At the same time, Iran claimed another advance in its nuclear program: The number of centrifuges carrying out uranium enrichment had increased to 6,000, the country's nuclear chief said - up from 5,000 in November.
His announcement was the latest defiance of United Nations' demands that Tehran suspend its enrichment program because of fears it could be used to produce material for a warhead. Iran denies it seeks to build a nuclear bomb, saying its nuclear program aims only to generate electricity.
The power plant in the southern port city of Bushehr, built with Russian help, is meant to be the first in a number of reactors for an energy program. But the opening of the 1,000-megawatt light-water reactor has long been delayed by construction and supply glitches. The United States for a time tried to dissuade Russia from helping the project.
It's unclear when the reactor could be switched on.
The tests, which began 10 days ago, "could take between four and seven month," the nuclear chief, Vice President Gholam Reza Aghazadeh, told reporters at Bushehr. It was not known how long after the tests the reactor could start up.
The plant, which will run on enriched uranium imported from Russia, has worried the West because the spent fuel could be turned into plutonium, a potential material for nuclear warheads.
U.S. concerns over the reactor softened after Iran agreed to return spent fuel to Russia to ensure Tehran does not reprocess it into plutonium. Washington largely dropped its opposition to the project and argued instead that the Russia fuel deal shows that Iran does not need its own domestic uranium enrichment program. Russia's fuel deliveries to Iran began in 2007.
"Although there appeared to be some indirect diplomatic overtures to Iran by the Obama administration during the past few weeks, the Iranian government - faced with elections for President in four months - is challenging the international community and the U.S. by upping the ante with their first test run of a nuclear plant," said CBS News foreign affairs analyst Pamela Falk from the U.N.
Enrichment is a concern because while low-enriched uranium is used as fuel for a reactor, higher-enriched uranium can be used to build a bomb. In the enrichment process, uranium gas is pumped through a series of centrifuges and spun at supersonic speeds to remove impurities.
Aghazadeh announced that 6,000 centrifuges were now operating at Iran's enrichment facility in the town of Natanz. He said Iran hopes to install over 50,000 centrifuges there over the next five years. "We are doing what we need to do in Natanz on the basis of a specific time schedule," he told a press conference.
Iran says it intends to use the enriched uranium fuel in its first domestically made nuclear power plant, in the town of Darkhovin, which it wants to start operating in 2016. Aghazadeh said any delay in enrichment will mean a delay in opening Darkhovin.
The tests at Bushehr are a computer run of the equipment to ensure there are no malfunctions in the future when enriched uranium fuel is introduced into the reactor. No electricity is produced during the testing.
In the first stage of the test, technicians for the past 10 days have been loading a "virtual fuel" into the reactor. The virtual fuel consists of lead, which imitates the density of enriched uranium, said Iranian nuclear spokesman Mohsen Shirazi.
Once the fuel is fully loaded, "we will check to see how the reactor will operate," said Russian nuclear agency chief Sergei Kiriyenko, who was inspecting the process. "This (test) is one of the major elements of an extensive project," he said.
Kiriyenko said Bushehr witnessed "remarkable progress in recent months" but that work remains to be done to "speed up the launching of the site." The Russian-Iranian team was "approaching the final stage" before the plant becomes operational, he said.
Aghazadeh, who was accompanying Kiriyenko, said the test was going well and engineers told him they expected no problems.
"Today was one of the most important days for the Iranian nation," Aghazadeh said. "We are approaching full exploitation of this plant."
In Israel, Foreign Ministry spokesman Yigal Palmor said, "Iranians are showing again that they are making progress in their nuclear race."
"This should be understood as very bad news for the whole of the international community," Palmor said, calling for "immediate and very determined steps in order to prevent Iran from becoming a nuclear power."
The Bushehr reactor was meant to start up in 2008, and some 700 Iranian engineers were trained in Russia over four years to operate the plant.
The Bushehr project dates backs to 1974, when Iran's U.S.-backed Shah Mohammed Reza Pahlavi signed an agreement to build the reactor with the German company Siemens. The company withdrew from the project after the 1979 Islamic revolution toppled the shah. In 1992, Iran signed an agreement with Russia to complete the project and work began on it in 1995.
Russia says there is no evidence that Iran is seeking nuclear weapons and has joined China in weakening Western-backed sanctions in the U.N. Security Council, arguing that punishing Tehran too harshly for its nuclear activities would be counterproductive.
The U.N. Security Council has passed three sets of sanctions against Iran over uranium enrichment and is considering further measures.