Iran on Monday was finally taking control of its civilian nuclear reactor at Bushehr, a project begun 37 years ago by West Germany, wracked by setbacks, and finished by Russia.
The Islamic republic's atomic agency chief Ali Akbar Salehi confirmed Russia was handing over the 1,000-megawatt plant but said its experts would remain in Bushehr as part of a guarantee scheme for the operation.
"A team of Iranian engineers will take control of the Bushehr power plant beginning today," he told state television, ahead of the handover ceremony in the southern city located east across Gulf waters from southern Kuwait.
Construction of the facility -- championed by Tehran as an illustration of its peaceful nuclear intentions -- began in 1975 with the help of West German company Siemens, which quit the project after the 1979 Islamic revolution over concerns about nuclear proliferation.
Work was hampered during the 1980-1988 Iran-Iraq war, until Russia agreed in mid-90s to take up the construction baton due originally for completion in 1999.
It is not clear how much the plant has finally cost Iran.
Finally finished more than a decade late and inaugurated in 2010, Bushehr did not come into service until 2011 because of repeated technical problems.
And since then, difficulties have stopped its operation periodically.
"There will be a two-year warranty period, during which Russian experts will be present at the Bushehr plant ... Should any problem arise the Russian contractor is responsible for removing it," Salehi said Monday.
He added that after that period Iran will be "fully responsible" for the plant's operation.
Moscow has also agreed to provide its fuel for 10 years, with the supply deal committing Tehran to returning the spent fuel, amid Western concerns over its controversial uranium enrichment programme.
Tehran's nuclear ambitions have been at the heart of its troubled relations with world powers for years.
Western powers and Israel suspect that Iran's declared peaceful programme of uranium enrichment masks a covert weapons drive, a charge vehemently denied by the Iranian leadership.
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Some Iranian officials accuse Russia of foot-dragging in Bushehr under pressure from the United States, which had sought in vain to prevent the project from reaching fruition.
Construction of the Bushehr facility has sparked concern among Gulf Arab states, but both Iran and Russia say it is subject to safeguards of United Nations watchdog, International Atomic Energy Agency (IAEA).
Foreign experts say Tehran attaches great importance to Monday's handover, as it illustrates its self-sufficiency in harnessing civilian nuclear power, no longer dependent on outside help.
But neighbouring nations and the West have concerns about Bushehr, given its location in an earthquake-prone zone on the Gulf, especially since Japan's Fukushima disaster of 2011.
As the crow flies, the plant is far closer to Iran's neighbours than it is to its own capital, one foreign diplomat pointed out, adding: "The prevailing winds go towards Dubai, and marine currents towards Kuwait."
Iran sits astride several major fault lines and is prone to frequent earthquakes, some of which have been devastating.
On April 9, a 6.1-magnitude quake rocked the south, with an epicentre around 100 kilometres (60 miles) from Bushehr.
However, a spokesman for Russia's state nuclear corporation Rosatom -- which finished the plant -- at the time said "they did not even feel the tremors" in Bushehr.
Western concerns also include Iranian engineers' ability to run a power plant constructed of components from three different sources -- German, Russian and domestic.
Iran has said it wants to produce 20,000 megawatts of electricity from nuclear power, which would necessitate building 20 1,000-megawatt reactors.
According to Iranian officials, negotiations with Russia are underway for cooperation on future plants.
Salehi said Sunday he expected work to start soon on a second plant upon completion of talks with Moscow, saying: "Negotiations are continuing and are well-advanced."
"Work will start soon," he added, without specifying a date.