A convoy of police and army vehicles and motorcycle outriders accompanied the vehicle, which had left the nuclear reprocessing centre at La Hague shortly before dawn on Thursday.

The itinerary across some 1,200 kilometres (750 miles) was 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) -- from demonstrating within 500 metres (yards) of the perimeter fence there.

Protesters said the conditions in which the material was being transported were unacceptably dangerous.

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

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