Iranian and Russian engineers began loading fuel Saturday into Iran's first nuclear power plant, which Moscow has promised to safeguard to prevent material at the site from being used in any potential weapons production.
After years of delays, the fueling of the Bushehr plant in southern Iran marks the startup of a facility for energy production that the U.S. once hoped to block as a way to pressure the country to stop separate nuclear activities of far greater concern.
There have not been strong objections to the Bushehr plant itself as there have been with Iran's separate efforts at other sites to accelerate uranium enrichment - a process that makes the fuel for power plants but which can also be used in weapons production.
Even as Iran's nuclear chief said the plant demonstrated the country has only peaceful aims, he celebrated it as a defiant "symbol of Iranian resistance and patience" in the face of Western pressure.
"Despite all pressure, sanctions and hardships imposed by Western nations, we are now witnessing the startup of the largest symbol of Iran's peaceful nuclear activities," Ali Akbar Salehi told reporters inside the plant.
Washington and other nations do not oppose Iran's stated aim of producing nuclear energy, but are concerned that if Iran masters the enrichment cycle it would have a pathway to weapons production under the convenient cover of a peaceful energy program. Iran denies such an intention.
It is the enrichment work that has been the target of four rounds of U.N. Security Council sanctions.
Russia, which helped finish building Bushehr, has pledged to prevent spent nuclear fuel at the site from being shifted to a possible weapons program. After years of delaying its completion, Moscow says it believes the Bushehr project is essential for persuading Iran to cooperate with international efforts to ensure Iran does not develop the bomb.
The United States, while no longer formally objecting to the plant, disagrees and says Iran should not be rewarded while it continues to defy U.N. demands to halt uranium enrichment.
On Saturday, a first truckload of fuel was taken from a storage site to a fuel "pool" inside the reactor building under the watch of monitors from the U.N.'s International Atomic Energy Agency. Over the next two weeks, 163 fuel assemblies - equal to 80 tons of uranium fuel - will be moved inside the building and then into the reactor core.
Workers in white lab coats and helmets led reporters on a tour of the cavernous facility.
It will be another two months before the 1,000-megawatt light-water reactor is pumping electricity to Iranian cities.
The Bushehr plant is not considered a proliferation risk because the terms of the deal commit the Iranians to allowing the Russians to retrieve all used reactor fuel for reprocessing. Spent fuel contains plutonium, which can be used to make atomic weapons. Additionally, Iran has said that IAEA experts will be able to verify that none of the fresh fuel or waste is diverted.
Of greater concern to the West, however, are Iran's stated plans to build 10 new uranium enrichment sites inside protected mountain strongholds. Iran said recently it will begin construction on the first one in March in defiance of the U.N. sanctions.
Nationwide celebrations were planned for Saturday's fuel loading at Bushehr.
"I thank the Russian government and nation, which cooperated with the great Iranian nation and registered their name in Islamic Iran's golden history," Salehi said. "Today is a historic day and will be remembered in history."
He spoke at a news conference inside the plant with the head of Russia's state-run nuclear corporation, Sergei Kiriyenko, who said Russia was always committed to the project.
"The countdown to the Bushehr nuclear power plant has started," Kiriyenko said. "Congratulations."
Iran's hard-liners consider the completion of the plant to be a show of defiance against U.N. Security Council sanctions that seek to slow Iran's other nuclear advances.
Hard-line leader Hamid Reza Taraqi said the launch will boost Iran's international standing and "will show the failure of all sanctions" against Iran.
President Mahmoud Ahmadinejad reiterated Friday that Tehran was ready to resume negotiations with the six major powers trying to curb Iran's enrichment work - the five permanent Security Council members plus Germany.
Ahmadinejad, however, insisted Iran would reject calls to completely halt uranium enrichment, a key U.N. demand. The president had earlier said the talks could start in September, but in an interview with Japan's biggest newspaper, The Yomiuri Shimbun, he said the talks could start as early as this month.
Russia signed a $1 billion contract to build the Bushehr plant in 1995 but has dragged its feet on completing the work.
Moscow had cited technical reasons for the delays, but analysts say Russia used the project to try to press Iran to ease its defiance over its nuclear program.
The uranium fuel Russia has supplied for Bushehr is well below the more than 90 percent enrichment needed for a nuclear warhead. Iran is already producing its own uranium enriched to the Bushehr level - about 3.5 percent. It also has started a pilot program of enriching uranium to 20 percent, which officials say is needed for a medical research reactor.
The Bushehr plant overlooks the Persian Gulf and is visible from several miles (kilometers) away with its cream-colored dome dominating the green landscape. Soldiers maintain a 24-hour watch on roads leading up to the plant, manning anti-aircraft guns and supported by numerous radar stations.
There are several housing facilities for employees inside the complex plus a separate large compound housing the families of Russian experts and technicians. The site is about 745 miles (1,200 kilometers) south of Tehran.
Russians began shipping fuel for the plant in 2007 and carried out a test-run of the plant in February 2009.
Iran says it plans to build other reactors and says designs for a second rector in southwestern Iran are taking shape.
The Bushehr project dates backs to 1974, when Iran's U.S.-backed Shah Mohammed Reza Pahlavi contracted with the German company Siemens to build the reactor. The company withdrew from the project after the 1979 Islamic Revolution toppled the shah.
The partially finished plant later sustained damages after it was bombed by Iraq during its 1980-88 war against Iran.
Before making the Russian deal to complete Bushehr, Iran signed pacts with Argentina, Spain and other countries only to see them canceled under U.S. pressure.