What is Mox fuel?

It is the result of reprocessing spent nuclear fuel rods by releasing the uranium and plutonium they contain and recombining them to create new, usable fuel rods.

The rods - kept inside 14-inch thick lead and steel flasks - contain thimble-sized fuel pellets, which owners British Nuclear Fuels Ltd (BNFL) say each produce as much energy as a ton of coal.

Why is this consignment of Mox coming to Barrow from Japan?

The rods were originally processed out of spent fuel from Japanese reactors at BNFL's Sellafield power plant in Cumbria, close to Barrow.

They were sent back by sea to Japan in 1999, ready for use in nuclear power stations there. But it was a sensitive time following a major nuclear accident at one plant, which killed one worker and exposed hundreds to radioactive fall-out.

Then a huge row erupted when BNFL admitted staff at Sellafield faked safety records. Japan said it no longer wanted the fuel, demanding BNFL take it back. The British firm eventually agreed and the shipment of unused rods set sail for its return journey to the UK in July.

Why has the shipment itself been so controversial?

Because of fears over security and environmental damage.

In theory the plutonium in the Mox rods could be converted for use in a nuclear weapon. It would certainly be easier than extracting it from the original, used fuel.

But BNFL point to elaborate security measures to protect the cargo, and say that in practice it would be extremely difficult to convert the plutonium as a major reprocessing operation in a large purpose-built facility would be needed.

Environmental groups such as Greenpeace and Friends of the Earth are more concerned about the potential impact of an accident on the marine life and coastal areas along the ships' route. They say there are concerns over the possibility of corrosion in both the ships' hulls and the flasks containing the rods.

BNFL say those concerns are unfounded and that the cargo is insoluble. In the unlikely event of an accident, the firm claims, and in the unlikely event of the rods coming free from their protective casings "it would be like dropping a marble into a glass of water" in terms of the amount of time needed for them to dissolve and pose an environmental hazard.

What protection measures are in place?

The purpose-built ships, Pacific Pintail and Teal, are each armed with two 30mm cannons. Armed officers from the UK Atomic Energy Constabulary are also on guard against boarders.

Beyond them, would-be thieves would have to crack open the vessels' reinforced hatch covers and unload the flasks without the aid of deck cranes, which were removed before the cargo departed.

Greenpeace claims the ships may also have been escorted by a submarine.

To guard against accidents, the ships have five holds and it is claimed that even if every one was flooded, the vessels would remain afloat.

They also have double hulls to withstand collision damage as well as twin radars, navigation and propulsion systems to avoid the danger of collision or grounding in the first place.