By Fredrik Dahl
VIENNA (Reuters) - U.N. inspectors are to visit an Iranian plant on Sunday linked to a planned heavy-water reactor that could yield nuclear bomb fuel, taking up an initial gesture by Iran to open its disputed nuclear program up to greater scrutiny.
The increased transparency is one of the various spin-offs from a dramatic diplomatic rapprochement over the past month highlighted by a deal Iran struck with six world powers to curb its nuclear program in return for some easing of sanctions.
It will be the first time in more than two years that the International Atomic Energy Agency (IAEA) is allowed to go to the Arak heavy-water production plant, which is designed to supply a research reactor under construction nearby.
The improved access will enable the Vienna-based U.N. nuclear watchdog to better "understand" the activities there, IAEA Director General Yukiya Amano said last week when he announced that Iran had invited his experts to come on December 8.
Iran's atomic energy organization said this underlined the country's "goodwill to remove ambiguities about the peaceful nature of its nuclear energy program", Press TV, Iran's English-language state television, reported on Wednesday.
But Western diplomats and nuclear experts stress that Iran must do much more in order to fully address suspicions that it has been trying to develop the capability to assemble nuclear weapons, a charge the Islamic Republic denies.
Iran's heavy water-related work is of great concern for the West: plutonium, a nuclear bomb ingredient, can be extracted from the spent fuel of a reactor that is powered by natural uranium and uses heavy water as a coolant and moderator.
Iran, which says the reactor will make medical isotopes, promised last month to stop installation work there for six months as part of its breakthrough pact with global powers.
Tehran also pledged in the November 24 Geneva interim accord to halt its most sensitive uranium enrichment, activity which it says is for peaceful energy only but which could also be applied to creating the fissile core of atomic bombs.
Iran has moved quickly since Hassan Rouhani, a relative moderate, became president in August on a pledge to allay international concern about its nuclear program in exchange for relief from sanctions battering its oil-dependent economy.
After years of sharpening confrontation that raised fears of a new Middle East war, Rouhani's election created a rare diplomatic opportunity to smooth Iran's troubled relations with Western states and end its isolation.
In the course of a few weeks of intensive diplomacy, Iran struck two separate but still closely linked nuclear accords: one on November 11 with the IAEA on more transparency and a broader diplomatic pact with the United States, France, Germany, China, Russia and Britain in Geneva 13 days later.
Both are seen as important first steps towards ending a decade-old standoff over Tehran's atomic activities. But diplomats say many difficult hurdles remain to be overcome to reach a final settlement of the dispute, including differences over the scope and capacity of Iran's nuclear program.
The IAEA regularly goes to Iran's uranium enrichment plants at Natanz and Fordow and other nuclear sites, but it wants wider inspection powers to make sure there are no hidden atomic activities and no diversion of atomic material.
The Arak visit is the first of six concrete steps that Iran agreed to implement under its cooperation pact with the IAEA, which is investigating allegations that Iran has been studying how to make nuclear bombs. Iran denies any nuclear weapons aim.
The other measures Iran committed to take within three months include allowing the IAEA to see a uranium mine, Gchine, as well as furnishing information about more enrichment plants and reactors that it has previously said it plans to build.
The IAEA needs such access and data to gain a more complete picture of the Iranian nuclear program, experts say.
Western diplomats have described the six steps as relatively easy for Iran to carry out - "low-hanging fruit" in the words of one envoy - and they say future action sought by the IAEA would probably be more difficult.
For example, the IAEA has made it clear it still wants to visit the Parchin military base southeast of Tehran where the U.N. agency believes nuclear weapons-relevant explosives tests were conducted, possibly a decade ago.
Last month's IAEA-Iran deal signaled a change in tactics after almost two years of fruitless negotiations focused on Parchin and other sensitive issues as part of the U.N. agency's investigation into suspected atomic bomb research by Tehran.
"This new approach starts with less controversial transparency issues but in subsequent phases...it will address the main IAEA concerns over the possible military dimensions to Iran's nuclear program," the Institute for Science and International Security (ISIS), a U.S. think-tank, said.
The six initial measures "remain far from being enough to satisfy" U.N. inspectors' concerns, it added in an analysis.
The existence of the Arak heavy-water plant, which has the capacity to produce 16 tonnes per year, was first revealed by an exiled Iranian dissident group 11 years ago.
Since its last visit in August 2011, the IAEA has been monitoring the site southwest of Tehran via satellite imagery. It said in August the plant appeared to still be in operation.
(Editing by Mark Heinrich)