By Fredrik Dahl
VIENNA (Reuters) - By dropping earlier demands that Iran shut down an underground uranium enrichment plant and ship material out of the country as part of a preliminary deal, nuclear negotiators have kicked some of the toughest questions forward to talks for the next year.
The curbs to its nuclear program that Iran agreed to on Sunday are easier to reverse than measures that were previously called for by the six global powers seeking to prevent Tehran from developing an atomic bomb, experts say.
To opponents of the deal, like Israel, which branded it an "historic mistake", that is a fatal flaw. But supporters say the compromise was necessary to halt Iran's nuclear advances so that the real bargaining could begin, and should help keep both sides focused on the final negotiations which lie ahead.
A senior Western diplomat acknowledged that Iran could resume its most controversial activity - production of 20 percent enriched uranium - if it should decide to abandon the deal or if final talks fail.
But by making it easier for inspectors to detect any such move, the preliminary accord requires Tehran to demonstrate its sincerity while a final deal is hammered out.
"This is all about testing their good faith. We would pick that up very quickly if they did it," the envoy said.
"Any agreement like this represents an element of compromise. Given where we were six months ago, to get the two sides together to agree something, there had to be some compromise from both sides."
"STOP", BUT NOT "SHUT" OR "SHIP"
Instead of requiring Iran to take steps that would be hard to undo, the powers' demands focused on stopping the higher-grade enrichment and halting future progress in other parts of the nuclear program for six months, while increasing inspections to determine if Iran is complying.
For their part, the United States and European Union have protected their future negotiating position by leaving most of their economic sanctions against Iran in place.
"Each side would retain enough leverage - one, in the form of continued economic penalties; the other in the form of a continued nuclear program - to maintain incentives for a grander bargain and guard against the other's potential reneging," said Iran expert Ali Vaez of the International Crisis Group think-tank.
The most controversial part of Iran's nuclear program has been its enrichment of uranium, which is first turned into a gas and then spun at high speeds in centrifuges to increase the concentration of the fissile isotope that is needed to make either fuel for a reactor or the core of an atomic bomb.
Tehran says it is refining uranium only for peaceful purposes and has the right to do so under international treaties. Western countries believe it has no such right and no legitimate need for an enrichment program of its own.
In addition to lower-grade work which began in 2007, Iran has since 2010 been enriching uranium to 20 percent purity, which Western countries see as a small technical step from reaching the 90 percent level needed to make a bomb.
In fruitless meetings during 2012, the powers sought a confidence-building, interim deal that would require Iran to stop its higher-level enrichment, close its Fordow enrichment site and send its stockpile of the higher-level uranium abroad.
Those demands were dubbed "stop, shut, ship" by diplomats. In the end, the November 24 deal in effect dropped two of the three demands: it obliges Iran to "stop" 20 percent enrichment but says nothing about "shutting" Fordow or "shipping" material out.
The same number of centrifuges can continue to spin, producing lower-level enriched uranium at Fordow - built deep inside a mountain near the holy Shi'ite Muslim town of Qom to shield it from any military attacks - and at Iran's other enrichment plant close to the central town of Natanz.
And instead of sending out the stockpile of 20 percent uranium, Iran will dilute it or convert the gas to a less proliferation-sensitive oxide powder.
The United States says this will "neutralize" the material. But experts say Iran could in theory convert the powder back, although it has agreed not to build a facility to do so.
"This is not a roll-back of the program," said Olli Heinonen, former deputy director of the International Atomic Energy Agency (IAEA) and now an expert at Harvard University. Instead, he said, it represents a "temporary halt" of many of the nuclear program's elements.
Apart from the enriched uranium, Western countries are also concerned that Iran could produce plutonium at Arak, an unfinished research reactor where Tehran says it intends to make medical isotopes. Plutonium can be used as an alternative to enriched uranium to build a bomb core.
Sunday's deal requires Iran to halt activity at Arak, although it may contain a loophole allowing it to build components off-site. In comments unlikely to go down well in Western capitals, Iranian Foreign Minister Mohammad Javad Zarif said on Wednesday that construction would continue at Arak, though he said there would be no new equipment installations.
Western officials and experts accept that the deal leaves Iran's nuclear program largely in place for now.
"For the time being, Iran will be allowed to retain most of its current infrastructure, which will have to be substantially reduced at a later stage," said Robert Einhorn, the U.S. State Department's non-proliferation adviser until earlier this year.
"But the first step will prevent Iran from sharply ramping up its capabilities in the next six months," he wrote in Israel's Haaretz newspaper.
Former chief U.N. nuclear inspector Herman Nackaerts said Iran, if it wanted to, could quickly resume higher-level enrichment at Fordow, but because of expanded inspections, including daily visits by IAEA monitors, it would easily be caught if it did so.
"It is technically easy to do that and it can quickly be done," Nackaerts, who retired in September, told Reuters. "Of course, when the inspectors are there every day they will notice that."
Western diplomats acknowledge that Iran's commitments are largely reversible so far, but say the deal takes care of the most urgent concerns while talks are under way.
"This first stage is one where the program is slowed in some ways, capped in others, but Iran can resume quickly," said a second Vienna-based diplomat.
"The main issues that we were concerned about are all covered by this. As we move on we will tackle more and more difficult things," said the first senior Western diplomat.
(Corrects attribution of quotations in final two paragraphs to make clear they were spoken by two separate diplomats)
(Additional reporting by Dan Williams in Jerusalem and Justyna Pawlak in Brussels; Editing by Peter Graff)