France's nuclear safety institute, IRSN, picked up faint traces of ruthenium 106, a radioactive nuclide that is produced when atoms are split in a nuclear reactor and which does not occur naturally, in three of its 40 monitoring stations late September.

Faint traces were also detected in Germany, Austria, Italy and Switzerland.

There has been no impact on human health or the environment in Europe, a French official stressed, but he added that detection of such a cloud was "absolutely not normal".