For Decades, One Design Has Dominated Nuclear Reactors While Potentially Better Options Were Left by the Wayside

BY M. MITCHELL WALDROP

RADICAL REACTORS For decades, one design has dominated nuclear reactors while potentially better options were left by the wayside. Now, the alternatives might finally have their day.

ack in 2000, when Kirk Sorensen was a NASA engineer looking at nuclear- power options for future colonies on the Moon, he came across a book that described the molten-salt reactor: an energy source in which the nuclear fuel was liquid. BIt sounded bizarre, says Sorensen. Every reactor he had ever heard of used some form of solid uranium fuel — starting with the ‘light-water’

© 2012 Macmillan Publishers Limited. All rights reserved FEATURE NEWS reactors that currently dominate the nuclear- expensive. It’s slow. And the stakes are very reactor fuel. Governments around the world power industry. But the book explained that the high, because safety has to be a factor.” suddenly had to face the geopolitical realities: molten-salt technology had been demonstrated But those involved share a conviction that large-scale commercial reprocessing would some three decades earlier at the Oak Ridge the best hope for the nuclear industry’s future invite rampant nuclear-weapons prolifera- National Laboratory in Tennessee — and that is to reclaim its past. As Sorensen points out tion. Because each reprocessing plant would the fluid uranium- or thorium-containing fuel about the cancellation of the molten-salt pro- be working with bomb-grade plutonium by offered major advantages. Molten-salt reactors gramme: “Nobody ever said, ‘Maybe we made the tonne, how could inspectors ever be sure would be impervious to catastrophic meltdown, a mistake. Maybe we should go back and revisit that no one had diverted the 4–6 kilograms for example, and instead of producing nuclear that decision’.” required for a weapon? waste laced with plutonium and other long- So in April 1977, US President Jimmy Carter lived radioisotopes, they could destroy those FIRST, NOT BEST banned commercial reprocessing. President isotopes almost completely. Light-water reactors achieved their dominance Ronald Reagan lifted that ban a few years later, The list of advantages went on and on, says not because they were best, but because they but the costs of the facilities were so high that Sorensen: the molten-salt idea “had the poten- were first. Originally developed in the late only two commercial reprocessing plants have tial to solve almost all the problems of nuclear 1940s as a compact power source for nuclear been opened for reactor fuel since then, both energy in a far, far more elegant way” than ships and submarines, the light-water design in France. Research on breeder reactors largely light-water reactors. “So why didn’t we do it was adapted and scaled up during the 1950s, ceased, because they seemed to make little this way in the first place?” when the United States sought to put a peaceful sense without reprocessing. And engineers A lot of people have been asking that face on atomic energy by creating a commer- found themselves left with a complicated dis- question in the past decade — and not just cial nuclear-power industry. ‘Light water’ is posal problem: they would now have to isolate about the molten-salt reactor. That particular ordinary H2O, which flows through the reactor tens of thousands of tonnes of spent fuel for technology was abandoned in 1976 because of core, absorbs its heat and circulates it to a con- hundreds of centuries, thanks to the 24,100- warring agendas within the US research pro- ventional steam turbine that turns the heat into year half-life of plutonium-239. No one has yet gramme. But it was just one of several alter- electricity (see ‘The nuts and bolts of nuclear’). worked out how to guarantee isolation on that native technologies to be sidelined during the Eventually, such reactors were meant to be timescale (see Nature 473, 266–267; 2011). first rush to commercialize nuclear power. part of a larger system that would make up for a Meanwhile, the 1970s also brought an Others include ‘fast’ reactors that would also basic inefficiency: left alone, any nuclear reactor increasing outcry over safety. If the flow of water burn up nuclear waste, and high-tempera- will quickly poison itself. As the chain reaction through a light-water reactor is interrupted ture reactors that could take a huge bite out proceeds, the fuel accumulates more and more for any reason, then heat becomes trapped in of greenhouse-gas emissions by