have done to an Iraqi reactor in 1981 and to a Syrian facility in 2007.
But experts say that the reactor, plagued by delays in construction, is a long way from being even close to as much concern as uranium enrichment.
Iran has almost enough uranium enriched to purities of 20 percent -- close to weapons-grade -- for a bomb's worth, if it chose to further enrich to this level.
At present, such a "break out" would be detected by the International Atomic Energy Agency, the UN watchdog.
But with the uranium stockpile growing and Iran installing more and more centrifuges, some more modern and faster, the worry is that one day it could do this so quickly that the IAEA would not notice in time.
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Living with Arak
The IAEA, which monitors Arak, said in August that a planned start-up in the first quarter of 2014 was no longer achievable, and it remains unclear when it will come into operation.
Once it is, it needs to run for 12-18 months to produce spent fuel that could be used to extract plutonium, said Shannon Kile from the Stockholm International Peace Research Institute (SIPRI).
Moreover, Iran does not have a declared reprocessing facility to extract the plutonium, and a secret one would quickly be detected.
"Reprocessing facilities are large and produce radionuclide gaseous products which can be detected by environmental sampling, and that's true whether you have (IAEA) inspectors on the ground or not -- it can be done by airborne means for example," Kile told AFP.
Many analysts believe therefore that world powers at the next meeting in Geneva on November 20 should tolerate Iran completing and operating the IR-40 -- provided there are additional agreements.
These could include an undertaking by Iran to remove spent fuel from Arak to a third country or converting it to a less alarming "light water" reactor.
"The powers, including France and Israel and everyone else, could live with Iran completing and operating the reactor," if such safeguards are in place, Mark Hibbs from the Carnegie Endowment for International Peace told AFP.
"Arak represents a long-term proliferation risk, not a near-term risk, and it can be addressed in the 'final phase' of negotiations" between Iran and the six world powers, said Daryl Kimball from the Arms Control Association.
"France and the other P5+1 powers would be making a mistake if they hold up an interim deal that addresses more urgent proliferation risks over the final arrangements regarding Arak," he said.