At the Russian measuring station Argayash, the highest level measured was 986 times that of the previous month, the Russian weather service said in a statement. At the Novogorny station, the measured levels were 440 times higher.
In mid-October Russian nuclear agency Rosatom denied that any of its facilities experienced any incidents — today, it again denied this.
Officials at Mayak have said the dose of radiation is 20,000 times smaller than the "allowed annual dose," and therefore "poses no danger to human health and lives."
In a statement released on Tuesday, environmental group Greenpeace demanded an in-depth inquiry "into potential concealment of a nuclear incident" and an investigation into public health risks.
Mayak was the location of the Kyshtym disaster in 1957, which remains the third-most serious nuclear accident ever recorded.