A convoy of police and army vehicles and motorcycle outriders was accompanying the vehicle, which left the nuclear treatment centre at La Hague shortly before dawn. Late morning it was travelling by motorway between the cities of Nantes and Bordeaux.

The itinerary across some 1,200 kilometres (750 miles) was being kept secret in order to deter protests from environmental activists who have staged small-scale demonstrations since the arrival of the material by ship at the port of Cherbourg early Wednesday.

The Atomic Energy Commission and the company Areva which runs Cadarache were seeking an injunction to prevent two anti-nuclear groups -- Greenpeace and Sortir du Nucleaire (Get out of nuclear power) -- demonstrating within 500 metres of the perimeter fence there.

Protesters say the conditions in which the material is being transported are unacceptably dangerous.

The 140 kilograms (308 pounds) of plutonium from US weapons arsenals are to be transformed into two tonnes of fuel used in civilian power plants known as mixed oxide, or Mox, and returned to the United States.

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