DOES the world need more nuclear power or less? Seared by the disaster at the Fukushima Dai-ichi nuclear plant in March 2011, Japan has now taken all its commercial reactors offline. The last was powered down on September 16th. Tokyo Electric Power, owner of the ill-fated reactors on the Fukushima coast, still hopes to restart an idled nuclear plant in Niigata prefecture next July—if it can overcome entrenched local opposition. Meanwhile, measures are underway in Germany and Switzerland to phase out their nuclear stations. Another 11 European countries, plus Australia and New Zealand, remain adamantly opposed to nuclear power. In recent years, more reactors around the world have closed than opened. Yet nuclear reactors do one thing no other mainstream source of electricity can boast: they generate large blocks of power without producing carbon dioxide in the process. Hydro-electricity is largely carbon-free, but most suitable sites have long since been exploited. Certainly, renewables like solar, wind and biomass can deliver power largely free of greenhouse gases. But renewables are nowhere near reliable nor cheap enough to displace conventional fuels—be they coal, natural gas, oil or nuclear. Nor can they be scaled up fast enough to meet the world’s insatiable demand for electricity. Overall, opposition to nuclear power—despite the graphic footage of the nuclear disaster in Fukushima—seems to be on the wane. Last year, The Economist held an online debate on whether the world would be better off without nuclear power. Readers voted 61% to 39% in favour of keeping it (see “Debate on nuclear power”, April 15th 2012).

All told, 40-odd countries—mainly in the Middle East and Asia—have now committed themselves to building their first atomic-power plants, or to adding new ones to their existing nuclear capacity. As the poster-child for pollution, China is keenly aware that it cannot go on building dirty coal-fired power stations indefinitely and needs a cleaner alternative. Hence the 32 new reactors China has under construction, which will add 70 gigawatts of nuclear capacity by 2020. Russia is building ten new ones and India seven. Britain is about to start work on its first nuclear reactor since 1995. With eight of its nine nuclear plants now reaching the end of their lives, Britain plans to build a dozen new ones by 2030.

Today, the nuclear industry’s prospects look brighter than at any time since 1979. That was when a partial meltdown at the Three Mile Island plant in Pennsylvania sent shockwaves around the world—and further orders for nuclear generating capacity began to dry up. The latest reactor designs are far safer. The AP1000, an “advanced passive” reactor from Toshiba’s Westinghouse division, has a huge reservoir of water above the reactor, which is dumped by gravity into the core in an emergency. Westinghouse claims the AP1000 is 100 times safer than present reactors. The broadly similar European Pressurised Reactor, designed by Areva, a French firm, has four redundant safety systems instead of the more usual two or three.

But to Babbage’s mind, the question is not whether their “passive” designs—ie, emergency cooling systems that work by gravity and natural convection instead of electrical valves, relays and pumps—can make them safer. Of that there is no doubt. The question, rather, is why such an inherently flawed design as the light-water reactor (LWR) is still, after all these years, the preferred technology?

Most of today’s reactors, whether they use boiling water or pressurised water, trace their ancestry back to the USS Nautilus, the world’s first nuclear submarine, launched in 1954. At the time, the LWR was just one of many reactor designs that existed either on paper or in the laboratory—using different fuels (uranium-233, uranium-235 or plutonium-239), different coolants (water, heavy water, carbon dioxide or liquid sodium) and different moderators (water, heavy water, beryllium or graphite).

The light-water reactor of the day, with its solid uranium-dioxide fuel and water for both moderator and coolant, was by no means the best. But Admiral Hyman Rickover, the father of America’s nuclear navy, chose it because it could be implemented faster than any of the others, making it possible for Nautilus to be launched on time. The LWR also appealed to Rickover because it produced a lot of bomb-making plutonium as a by-product.

After that, the die was cast. America’s first commercial reactor, the 60-megawatt Shippingport station in Pennsylvania, which started in 1957 (one year after the Calder Hall power station in Britain), used essentially the same light-water design as Nautilus. Henceforth, the rest of America’s commercial reactors would follow suit. Other countries subsequently copied or licensed much the same light-water technology.

