* Iran's higher-grade nuclear stockpile worries West, Israel
* Iran says needs 20 percent uranium to fuel reactor
* Oxide conversions keep stockpile below Israel "red line"
By Fredrik Dahl
VIENNA, Nov 15 (Reuters) - Iran appears to have movedquickly to prevent a large increase in its most disputed nuclearstockpile, a new U.N. watchdog report indicates, in what may bean attempt not to undermine talks on a nuclear deal with sixworld powers next week.
The Islamic Republic's holding of uranium gas refined to afissile concentration of 20 percent is closely watched by theWest as it represents a relatively short technical step awayfrom the level required for the core of an atomic bomb.
Israel, which has long warned it could use force to preventIran from gaining nuclear weapons, has said its foe must notobtain enough of this higher-grade uranium for one warhead ifprocessed further. Iran says its work is peaceful and that it isIsrael's assumed nuclear arsenal that threatens peace.
The powers, which are due to resume negotiations with Iranin Geneva on Nov. 20 on a preliminary deal towards ending thedecade-old standoff over its nuclear programme, want Tehran tostop 20 percent enrichment and neutralise the stockpile.
Iran has over the past year in effect kept the amount of its20 percent reserve well below Israel's so-called "red line" byconverting a large part of the uranium gas into oxide to makefuel for a medical research reactor in Tehran.
"Iran does not want to provoke Israel to attack Iran.Especially now," said nuclear expert Mark Hibbs of the CarnegieEndowment for International Peace think-tank.
But conversion work was halted between Aug. 20 and Nov. 5,in part for maintenance reasons, according to the quarterlyreport by the International Atomic Energy Agency (IAEA), issuedto member states late on Thursday.
As Iran continued its production of 20 percent uranium gas,the stockpile would probably have grown steadily during much ofthe August-November period covered by the report, perhaps tosignificantly above 200 kg, analysts and diplomats said.
IRAN SIGNALS RESTRAINT
The IAEA data suggests, however, that Iran moved fast onceit resumed conversion early this month, leading to a more modestrise to 196 kg in the Nov. 14 report, up by about 10 kg sincethe previous one issued in late August.
Tehran may have done so by attaching a full cylinder ofuranium gas to the conversion process, thereby reducing thestockpile, one nuclear expert said.
"There are rumours it got quite high - though not over the'red line'," one Western diplomat in Vienna, where the IAEA isbased, said. "I think the decision to blend down is politicallydriven."
Iran faced a "delicate balancing act," the envoy said: if itstops refining to 20 percent it gives in to the powers' demandfor nothing. But if it fails to convert enough it risksprovoking Israel and sends the wrong message to the West.
A senior diplomat familiar with the IAEA's report saidIran's move to stop converting during a couple of months formaintenance was "normal ... nothing exceptional".
The IAEA report showed that since Hassan Rouhani, a relativemoderate, became president in August Iran had virtually stoppedthe expansion of its overall uranium enrichment capacity.
Iran says it is refining uranium to produce energy. But itsrefusal so far to scale back its nuclear programme and open itup to unfettered IAEA inspections has drawn tough sanctions thathave severely damaged its oil-dependent economy.
However, the marked slowdown in the growth of activities ofpossible use in developing nuclear bombs may be meant to putsubstance in Rouhani's warmer tone towards the West after yearsof confrontation and strengthen Tehran's bargaining position.
"I think they were desperate to show that during this firstIAEA report under Rouhani's tenure they were able to cap thecapacity," proliferation expert Mark Fitzpatrick of theInternational Institute for Strategic Studies think-tank said.
"That whole image could have come undone if the amount ofgasified 20 percent product had crept too close to 250 kg, sothey had to make a rush effort to lower the amount."
The six powers - the United States, France, Russia, China,Germany and Britain - want Iran to curb its nuclear programme inexchange for limited sanctions relief as part of aconfidence-building accord that would buy time for negotiationson a more far-reaching settlement. (Editing by Mark Heinrich)
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