VIENNA - Sunday's agreement to curb Iran's nuclear program contains an apparent gap that could allow Tehran to build components off-site to install later in a nuclear reactor where it has promised to halt work, experts said.
They said any impact of the omission is likely to be small if Iran follows other undertakings in the interim accord, which is designed to restrain Tehran's nuclear program for six months in return for limited sanctions relief.
But the gap, which one diplomat described as a potential "loophole", could provide a test of Iran's intentions, and demonstrates how difficult it will be to reach a final deal to resolve Iran's nuclear standoff with the West once and for all.
Iran's uncompleted heavy-water research reactor near the town of Arak emerged as one of the most important issues in marathon negotiations in Geneva last week that ended early on Sunday with a breakthrough deal.
Tehran has earlier said it could open the reactor as soon as next year. It says its purpose is only to make medical isotopes, but Western countries say it could also produce plutonium, one of two materials, along with enriched uranium, that can be used to make the fissile core of a nuclear bomb.
Much of the final day of negotiations was taken up with the major powers pressing hard for language that would stop Iran from completing the reactor.
In the deal, Iran agreed that it will "not make any further advances of its activities" at Arak, language that also covers its two big uranium enrichment plants, Fordow and Natanz.
Footnotes hammered out in the final hours of the talks set out a range of activities that would be forbidden at the reactor. For the half year covered by the agreement, Iran is barred from starting the reactor up, bringing fuel or heavy water to it, testing or producing more fuel for it, or installing any remaining components.
But no language explicitly prevents it from making components elsewhere, which could then be installed later.
Former chief UN nuclear inspector Olli Heinonen, now at Harvard university, said the measures were good, but could have been better: "I would have also included the manufacturing of key components," he told Reuters in an e-mail.
One Western diplomat, who deals with nuclear issues but is not from one of the six world powers that negotiated the deal with Iran, said he did not see the gap as big.
While it was one of several possible "loopholes" in a very complicated agreement, the accord would still achieve its main aims, provided that Iran abides by it.
"If Iran is committed then none of these loopholes are fatal," said the diplomat, who is based in Vienna, headquarters of the International Atomic Energy Agency which will play an expanded role monitoring Iran's nuclear program.
Among other steps, Iran has agreed to the suspension of its most sensitive enrichment of uranium, to constraints on other atomic activities and to improved monitoring by the IAEA.
International inspectors say they are confident they can keep tabs on Iran's declared nuclear activities at known sites, although without wider access they cannot rule out undeclared activity at secret locations.
The diplomat said the most important work to complete Arak is the work to be done at the plant which is barred by the accord, meaning that any manufacturing of components at another location may not be that significant for the timeline.
"The estimate of one to two years to actually get the thing going assumes everything required offsite is already procured and/or manufactured," the diplomat said.
Mark Fitzpatrick, director of the non-proliferation program at the International Institute for Strategic Studies (IISS) think-tank, also noted the lack of prohibition on the manufacture of components but said most parts had probably already been built.
"I expect that most of the work on those components has already been completed, but no doubt some such work will continue," he said. "Iran adheres to the principle that what is not prohibited is allowed."
"DEVIL IN THE DETAIL"
Iran appears to have largely built the facility's external structure in a valley among barren desert highlands, gradually installing key components over the years.
In May, UN nuclear inspectors observed that the reactor vessel had been delivered to the site.
But the IAEA's latest quarterly report on Iran said other major parts - such as control room equipment, the refuelling machine and reactor cooling pumps - had yet to be put in place.
IAEA Director General Yukiya Amano told Reuters on Nov. 13 that Iran still had "quite a lot to do" to complete the plant, which the U.S. Institute for Science and International Security (ISIS) said has been under construction since mid-2004.
While attention has long focused on Iran's established uranium enrichment work, its progress at Arak also rang alarm bells, raising concern that Tehran could pursue both possible bomb core alternatives - uranium and plutonium - simultaneously.
To make a plutonium bomb, Iran would also need to build a reprocessing plant to extract the material, and it has no declared plans to do so.
Nuclear analyst Mark Hibbs of the Carnegie Endowment think-tank said Iran might be able to do some Arak-related work off-site under Sunday's interim accord.
"But the agreement puts a firewall around the reactor, meaning that no equipment will be installed ... and no fuel will be loaded," Hibbs said.
Middle East expert Shashank Joshi of the Royal United Services Institute (RUSI) said it could be argued that the deal also covers building components at another location.
"Of course, the fact that we are having this argument is itself acknowledgment of a possible loophole. Remember the US-DPRK 'leap day' deal? Devil in the detail," Joshi said, referring to an ultimately failed agreement between North Korea and the United States early last year.