The West, which suspects Tehran of seeking to produce its own nuclear bomb, has been critical of Russia's involvement in building Iran's first nuclear power plant. Russia says it is purely civilian and cannot be used for any weapons programme.
The head of Russia's state nuclear company, Sergei Kiriyenko, said this month Russia aimed to start up a nuclear reactor at Bushehr by the end of the year. The plant is located on the Gulf cost in Iran's southwest.
Iranian media said Kiriyenko and the head of Iran's Atomic Energy Organisation, Gholamreza Aghazadeh, would attend Wednesday's pre-commissioning at the site.
Mohammad Saeedi, Aghazadeh's deputy, described it as a "preliminary phase" for starting the plant and told Reuters it would be followed by its commissioning and launch, without giving a timetable.
State radio said it would include testing of all of Bushehr's activities with computer software.
"Pre-commissioning is an important step before the actual commissioning of the power plant," the radio quoted Mohsen Delaviz, a spokesman for Iran's Atomic Energy Organisation, as saying.
The launch of the Bushehr plant's nuclear reactor has been delayed frequently, though Russia last year completed delivery of nuclear fuel to the station under a total contract estimated to be worth about $1 billion.
Iran's official IRNA news agency said the plant "is in the final stages of its construction" and the Russian side had boosted the number of staff to "increase the speed of work".
Analysts say Iran could become a central issue in relations between Russian President Dmitry Medvedev and new U.S. President Barack Obama, who has said that the United States was prepared to talk to Tehran.
They say Russia has used Bushehr as a lever in relations with Tehran, which is suspected by the United States and some European countries of seeking to build nuclear weapons.
Iran, the world's fourth-largest crude producer, rejects such allegations and says its nuclear programme is aimed at generating electricity so that it can export more oil and gas.
Russia started deliveries of nuclear fuel for the plant in late 2007, a step both Washington and Moscow said removed any need for Iran to have its own uranium enrichment programme.
Switching on the Bushehr plant could dismay some in the United States, Israel and Europe who are deeply suspicious of Iran's intentions.
Moscow says the plant poses no proliferation risk as Iran will return all spent fuel rods to Russia. (Reporting by Hashem Kalantari and Parisa Hafezi; Writing by Fredrik Dahl; Editing by Alison Williams)