In hindsight, that was a terrible mistake. Producing copious quantities of plutonium is just about the last thing a commercial reactor needs to do. It creates huge handling and storage problems as well as all manner of security and proliferation headaches. On top of that, the LWR’s other drawbacks ensured that commercial reactors would henceforth be more expensive to build and costlier to operate than might otherwise have been the case.

For instance, the cooling water in an LWR is not only radioactive and corrosive, but also under extremely high pressure. As a consequence, light-water reactors need to be housed in fortress-like containment buildings in case the cooling system fails and radioactive steam is released into the atmosphere.

Another problem concerns the bundles of rods that contain the uranium-dioxide fuel. These have to be removed from the core after only a few years of burning and stored in cooling ponds, even though no more than 3-5% of the energy in their uranium has been consumed. Their zirconium cladding swells and distorts as a result of temperature differences and radiation damage. There is always the danger of fuel rods rupturing if left in the reactor too long.

Within the fuel rods themselves, the fissile material becomes steadily poisoned by short-lived byproducts, such as xenon-135. This causes dangerous instabilities that make managing the reactor tricky. Such instabilities are what caused the Chernobyl reactor to explode. As if all that were not enough, dealing with a light-water reactor’s long-lived radioactive byproducts remains a Faustian nightmare.

Passive safety features aside, the new generation of reactors being hawked around the world are still basically old-fashioned light-water reactors with solid-fuel cores that are cooled and moderated by water. “Maddeningly,” say two leading light-water critics, “historical, technological and regulatory reasons conspire to make it hugely difficult to diverge from our current path of solid-fuel, uranium-based plants.”

In what has become a classic account of America’s missed opportunity to make nuclear power cleaner, safer and potentially an alternative to coal, Robert Hargraves of Dartmouth College and Ralph Moir, formerly of Lawrence Livermore National Laboratory, have made the most compelling case yet (in the July-August 2010 issue of American Scientist) for reactors that use a liquid fuel instead of a solid one. “Knowing what we now know about climate change, peak oil, Three Mile Island, Chernobyl, and the Deepwater Horizon oil well gushing in the Gulf of Mexico in the summer of 2010, what if we could have taken a different path?” asked the authors.

One advantage of liquid fuels is that they are not subjected to the radiation damage or structural stresses that cause the fuel rods in conventional reactors to swell and distort. Also, because they use a liquid fluoride salt for a coolant, there is no high-pressure water to deal with. Operating at atmospheric pressure, no containment vessel is therefore needed. The xenon gas that poisons the fuel rods in a conventional reactor simply bubbles out of a liquid fuel, while other fission products precipitate out and cease absorbing neutrons from the chain-reaction underway.

The spent fuel from a light-water reactor contains radioactive plutonium with a half-life of over 24,000 years. The fuel used in a liquid-fuel reactor is liquid fluoride laced with thorium. The toxicity of what little waste it produces is 10,000 times less than that from a conventional reactor. Overall, the half-life of a liquid-fuel reactor’s byproducts is measured in hundreds rather tens of thousands of years.

The liquid-fluoride thorium reactor, developed at Oak Ridge National Laboratory in Tennessee during the late 1960s, ran successfully for five years before being axed by the Nixon administration. The reason for its cancellation: it produced too little plutonium for making nuclear weapons. Today, that would be seen as a distinct advantage. Without the Cold War, the thorium reactor might well have been the power plant of choice for utilities everywhere.

Today, the thorium reactor is a non-starter, at least in America and other countries that have invested heavily in light-water technology. But things are different in India, a country with no uranium but an abundance of thorium. India plans to produce 30% of its electricity from thorium reactors by 2050. Being plentiful and cheap, thorium is the only fuel that stands a chance of generating electricity as cheaply as burning coal. As such, it is the only fuel capable of weaning the world off the biggest single polluter of all.