generating of the fragments left over after the uranium the core. Even if the reactor is technically shut zero-carbon heat for industry. Taken together, atoms split, which in turn absorb more and down, the fission products can still produce these alternative technologies could eliminate more of the neutrons required to keep the reac- enough heat from radioactive decay to melt the most or all of nuclear energy’s drawbacks. But tion going. After perhaps 18 months, the fuel is fuel and escape into the environment. All light- they have received only fitful attention from ‘spent’ and has to be removed — even though it water reactors have emergency back-up cooling researchers over the decades, thanks to con- still contains much of its original energy. systems — but what if those systems fail? That stantly shifting agendas and funding levels. “So there was always this vision that there fear was realized in March 1979, when an acci- Now, change may be coming. Over the past would be a recycled-spent-fuel infrastructure dental loss of coolant triggered a partial melt- decade, the need for safe, carbon-free energy that would allow you to recover more of the down at the Three Mile Island nuclear power — especially in fast-developing nations such fuel’s energy,” says William Magwood, a former plant near Harrisburg, Pennsylvania — and as China — has sparked government interest director of the Office of Nuclear Energy at the dramatically confirmed at Fukushima Daiichi, in alternative nuclear technologies, along with US Department of Energy (DOE) and now a which saw a complete meltdown in March 2011 commercial efforts to revive and market some member of the US Nuclear Regulatory Com- (see Nature 483, 138–140; 2012). designs. Optimists think that even the nuclear mission. A worldwide network of reprocess- disaster at the Fukushima Daiichi power plant ing plants would take the spent fuel, chemically A SECOND CHANCE in Japan last year will ultimately boost the mar- extract the still-usable components — mostly The public and political backlash after the ket for safer alternative reactors. From start-ups uranium-235, plus the fissionable pluto- Three Mile Island incident created a worldwide such as Flibe Energy, which Sorensen founded nium-239 formed when neutrons are captured ‘nuclear brown-out’ that lasted for a quarter of last year in Huntsville, Alabama, to commer- by non-fissile uranium-238 — and then turn a century. Power companies scrapped their cialize the molten-salt reactor, to industry them into fresh reactor fuel. Ultimately, the nuclear expansion plans and cancelled almost giants such as General Electric-Hitachi Nuclear plan was to transition to a new generation of all of their reactor orders. And the industry Energy, which is developing a commercial fast ‘breeder’ reactors designed to maximize pluto- became even more reluctant to explore new reactor, companies hope to be ready. nium production. The only waste would be a technologies. “The industry is risk-averse to Reviving the technologies will not be quick comparatively small residue of intensely radio- moving beyond technology and materials they or easy. Although the basic designs were active fission products that would decay within have lots of experience with” and that they worked out decades ago, engineers hoping to a few centuries, and could be disposed of in, know can get regulatory approval, says Per put them into practice must develop things say, a well-designed concrete bunker. Peterson, a nuclear engineer at the University such as radiation-resistant materials, more- This vision became the dominant US strategy of California, Berkeley. efficient heat exchangers and improved safety in the 1960s and early 1970s, says Magwood, to With little interest from industry and no systems — and must then prove to regula- the point at which authorities terminated much practical hope for deployment, advanced- tors that all these systems will work. “Nuclear of the research funding for non-breeder reac- reactor research struggled with inconsistent is hard,” says Edwin tor designs — including the molten-salt reactor. direction and support. “It’s very hard to do NATURE.COM Lyman, senior global- And the scheme took off: of the 437 nuclear- planning and advanced engineering R&D Hear more about security analyst for the power reactors currently operating around the if you’re up and down, up and down,” says radical reactors on Union of Concerned world, 356 are light-water reactors. Michael Corradini, a nuclear engineer at the Nature’s podcast: Scientists in Cambridge, But then, in May 1974, India tested a nuclear University of Wisconsin-Madison. go.nature.com/jqcno5 Massachusetts. “It’s bomb made with plutonium extracted from This picture didn’t begin to change

