Rosatom said the power unit had been automatically shut down due to a malfunction of the turbine generator.
"The reasons are being investigated, the radiation background is normal," a Rosatom representative told Reuters.
According to Rosenergoatom, Rosatom's subsidiary, another two power units at the Rostov station, located near Volgodonsk, were operating normally, while another unit was undergoing planned maintenance.
The energy ministry said temporary restrictions had been imposed on power usage in the south of the country because of the incident.
The Rostov nuclear plant is the largest energy-generating facility in southern Russia with a total capacity of 4,071 megawatts. It provides around 75% of the power generation in the Rostov region and is connected to the grid of other southern regions.
The south of Russia has been affected by an abnormal heatwave, which has caused mass power outages in several regions.
