A ship carrying nuclear material bound for Sellafield left a Japanese port under tight security today, amid protests from anti-nuclear activists who said the cargo could be at risk of theft or attack.
The cargo will take six weeks to reach Britain and will travel up the Irish Sea.
The ship, carrying a potentially weapons-usable mix of plutonium and uranium oxides (MOX), sailed just days after the US State Department warned of the potential for extremist attacks today - US Independence Day.
An air of tension surrounded the pier near the nuclear power plant at Takahama, as the Pacific Pintail headed off on its secret course back to Britain.
Some 100 protesters gathered in front of the gate to the power plant, holding signs saying "Stop MOX" and "Plutonium equals atom bombs".
The MOX fuel is being returned to British Nuclear Fuels (BNFL) after Japan's Kansai Electric Power Co Inc discovered that data for a 1999 shipment from Britain had been deliberately falsified.
Minister for Environment, Mr Martin Cullen, expressed concern at the news. He said the shipment of nuclear materials to and from the Sellafield plant represented "an unacceptable risk to the environment of Ireland and the health and economic well being of its population."
Meanwhile, Greenpeace today withdrew its High Court application in London aimed at preventing BNFL shipping the plutonium from Japan.
The environment group said the consignment left Japan "just a couple of hours before a High Court judge in London was due to decide whether to grant an injunction to stop them".
Greenpeace claims the fuel, a mixture of plutonium and uranium oxides, contains 255 kg of plutonium - enough to make 50 nuclear weapons. The group is also seeking High Court permission to apply for a judicial review of the Environment Agency's decision not to treat the material, known as MOX, as radioactive waste.
A spokesman said: "To send highly radioactive materials on a six-week trip on the high seas was a stupid idea before September 11th. In today's context it can only be described as insane."
Meanwhile Britain launched a new body today to take on the liabilities of BNFL which have swelled to almost £50 billion.
In a move industry experts view as a first step towards selling off the state-run operator, Energy Minister Mr Brian Wilson published a policy White Paper creating a Liabilities Management Authority to manage nuclear "clean up" and the massive costs involved for decades to come.