until around the turn of the millennium. what the name implies: they generate steam at that collectively make disposal of spent fuel a “Nuclear construction had taken off in China up to 1,000 °C, much hotter than the roughly nightmare. Fast neutrons, by contrast, rarely and south Asia — any place that doesn’t have 300 °C available from light-water reactors. This get absorbed. They don’t hit their targets often, oil and gas,” recalls Charles Forsberg, a nuclear requires some radically different design choices, but when they do, that target almost always engineer at the Massachusetts Institute of such as the use of helium gas instead of water splits. As a result, fast reactors not only avoid Technology (MIT) in Cambridge. (There to extract heat, and the use of a heat-resistant the problem of producing long-lived isotopes, are currently 64 reactors under construc- fuel made from oxides and carbides of uranium. but can even destroy them in spent fuel. tion around the world, with hundreds more Such reactors cannot melt down: the fuel Building a fast reactor is tricky, says Peterson, planned.) In the United States, he says, “the is stable up to 1,600 °C, hundreds of degrees not least because it has to be cooled by liquid feds realized that, if we’re not doing anything hotter than the core would become even if all sodium or some other substance that won’t slow on nuclear, we won’t be at the table”. Climate power and coolant were lost. The high temper- the neutrons down as water does. This can make change, too, drove renewed interest in nuclear atures would make the reactors more efficient for a bulky design. “And it’s very challenging to technology in the United States and Europe. at producing electricity. And they could slash build heat exchangers” to make steam for the Given the erratic output of both wind and solar carbon emissions by supplying heat for indus- power turbines, he says, because sodium reacts generators, says Forsberg, “if you’re going to trial processes. In the United States, roughly violently with moisture to produce explosive get off fossil fuel, you have to have a serious 23% of all energy is used in industrial applica- hydrogen gas. Researchers are actively study- nuclear programme”. tions such as petroleum cracking and plastics ing other, less reactive options for cooling, such manufacture, many of which need tempera- as lead and supercritical carbon dioxide, he says. RADICAL INVESTMENT tures of at least 700 °C. Currently, those tem- Nevertheless, some 20 fast reactors have been One result of this renewed focus was the US peratures tend to be generated by burning operated over the years — many of them fol- Nuclear Power 2010 programme. Announced natural gas; high-temperature reactors could lowing the 1970s breeder design that was built by the DOE in February 2002, this govern- provide a zero-carbon alternative. to maximize plutonium production instead of ment–industry cost-sharing plan was designed A number of commercial high-tempera- consuming it — and at least four manufactur- to help manufacturers to develop and license ture reactors are under development around ers are developing small fast reactors for spent- light-water reactors with advanced safety fea- the world. But this year, a consortium of fuel consumption. A leading example is the tures, such as the ability to keep the coolant petrochemical companies and reactor man- Super Power Reactor Innovative Small Mod- moving during an accident, using gravity and ufacturers agreed to back the Antares high- ule (S-PRISM) from General Electric-Hitachi natural convection. Several such reactors are temperature reactor design from the French in Wilmington, North Carolina. It calls for a now being planned around the world, includ- company AREVA, based in Paris. “All that’s compact sodium-cooled fast reactor, integrated ing four under construction in the United left is about $800 million of work design and with a recycling unit that would take the reac- States — the first new reactors there in a gen- licensing effort required to get the technol- tor’s spent fuel, remove the fission products that eration. ogy to the point where the Nuclear Regula- poison the nuclear reaction, and put the reju- Even more radical designs might find an tory Commission could approve it,” says Fred venated fuel back into the reactor. At no point opening with the DOE’s cost-sharing pro- Moore, head