Russia agreed in 1995 to build the Bushehr plant on the site of a project begun in the 1970s by German firm Siemens, but delays have haunted the $1 billion project and diplomats say Moscow has used it as a lever in relations with Tehran.
The United States has criticized Russia for pushing ahead with the Bushehr project at a time when major powers including Russia are pressing Tehran to allay fears that its nuclear energy program may be aimed to develop weapons.
But Western fears that the Bushehr project could help Tehran develop a nuclear weapon were lessened when Moscow reached an agreement with Iran obliging it to return spent fuel to Russia. Weapons-grade plutonium can be derived from spent fuel rods.
Russian and Iranian specialists are to begin loading uranium-packed fuel rods into the reactor on Aug. 21, a process that will take about 2-3 weeks.
"This will be an irreversible step," Sergei Novikov, a spokesman for Russia's state nuclear corporation, Rosatom, said by telephone. "At that moment, the Bushehr nuclear power plant will be certified as a nuclear energy installation," he said.
"That means the period of testing is over and the period of the physical start-up has begun, but this period takes about two and a half months," he said, adding that the first fissile reaction would take place in early October.
The head of Iran's nuclear energy agency, Ali Akbar Salehi, said a ceremony inaugurating the plant would be held in late September or early October, when the fuel is moved "to the heart of the reactor."
The reactor will be linked to Iran's electricity grid about six weeks later when it is powered up to a level of 50 percent, Salehi told the semi-official Mehr news agency.
Diplomats say the Bushehr plant, monitored by the United Nations nuclear watchdog, poses little proliferation risk and has no link with Iran's secretive uranium enrichment program, seen as the main "weaponization" threat, at other installations.
Russia started the delivery of nuclear fuel to the Bushehr plant in late 2007 and deliveries were completed in 2008.
Moscow and Washington agree that importing fuel makes unnecessary Iran's own enrichment project — the main focus of Western concerns that Tehran is trying to make a nuclear bomb.
Iran, the world's fourth-largest crude oil producer, rejects such allegations and says its nuclear program is aimed only at generating electricity or producing isotopes for medical care.
Prime Minister Vladimir Putin had said on March 18 that Russia planned to start up the reactor at the Bushehr plant in the summer of 2010.
