Greenpeace called for an investigation on Tuesday into the possible concealment of a nuclear incident in southern Russia, after Russian meteorologists confirmed on Tuesday that they recorded a rise in radiation coming from the Urals region in late September. The spike in radioactive ruthenium-106 isotopes coincided with a radioactive cloud that formed over much of Europe—but a reprocessing plant denied any involvement.

"An emergency discharge of ruthenium could be connected with the process of nuclear waste vitrification. Another possibility is that materials containing ruthenium-106 were placed in a metal remelting furnace. Both these activities take place in the Rosatom complex at Mayak," said Greenpeace in a statement.

The environmental group's Russian arm demanded that Russia's nuclear agency conduct "an in-depth inquiry and publish the results about the incidents at Mayak." Greenpeace also called for an examination into whether officials had been sufficiently transparent "and whether public health around a possible release of ruthenium 106-was sufficiently protected."

According to Radio Free Europe, officials in Chelyabinsk, a city in the southern Ural region where Mayak is located, "received no official information about dangerous levels of radiation in September."

But according to the meteorological service, levels of ruthenium-106 were found to be nearly 1,000 times higher than normal in parts of Russia in late September.

In the first week of October, both Austrian and German officials detected unusually high levels of radiation in their airspace. French researchers at the Institute for Radiological Protection and Nuclear Safety (IRPS) suspected that the radiation had come from Russia, but Russian officials denied that there had been any recent incidents.

The IRPS has said that the isotope posed no health risks to people in any of the countries that saw elevated levels, but Greenpeace urged transparency about the possible nuclear leak.

Mayak was the site of one of the worst nuclear disasters in history in 1957—an incident which was concealed by the Soviets. Tens of thousands of people were contaminated after a storage tank containing radioactive waste exploded.

Jan Haverkamp, a nuclear energy expert consultant at Greenpeace Central Eastern Europe also spoke in favor of transparency and an in-depth investigation.

"Mayak looks like a logical source for this release, but the data do not yet exclude also other possibilities, including in neighboring Kazakhstan," Haverkamp said. "It is important there will be clarity as soon as possible, also to see if potential exposed people need support."