Russia’s nuclear agency said there were no open flames and no threat of a radiation leak.
The fire was in a basement at the Institute of Theoretical and Experimental Physics in south-western Moscow, said Sergei Vlasov, spokesman for the emergencies ministry.
Smoke rose above the institute, and an acrid smell filled the air. Dozens of emergency vehicles, including fire engines and ambulances, stood inside and outside the gate, witnesses said.
Sergei Novikov, spokesman for the state nuclear agency Rosatom, said there were no open flames, only smoke from an area housing power cables that could not affect any nuclear materials.
Greenpeace Russia official Ivan Blokov said the incident was potentially very dangerous. “It shows there has been a major failure in operations,” he said.