Russian troops dug a five-mile canal on Saturday to protect a nuclear arms site from wildfires caused by a record Leatwave.
The fires have aroused fears that radioactive fallout from the Chernobyl nuclear disaster would be returned to the air again.
The canal was dug at Sarov, whose nuclear site, ringed by forest now, produced the first Soviet atomic bomb in 1949 and remains Russia's main nuclear design and production facility.
The emergencies ministry said that the situation in Sarov had “stabilised”.
The forest and peat fires have killed at least 52 people, made more than 4,000 homeless, diverted many flights and pushed air pollution in Moscow to six times its normal level, with the smoke reaching even the underground metro stations.