of the division that provides would it isolate bomb-ready plutonium. gramme for Small Modular Reactor develop- power and steam for the Dow Chemical Com- The potential market is substantial, says Eric ment, launched this year. That scheme’s goal pany, headquartered in Midland, Michigan. Loewen, head of advanced-reactor develop- is to move away from the current multi-giga- He estimates that this should take 5–7 years. ment for General Electric-Hitachi. “We have a watt nuclear plants, which can cost between If all goes to plan, high-temperature systems usability study going on with the United King- US$10 billion and $15 billion to build, towards will be among the first advanced reactors to dom, where we would take the 100 tonnes of plants of 250 megawatts or less — small enough be deployed, starting in the 2020s. plutonium from their reprocessing plants and to mass-produce in a factory and ship to the Not far behind would be fast reactors, which turn it into an energy resource,” he says. And in intended site. Four reactor vendors, all with tackle a problem that high-temperature reac- the United States and elsewhere, he says, “our advanced light-water designs, competed for tors cannot: spent nuclear fuel. Fast reactors vision is a network of advanced recycling cen- tres”, each with six S-PRISM reactors and one recycling centre that could keep up with the waste from between “IF YOU’RE GOING TO GET OFF FOSSIL FUEL, YOU one and three light-water reactors, and get rid of the backlog currently sitting HAVE TO HAVE A SERIOUS NUCLEAR PROGRAMME.” in storage. That network will not be cheap. But the fundamental challenge is the award, which on 20 November went to a could consume the stuff, turning waste into political, says Loewen, echoing Forsberg and consortium headed by the Babcock and Wilcox energy and easing the disposal problem. many other experts: what is needed is “a policy Company of Charlotte, North Carolina. Fission neutrons are ‘fast’ when they have framework that lets people see spent fuel as But other designs could also benefit, says just emerged from newly split nuclei at a mean an asset, rather than something to be thrown Peterson. “If we can generate a market for energy of roughly 2 million electronvolts. In away.” light-water small modular reactors,” he light-water reactors, collisions with the hydro- says, “that makes it much easier to develop gen nuclei in the coolant water quickly slow MOLTEN-SALT REACTORS a market for prototype advanced reactors.” the neutrons to just a fraction of an electron The great virtue of solid reactor fuel is its Power companies could experiment with the volt, which makes them more likely to trigger predictable geometry. The great drawback is new technology by simply sliding in another another fission reaction. But slow neutrons its complexity. The intensity of neutron bom- module. If it works, great, says Peterson. If it have a drawback: instead of splitting the tar- bardment, the distribution of fission products, doesn’t, not much has been lost. “This lowers get uranium nuclei, they often get absorbed, the radiation damage to the fuel’s crystalline their whole risk threshold,” he says. transforming the nuclei into long-lived iso- structure: everything varies from point to Prime candidates for slide-in modules are topes of plutonium, neptunium, americium, point. This is a constant headache for design- high-temperature reactors, which do exactly curium or other heavy elements — the ones ers trying to ensure that the reactor operation

THE NUTS AND BOLTS OF NUCLEA ADVANCED EACTOS All nuclear reactors rely on ssion to make heat, which can be used for generating electricity. But These decades-old reactor designs engineers have come up with many alternatives to the commonly used light-water reactor. have been revived and modernized in recent years.

Steam generator and turbine turn heat into electricity. HIGH-TEMPEATUE EACTOS Deliver heat at up to 950 °C for CONTAINMENT STUCTUE industry, cutting carbon emissions. UANIUM OXIDE/CABIDE MIX COATED WITH SILICON CABIDE; STEAM STEAM IN STABLE TO ,600 ˚C GENEATO HELIUM GAS O FLIBE CONVENTIONAL (MIXTUE OF LITHIUM FLUOIDE STEAM TUBINE AND BEYLLIUM FLUOIDE) GAPHITE FUEL WATE OUT FAST EACTOS Use unmoderated ‘fast’ neutrons to burn up heavy elements in spent nuclear fuel. METALLIC O CEAMIC-CLAD UANIUM HELIUM GAS; LIQUID SODIUM METAL; MOLTEN LEAD; O MOLTEN LEAD–BISMUTH MIX Water carries heat from the reactor core and NONE serves as a ‘moderator’, slowing neutrons to help them to trigger further ssion events. MOLTEN-SALT EACTOS LIGHT-WATE NUCLEA EACTOS