A small amount of uranium was detected in a package that arrived at London's Heathrow Airport on Dec. 29, British police said in a statement, adding that it did not appear to be linked to any direct threat.  The amount of material, detected by routine screening, was extremely small and has been assessed by experts as posing no threat to the public, Richard Smith, head of London police's Counter Terrorism Command said in a statement.  "Although our investigation remains ongoing, from our inquiries so far, it does not appear to be linked to any direct threat," Smith said. Police said no arrests had been made.  "We will continue to follow up on all available lines of enquiry to ensure this is definitely the case."  Heathrow Airport did not respond to a Reuters request for comment.  Uranium can be used for civilian power generation and scientific purposes and is a key ingredient in nuclear weapons. Certain isotopes emit radiation that can be harmful to humans, and the metal itself is toxic if ingested or inhaled.