France says a cloud of radioactive pollution over Europe could mean a nuclear accident took place in Russia or Kazakhstan.
The substance Ruthenium 106 has been detected since late September.
It was detected by several European networks, France's Institute for Radiological Protection and Nuclear Safety (IRSN) says.
The IRSN is involved in the monitoring of atmospheric radioactive contamination.
It says an investigation suggests there could have been a nuclear accident in Russia or Kazakhstan two months ago.
It says the most plausible release zone lies between the Volga River and the Ural Mountains.
Map showing the plausibility of the origin of the release | Image: IRSN
The IRSN says it mobilised "all its means of radiological monitoring" as soon as it became aware of the first detections.
It says from September 27th to October 13th, only its stations of Seyne-sur-Mer, Nice and Ajaccio found the presence of Ruthenium 106 in trace amounts.
Since October 13th, the substance has no longer been detected in France.
The results showed "a steady decrease" in Ruthenium 106 levels. It says the substance is no longer detected in Europe.
"The concentration levels of Ruthenium 106 in the air that have been recorded in Europe and especially in France are of no consequence for human health and for the environment", it says.
Read the full IRSN report here