Use liquid fuel to increase safety and This design, dating from the 1940s–50s, currently dominates the nuclear industry. minimize long-term nuclear wastes. FUEL: UANIUM OXIDE ENCASED IN ZICONIUM METAL UANIUM O THOIUM FLUOIDE COOLANT: ODINAY WATE FLIBE MODEATO: WATE GAPHITE is stable — and trying to convince regulators reactor after a four-decade hiatus is a daunting for the Nuclear Energy Institute trade group in that even the worst meltdown won’t allow any task. “We have to rebuild a knowledge base Washington DC, takes the long view. “We did part of the fuel to collapse into a critical mass. that has largely gone away,” says Sorensen. He the light-water reactors first, to get going,” he But all these concerns go away when the fuel founded Flibe Energy to try, though. The com- says. Next, in the 2020s, will come advanced is already a liquid — one major reason why pany is developing a 40-megawatt reactor that light-water reactors for increased safety, fol- Oak Ridge wanted to develop the molten-salt might be used on military bases so that they lowed closely by high-temperature reactors reactor back in the 1960s. ‘Molten-salt’ refers can operate independently of the grid. that expand the attack on carbon emissions. to the fuel, usually uranium tetrafluoride, “And then we build fast reactors to consume which is liquid at operating temperatures when SOLID CHANCE the waste.” blended with ‘FLiBe’: a mixture of lithium flu- In September 2011, Forsberg, Peterson, MIT's Molten-salt reactors are something of a wild oride and beryllium fluoride that serves as a Lin-wen Hu and Todd Allen, a nuclear engi- card, says Genoa, but are worth developing. coolant. “It’s a pot — a big, dumb, pot,” says neer at the University of Wisconsin-Madison, Some even wilder cards are under investiga- Forsberg. “You throw fuel in, it’s mixed, and the became principal investigators on a 3-year, tion: one notable example is the accelerator- overall composition changes not at all.” DOE-funded project that could be a step on the driven reactor, which would drive fission Liquid fuel has another big advantage, says way to the molten-salt reactor: a FLiBe-cooled reactions using neutrons from a high-energy Sorensen: “You don’t have to remove it from high-temperature reactor. “No one has ever particle accelerator. It could be fuelled with the reactor until it’s completely consumed.” built a salt-cooled solid-fuel reactor,” says Peter- thorium, and shut down instantly by switch- Instead, the fuel is circulated through an exter- son. But if the project works, the reactor core ing off the accelerator. nal recycling unit that extracts the fission prod- could be four to five times smaller than those But will nuclear energy really evolve? Those ucts continuously, keeping the fuel from being in other designs and, because of the stability of in the field see reason for optimism, particu- poisoned. The design also allows for an elegant the FLiBe salt, it would “always be hundreds of larly if the increasingly tangible consequences approach to safety, says Sorensen: at the bottom degrees below the failure limits”, he says. of climate change force governments to put a of the reactor is a hole, plugged with a chunk of Peterson says that the company could have price on carbon. Even the Fukushima disaster fuel that is kept solid by a refrigeration unit. If a test reactor within a decade, although “that could ultimately spur new nuclear technolo- the reactor loses power in an emergency, the assumes abundant resources”. That is a big gies, says Genoa. “It scared people and made refrigeration will cease, the plug will melt and assumption: the global economic crisis has them concerned about nuclear energy,” he says. the fuel will safely drain into underground made financing for all advanced reactors But as people looked more closely, “they said, holding tanks. Finally, the molten-salt design much harder to come by. Furthermore, notes ‘Hey, those were 30-year-old plants’”. In time, can accommodate a variety of fuels, ranging Corradini, pointing to the sudden abundance he says, smart, new reactors will look a whole from conventional uranium to raw nuclear of shale gas in the United States, “cheap fossil lot more appealing. ■ SEE COMMENT P.31 waste or thorium — an element that is roughly fuels have postponed many of the clean-energy three times more abundant than uranium. projects in the United States, not just nuclear”. M. Mitchell Waldrop is a features editor for For all of that, reviving the molten-salt Paul Genoa, director of policy development Nature in Washington DC.