"Rosatom, like the other public firms, is a state within a state and cannot be controlled by the the anti-monopoly service or the court," said Elena Panfilova, head of Transparency International Russia.
The group said it analysed 200 orders made by Rosatom for construction materials, and Panfilova said that firms often complained they were not allowed to take part in Rosatom tenders.
"Rosatom only allows chosen firms to take part in its offers, which are made at high prices in violation of its own norms," added deputy head of Transparency in Russia Ivan Ninenko.
Led by former prime minister Viktor Kiriyenko, Rosatom succeeded the Soviet Union's Ministry of Nuclear Energy and Industry in 1992. It was transformed into a state corporation in 2004.
Rosatom also controls nuclear weapons companies and oversees the country's construction of the first nuclear power plant in Iran.