The state nuclear agency operates 38 reactors -- mostly built during the Soviet period at 11 nuclear power stations across the country.

The second of four reactors at the Rostov Nuclear Power Plant was taken offline Thursday for repairs after a steam leak was detected at 21:54 GMT on Wednesday, said Rosenergoatom, a subsidiary of the state nuclear agency Rosatom.

The operator of the country's nuclear power plants said the leak was due to a welding defect on an 18-millimetre pipe carrying non-radioactive water in the reactor's steam generator.

Rosenergoatom spokesman Andrei Timonov told AFP the incident posed no danger.

"There was a thin trickle of steam," he said. "We will repair everything in two days," he said, adding that the pipe had to cool down for repairs.

Radiation levels at the plant near the city of Volgodonsk were "in line with normal background levels," Rosenergoatom said.

Russia is one of the world's largest producers of nuclear energy, and officials sporadically report operating faults.

The Kremlin is aiming to position itself as a key exporter of nuclear energy services, and Rosatom is working on projects in countries including Egypt and India.

The Soviet Union was home to the world's worst nuclear disaster when an explosion at the Chernobyl nuclear power plant in 1986 sent radioactive fallout across Europe.