ON 12 October 2000, the destroyer USS Cole hove into port at Aden in Yemen for routine refuelling.
As the vessel took on fuel oil, a small boat drew alongside it. Suicide bombers inside the boat detonated a cache of explosives, blasting a 20-metre hole in the destroyer's hull and killing 17 of its crew.
The attack was a stark reminder of the risks the crews of naval ships face when they are forced to put in at potentially unfriendly ports.
Now some members of Congress believe they have a way to keep ships out of harm's way and prevent similar incidents happening in the future. A bill recently passed by the House of Representatives aims to make many more of the ships in the US naval fleet nuclear powered, including amphibious assault ships that carry troops into combat. The benefit will be two-fold, argue proponents of the bill. Rocketing oil prices make nuclear power an economic way of funding naval expeditions, and thanks to the slow burn of the highly enriched nuclear fuel in marine reactors, ships will have no need to pull into potentially hostile ports to refuel.
However, critics claim the presence of a nuclear reactor on a ship would make it a terrorist target. "It beggars belief in these days of heightened terrorism alerts that people are seriously suggesting building nuclear-powered assault ships," says Ben Ayliffe, head of anti-nuclear campaigns at Greenpeace in the UK.
The new bill represents an escalation of recent efforts to get the navy to use more nuclear fuel. The rising cost of oil means it is getting close to the point at which it will be more economic for the navy to use nuclear power, says Representative Roscoe Bartlett, a Maryland Republican, who backs the proposed measures. "A 2007 study by the navy on alternative energy for ship propulsion indicated that the break-even price for nuclear propulsion for amphibious ships was an oil price of $178 dollars per barrel. We're now creeping up to that number - oil hit a new record of $133 a barrel today," he said in a statement on 21 May.
There is also the question of securing the military's energy supply, says Bartlett. Ninety-six per cent of the world's oil reserves are owned by countries other than the US. "Many of these countries, such as Saudi Arabia, Venezuela, Russia, Iran and Nigeria, are unstable and ambivalent or outright hostile to America and our allies," he says, pointing again to nuclear propulsion as the answer. "It offers greater power and unparalleled safety and operational endurance without the vulnerabilities of fossil fuel refuelling," he says.
The move towards a navy that relies more heavily on nuclear power is also being driven by the increasing use of ever more powerful radar and radio links. These are turning ships into energy guzzling data-processing centres, said a report by the US Defense Science Board task force on energy strategy in February. In its report, entitled "More Fight, Less Fuel", the task force said that a major reason nuclear power is being seen as an option for surface ships is because not enough is being done to ensure electronics and radar systems are energy efficient. This is forcing the navy to look for new power sources to ensure ships have a steady supply during combat, it says.
Congress upped the ante on the use of nuclear power across the fleet last year, when it passed the National Defense Authorization Act for 2008, an annual piece of legislation that tells the Pentagon how it should spend its budget. Under the act all future aircraft carriers, submarines and battle cruisers have to be built with a nuclear power system at their heart. The risk of a nuclear disaster does not necessarily increase significantly because such ships do not tend to get close to combat zones.
But the National Defense Authorization Bill for 2009, which the Senate has still to pass, aims to shift the process up a gear by adding various types of amphibious assault ships to the list of those that must be powered by nuclear reactors in the future. "We're dedicated to ensuring that the future forces of the navy are not hampered by access to, or the cost of, fossil fuels," says Representative Gene Taylor, the Mississippi Democrat who chairs the House subcommittee on sea power and expeditionary forces, which drafted the relevant section of the bill.
Amphibious ships come in various forms, from those that incorporate a dock for landing craft, to undersized aircraft carriers for helicopters and vertical take-off aircraft - or a mixture of both. The vessels' position in combat can also vary - from a "stand-off" over-the-horizon location to being moored to a pier in a combat zone.
All amphibious vessels have one thing in common, says Lisa Wright, a spokesperson for Bartlett: "They are combat vessels carrying marines into combat."
Equipping such ships with nuclear reactors would have another advantage in military operations, says Wright. "Assault ships are carrier escort vehicles and will no longer be holding up a carrier task force's progress by having to be refuelled every three to five days," she says.
Not everyone agrees with Bartlett's claim that nuclear-powered ships offer "unparalleled safety". The risk with building nuclear-powered assault ships is that nuclear fuel could fall into the wrong hands, says Igor Kudrik, a researcher at the Norwegian environmental pressure group, the Bellona Foundation, which monitors the ageing Russian Arctic fleet of nuclear submarines and their disposal.
"The small, very high-power pressurised water reactors contain highly enriched uranium - far higher than that at commercial nuclear power plants," he says. While civil reactor fuel is enriched with uranium-235 to around 5 per cent, military reactor fuel can reach 90 per cent enrichment levels to ensure ships have enough power to stay at sea for long periods. This could make reactor maintenance sites at US bases in ports around the world a tempting target for any thief intent on making weapons-grade fuel for a bomb.
Kudrik also thinks the US is being optimistic about how much nuclear reactors will reduce the need for ships to moor in potentially unfriendly ports. The very high-pressure water coolant in such reactors often leaks, meaning the reactor has to be shut down while the craft pulls into a nearby port for repairs.
What's more, the nuclear assault craft would present risks to the environment in combat situations, says Bellona nuclear physicist Nils Bøhmer, formerly with the Norwegian Radiation Protection Authority.
"Any fires in combat will easily release radioactivity into the environment. And from a terrorist's point of view a nuclear vessel will be a much more interesting target than a conventional ship. They will be able to release radioactivity and so create more panic and chaos," says B¿hmer.
He is worried that if the Senate agrees with the House in the next few weeks over amphibious vehicles it will start a trend. "If this bill goes through there will be pressure for still smaller navy ships to begin carrying nuclear power," he says.
For his part, Taylor touts the environmental credentials of naval nuclear power. "Nuclear systems emit no greenhouse gas emissions and are therefore the most environmentally friendly source of power available," he says.
But Ayliffe is not convinced. "They don't want a rogue state to get hold of fissile material, yet they equip assault ships with that very material. It's completely bonkers."