The solid, silver-coloured cylinder is just 8mm by 6mm but the authorities say it contains enough Caesium-137 to cause acute radiation sickness
A man with a knife wounded several people at the Gare du Nord train station in Paris on Wednesday morning, France Bleu radio reported.
The attacker was "neutralised" by police, interior minister Gerald Darmanin said on Twitter, without going into further detail.
The attack happened around 0640 CET (0540 GMT) inside the train station, radio franceinfo said, quoting rail operator SNCF.
The station is one of the busiest in Europe and a major link between Paris, London and the north of Europe.
Police have secured the area and trains were operating normally, franceinfo reported.
BFM TV said police had used a gun to stop the man.
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