Officials in Russia say there is no risk of a radiation leak after a fire broke out at a nuclear research centre in Moscow housing a non-operational 60-year-old atomic reactor.
But Greenpeace Russia has expressed concern, saying the incident was potentially very dangerous.
The fire broke out on Sunday in a part of the Alikhanov Institute of Theoretical and Experimental Physics in south-western Moscow containing a research collider, Reuters reports.
Institute officials say there were no radiation sources in the area of the collider, which had been shut off on 25 December last year, and no danger of a radiation leak.
They say personnel were evacuated and nobody was hurt.
Safety at Russia's nuclear facilities has been a concern since the deadly 1986 accident at the Chernobyl nuclear power plant in Ukraine, then a Soviet republic.
The authorities did not announce that disaster for two days.