AFTER the events of March 11th 2011, when an earthquake and tsunami led to a meltdown of three nuclear reactors at the Fukushima Dai-ichi power plant in Japan, you might be forgiven for concluding that atomic power and seawater don’t mix. Many engineers, though, do not agree. They would like to see more seawater involved, not less. In fact, they have plans to site nuclear power plants in the ocean rather than on land—either floating on the surface or moored beneath it.

At first, this sounds a mad idea. It is not. Land-based power stations are bespoke structures, built by the techniques of civil engineering, in which each is slightly different and teams of specialists come and go according to the phase of the project. Marine stations, by contrast, could be mass-produced in factories using, if not the techniques of the assembly line, then at least those of the shipyard, with crews constantly employed.

That would make power stations at sea cheaper than those on land. Jacopo Buongiorno, a nuclear engineer at the Massachusetts Institute of Technology, reckons that, when all is done and dusted, electricity from a marine station would cost at least a third less than that from a terrestrial equivalent. It would also make them safer. A reactor anchored on the seabed would never lack emergency cooling, the problem that caused the Fukushima meltdown. Nor would it need to be protected against the risk of terrorists flying an aircraft into it. It would be tsunami-proof, too. Though tsunamis become great and destructive waves when they arrive in shallow water, in the open ocean they are mere ripples. Indeed, were it deep enough (100 metres or so), such a submarine reactor would not even be affected by passing storms.

Water power

All these reasons, observes Jacques Chénais, an engineer at France’s atomic-energy commission, CEA, make underwater nuclear power stations an idea worth investigating. Dr Chénais is head of small reactors at CEA, and has had experience with one well-established type of underwater reactor—that which powers submarines. He and his team are now assisting Naval Group, a French military contractor, to design reactors that will stay put instead of moving around on a boat. The plan is to encase a reactor and an electricity-generating steam turbine in a steel cylinder the length of a football pitch and with a weight of around 12,000 tonnes.

The whole system, dubbed Flexblue, would be anchored to the seabed between five and 15km from the coast—far enough for safety in case of an emergency, but near enough to be serviced easily. The electricity generated (up to 250 megawatts, enough for 1m people) would be transmitted ashore by an undersea cable. For refuelling and maintenance unmanageable from a submarine, the cylinder would be floated to the surface with air injected into its ballast tanks. And, when a station came to the end of its useful life, it could be towed to a specialist facility to be dismantled safely, rather than requiring yet another lot of civil engineers to demolish it.

Naval Group has not, as yet, attracted any customers for its designs. But a slightly less ambitious approach to marine reactors—anchoring them on the surface rather than below it—is about to come to fruition in Russia. The first such, Akademik Lomonosov, is under construction at the Baltic Shipyard, in St Petersburg (see picture). According to Andrey Bukhovtsev of Rosatom, the agency that runs Russia’s civil nuclear programme, it is 96% complete. It will be launched later this year, towed to Murmansk, and thence transported to Pevek, a port in Russia’s Far East, where it will begin generating power in 2019.

Akademik Lomonosov consists of two 35MW reactors mounted on a barge. The reactors are modified versions of those used to power Taymyr-class icebreakers. As such, they are designed to be able to take quite a battering, so the storms of the Arctic Ocean should not trouble them. To add to their safety, the barge bearing them will be moored, about 200 metres from shore, behind a storm-and-tsunami-resistant breakwater.

Altogether, Akademik Lomonosov will cost $480m to build and install—far less than would have to be spent constructing an equivalent power station on land in such a remote and hostile environment. And, on the presumption that the whole thing will work, plans for a second, similar plant are being laid.

Nor is Russia alone in planning floating reactors. China has similar ambitions—though the destinations of the devices concerned are more controversial than those of Russia’s. Specifically, the Chinese government intends, during the 2020s, to build up to 20 floating nuclear plants, with reactors as powerful as 200MW, to supply artificial islands it is building as part of its plan to enforce the country’s claim to much of the South China Sea—a claim disputed by every other country in the area.

The firms involved in this project intend to tsunami-proof some of their reactors in the same way as the French, by stationing them in water too deep for massive tsunami waves to form. Because they are at the surface, though, that will not save them from storms—and locating them far from shore means the Russian approach of building sheltering breakwaters will not work either. That matters. Typhoons in the South China Sea can whip up waves with an amplitude exceeding 20 metres.

To withstand such storms, the barges will have anchors that are attached to swivelling “mooring turrets” under their bows. These will cause a barge to behave like a weather vane, always pointing into the wind. Since that is the direction waves come from, it will remain bow-on to those waves, giving it the best chance of riding out any storm that nature cares to throw at it. The barges’ bows will also be built high, in order to cut through waves. This way, claims Mark Tipping of Lloyd’s Register, a British firm that is advising on the plants’ design, they will be able to survive a “10,000-year storm”.

The South China Sea is also a busy area for shipping, so any floating power stations there will need to be able to withstand a direct hit by a heavy-laden cargo vessel travelling at a speed of, say, 20 knots—whether that collision be accidental or the result of hostile action. One way to do this, says Chen Haibo, a naval architect working on the problem at Lloyd’s Register’s Beijing office, is to fit the barges with crumple zones packed with materials such as corrugated steel and wood.

Not everyone is delighted with the idea of marine nuclear power. Rashid Alimov, head of energy projects at Greenpeace Russia, an environmental charity, argues that offshore plants could be boarded by pirates or terrorists, be struck by an iceberg or might evade safety rules that are hard to enforce at sea. On July 21st Greenpeace scored a victory when Rosatom said that Akademik Lomonosov’s nuclear fuel would be loaded in an unpopulated area away from St Petersburg.

That, though, is a pinprick. The future of marine nuclear power stations is more likely to depend on the future of nuclear power itself than on the actions of pressure groups such as Greenpeace. If, as many who worry about the climate-changing potential of fossil-fuel power stations think, uranium has an important part to play in generating electricity over coming decades, then many new nuclear plants will be needed. And if that does turn out to be the case, siting such plants out at sea may well prove a good idea.