Russia admits 'extremely high' levels of nuclear leak caused mysterious radioactive cloud over Europe

After several weeks of denial, Russia has finally admitted that "extremely high" concentrations of radioactive isotope ruthenium-106 caused the recent mysterious radioactive cloud over Europe. Moscow has conceded that the leak levels were 986 times the normal level, confirming earlier suspicion that pollution from a Russian nuclear plant could have caused the cloud formation.

The mysterious cloud was spotted in September and October and European reports blamed it on nuclear contamination from Russia. But Russian officials had denied any knowledge of leak at that time.

Reversing its stance on Monday, 20 November, Russia's meteorological service, Rosgidromet, said leak of Ru-106 – an isotope with a half-life of just one year and not found naturally on earth – was detected between 25 September and 1 October in the southern Urals. Russian officials say the highest concentration was registered in Argayash, a village in the Chelyabinsk region in the southern Urals.

"Probes of radioactive aerosols from monitoring stations Argayash and Novogorny were found to contain radioisotope Ru-106," Rosgidromet said. It said the isotope was detected in Tatarstan and southern Russia, and eventually reached "all European countries starting in Italy and toward the north of Europe" by late September.

Though the agency did not mention the exact source of the pollution, the monitoring station which picked up the highest levels of radiation is located about 20 miles from the Mayak nuclear facility. It was here that one of the worst nuclear disasters in history struck when more than 270,000 people were exposed to dangerous levels of radiation in 1957.

The thin cloud, which vanished within weeks, was spotted over more than a dozen nations, but reportedly did not pose a health risk.

According to Greenpeace, which has now called for an in-depth investigation into the latest incident, the Mayak plant still serves as a reprocessing site for nuclear fuel. "Greenpeace will send a letter asking prosecutors to open an inquiry into [the] potential concealment of a nuclear incident," read a statement released by the Russian arm of Greenpeace.

When Moscow's state-run nuclear corporation Rosatom released a statement earlier rejecting reports of a possible radioactive leak, it said the "radiation around all objects of Russian nuclear infrastructure is within the norm and the level of background radiation".