The plant in Bushehr in southern Iran has yet to receive necessary approval in Iran to operate at 100 percent capacity, the source in Russia's civilian nuclear power corporation, Rosatom, told the Russian news agency.
Under an agreement approved by the U.N. nuclear watchdog, the International Atomic Energy Agency, Russia will operate the plant, supply its fuel and take away spent fuel for two or three years. Iran will eventually take full control of the plant.
The Bushehr plant's construction, beset by delays, has taken more than three decades.
In 1998, Russia signed a contract with Iran to complete the plant.
Israel and Western countries have expressed concern Iran is working to develop a nuclear weapon but Iran says its nuclear program is for peaceful purposes only.