However, there still remains much troubled water between Takahama, where the Mox rods have sat unused since last September, and the BNFL's base in Cumbria.

The Pacific Teal carries a heavy load

While seeing BNFL's climb-down over the fuel rods as a vindication of their stance on nuclear reprocessing, environmental groups are still troubled by their impending homeward voyage.

Friends of the Earth's Mark Johnston says transporting the rods, which contain both plutonium and uranium, "raises serious environmental and security concerns".

Some governments on the route of the BNFL ships share these concerns.

Route or rout?

"The craft sometimes pass through the Panama Canal, sometimes go around the Cape of Good Hope. The route depends on the level of political storm BNFL expect. They'll take a longer route to avoid controversy," says Mr Johnston.

Click here to see graphic of the ship

Greenpeace, who are calling for the Mox to be treated as nuclear waste in Japan, say more than 50 nations protested when the ships made the 18,000-mile outward journey last year.

Transport of Mox

In western Europe by truck Intercontinental transport by sea Cask design approved by International Atomic Energy Agency Typical cask contains one tonne of Mox Source: Uranium Institute

South African and New Zealand politicians were particularly vocal in their calls for BNFL's Pacific Pintail to steer clear.

Critics of Mox reprocessing, where uranium and plutonium are released from spent nuclear fuel then recombined to create new fuel rods, say it raises nuclear proliferation issues.

While the plutonium in the Mox rods would be difficult for terrorists to convert into even a crude nuclear weapon, it is more accessible than left locked inside the original spent uranium fuel, says Professor William Walker of St Andrew's University.

Contained risk

"It would not be hard for a rogue state to extract plutonium from the mixed oxide and use it in weapons. But they would have the entire American military on their back if they decided to hijack the shipment."

Mox shipments have prompted worldwide protest

Nevertheless, BNFL take the security of their vessels seriously.

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.

Should the guard on both ships be over-powered, would-be thieves would have to crack open the vessels' reinforced hatch covers and unload the rods without the aid of deck cranes, which are removed before the cargo departs.

Red rag

Of course, those seeking to board the two craft may not wish to steal the cargo. The Pintail and Teal may prove a tempting target for green activists.

Environmental concerns are the real issue to have dogged BNFL's attempts to deliver its products to paying customers.

Fierce opposition by environmental campaigners means the current payload is only the third sea-level shipment of plutonium.

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Before the early 1980s, small payloads were carried by air. But as the nuclear debate hotted up, and America withdrew its consent, this became politically unacceptable.

Shipments of pure plutonium spark particular outrage. Placing the element inside Mox rods was mooted as way of appeasing these protests.

Safe passage

Last year's BNFL delivery to Japan was only the first of its sort. Despite the documentation debacle, the company says sea transportation of Mox is safe.

BNFL's Pacific Pintail carried the Mox to Takahama

BNFL spokesman Bill Anderton says the captains of the Pintail and Teal regard the ships as the safest they have ever commanded.

"The ships are divided into five holds. Even if every one was flooded, the ships would remain afloat."

Like many oil tankers, the vessels have double hulls to withstand collision damage.

Double check

Of course, with prevention the best medicine, they also have twin radars, navigation and propulsion systems to avoid the danger of collision or grounding in the first place.

Mr Anderton says an accident remains highly unlikely, but adds that even in a worst case scenario, the ship's cargo poses no danger.

BNFL has confidence in its transportation flasks

"The cargo is insoluble. The thimble-sized fuel pellets are contained inside a ceramic rod. These are kept inside 14-inch thick lead and steel flasks. The flasks are then bolted into the ships' holds."

Even if the rods were to be released from their protective housing, Mr Anderton is confident of their safety.

"It would be like dropping a marble into a glass of water. How long would that take to dissolve?"