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A small amount of the radioactive material was found with a shipment of scrap metal

Some of the worst contamination events were caused by people that just didn't know they were dealing with radioactive materials. The biggest example I can think of is the Goiania Incident. 
(Though that wasnt Uranium, but a much more radioactive material that was used for radiation treatment and was just carelessly abandoned when the medical center shut down. )
A small amount of the radioactive material was found with a shipment of scrap metal following a routine screening on 29 December.
Police said on Sunday that the man was arrested on suspicion of a terror offence on Saturday and was released on bail as part of their investigation.
He [Commander Richard Smith, who leads the Metropolitan Police's Counter Terrorism Command] added that despite the arrest, the incident does "not appear to be linked to any direct threat to the public", but detectives are continuing inquiries to ensure this is "definitely the case".