Russian troops have reportedly dug a canal eight kilometres long to keep wildfires that have been sweeping the region away from a nuclear site.
The fires caused by the month-long heatwave have been approaching the centre from nearby forests.
Sarov is the main nuclear design and production facility in Russia. The Emergencies Ministry said the situation had stabilised.
Earlier Russia’s nuclear chief said that as a precaution, all explosive and radioactive material had been removed from the site, which lies 350 kilometres east of Moscow.
In the capital, choking from smog caused by the fires, many people tried to carry on as usual. The opposition has called for factories to stop any activity that could cause further pollution.
At one of Moscow’s main airports to the south of the city, hundreds of people have been stranded after flights were cancelled. Many planes have been diverted elsewhere, some as far away as Ukraine.