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DWIGHT EISENHOWER observed in his “Atoms for Peace” speech in 1953 that nuclear technology originally developed for military purposes could also be put to peaceful uses, namely generating electricity. His speech led to the dissemination of nuclear technology for civilian purposes and the establishment of the first nuclear power stations. Many of these early reactors, built during the cold war, made a virtue of the “dual use” nature of nuclear technology. Designs were favoured that could create weapons-grade material as well as electricity.

Today those priorities have been reversed. America and Russia are taking steps to reduce their stockpiles of nuclear weapons, and the international community is trying to prevent their acquisition by new states. Under America's “Megatons to Megawatts” programme, weapons-grade material from retired warheads is being broken down to provide fuel for civilian nuclear power stations. With 53 new reactors under construction around the world and dozens more planned, the main difficulties facing nuclear scientists now are to reduce the threat of proliferation, improve efficiency and do something about the growing stock of nuclear waste in indefinite temporary storage.

These new priorities favour new sorts of reactor. Taking the lead in the development of the next generation of reactors is an international programme called the Generation IV International Forum (GIF), a collaboration between the governments of America, Argentina, Brazil, Britain, Canada, China, France, Japan, Russia, South Africa, South Korea and Switzerland, plus Euratom, the EU's nuclear body. Established in 2001, the GIF has drawn up a shortlist of six of the most promising designs, which range from updated versions of existing reactors to radically different approaches.

All nuclear reactors rely on nuclear fission, a process discovered in the 1930s. When certain heavy atoms are struck by a neutron, they absorb it, become unstable and split apart. This results in two lighter atoms, and two or three neutrons are ejected. The process releases large amounts of energy, much of it in the form of the kinetic energy of the fast-moving fission products. This energy is converted to heat as the fission products slow down. If the ejected neutrons hit other atoms nearby, those too can break apart, releasing further neutrons in a chain reaction. When enough neutrons produce further fissions—rather than escaping, bouncing off or being absorbed by atoms that do not split—the process becomes self-sustaining.

The technology underpinning civilian nuclear power-generation has not progressed much since the 1950s when a small number of prototype commercial reactors were first brought online. Based on the military reactors developed for weapons programmes and naval propulsion, these “generation I” systems pioneered the pressurised water reactor (PWR) design, which is the basis for most of the “generation II” nuclear reactors now in operation. In a PWR ordinary water, kept at a high pressure to prevent it from boiling, is used both to cool the reactor core and to “moderate” the nuclear reaction by reducing the speed of the neutrons in order to maximise their ability to cause further fissions. According to the International Atomic Energy Agency (IAEA), of the 436 nuclear reactors in operation today, 356 are either PWRs or boiling-water reactors—a simplified version of the same design.

The vast majority of current reactors use a “once through” fuel cycle, in which each batch of fuel spends a single term in the reactor core, and the leftovers are then removed and placed in storage. This spent fuel presents a storage problem, but it also offers an opportunity. According to the World Nuclear Association, an industry body, the spent fuel recovered from a reactor still contains around 96% of the original uranium, as well as plutonium that has been formed in the core. If the nuclear renaissance takes off at the rate that many are predicting, this inefficient use of the uranium fuel is likely to prove unsustainable, says Bill Stacey, a professor of nuclear engineering at the Georgia Institute of Technology.

The original series

In the near term most new reactors will continue to be PWRs. A forthcoming crop of “generation III” and “generation III+” reactors build on the light-water design with new safety mechanisms. Some can also run on mixed oxide (MOx) fuel, which is produced by reprocessing spent fuel to extract the plutonium and uranium and combining them to make a new fuel. But although MOx is currently used in around one-third of French reactors, the idea of reprocessing is controversial and has yet to gain widespread international support. Critics say it is uneconomic and increases the risk of proliferation.

The technology underpinning civilian nuclear power-generation has not progressed much since the 1950s

The six most promising “generation IV” designs identified by the GIF from an original list of over 100 concepts depart markedly from the light-water moderated, once-through models that dominate the existing fleet. Even those reactors that draw upon aspects of current designs add some new twists.

Start with the supercritical water-cooled reactor (SCWR). Although it uses water as the coolant, like existing designs, the water is at a much higher temperature (above 374{degree}C) and pressure. Under these conditions the water exists in a single, supercritical phase, rather than as liquid or steam. This eliminates the need to transfer heat from the coolant water to steam (via a secondary heat-exchanger) to drive a steam turbine, as is the case with current PWRs. Instead, supercritical water from the core drives a turbine directly.

Doing away with the need for separate pumps, pressurisers and steam generators results in higher thermal efficiency: 45% rather than the 33% of existing PWRs, according to Idaho National Laboratory. The simplicity of the design should also make it cheaper. The GIF estimates that an SCWR could be built at a cost of $900 per kilowatt of generating capacity—about a quarter of the expected cost of current generation III+ reactors. Some industry observers, however, are sceptical that these cost savings can be achieved.

Given that it builds on existing reactor designs, and also borrows from supercritical fossil-fuel boilers, which are also an established technology, the SCWR is likely to be one of the first generation-IV designs to be implemented. The GIF is aiming to have a demonstration version ready by 2022. But several technical challenges remain. In particular, says William Cook of the University of New Brunswick in Canada, “current reactor materials that do not crack corrode excessively, while materials that do not corrode excessively crack.” New alloys will be needed that do not crack or corrode under stress.

The second design with roots in existing technology is the Very High Temperature Reactor (VHTR). It has a once-through uranium cycle, but instead of water it uses graphite as the moderator and helium gas as the coolant. (Helium has the advantage that it is chemically inert and has only a limited tendency to become radioactive when exposed to neutrons.) As its name suggests, the VHTR is designed to run at very high temperatures, heating the coolant to around 950{degree}C, compared with 315{degree}C for a standard PWR, making it more thermally efficient.

Like the SCWR, the VHTR will require the development of new materials. Although the helium coolant presents fewer corrosion problems than supercritical water, creating core materials and fuel casings that can withstand the high temperatures involved is a daunting task. Nevertheless, the VHTR has sufficiently impressed the Obama administration, which in September announced $40m in funding for research and development of the Next Generation Nuclear Plant, a reactor based on the VHTR design.

Unlike the SCWR and the VHTR, which build on current reactors, the other four generation-IV designs take a completely different approach to the nuclear-fuel cycle. Three of them are “fast neutron” reactors, which do not include a moderator to slow down free neutrons during the fission process. With more free neutrons flying about, fast reactors can consume or “burn up” existing nuclear waste, a characteristic that endears them to waste-reduction advocates who see them as a means of “closing” the nuclear fuel cycle.

In keeping with the Janus-faced nature of nuclear technology, however, fast reactors can also be used to produce or “breed” new fissile material—converting uranium-238 into the notoriously dual-purpose plutonium, for example. Opponents of fast reactors worry about the costs and proliferation risks. But the prospect of being able to extract useful energy from nuclear waste, and also reduce its volume and toxicity, give fast reactors obvious appeal. The three shortlisted fast-reactor concepts—sodium-cooled, gas-cooled and lead-cooled—are differentiated primarily by their use of coolant. Each has its own pros and cons.

The most successful of the three designs to date has been the sodium-cooled fast reactor (SFR), which has racked up the highest number of reactor-years of operation in prototype form. One of the merits of the SFR is that “we really can build one,” says Robert Hill of America's Argonne National Laboratories. He points to the Russian BN600, a reactor that has been running since the 1980s. Sodium is favoured as a coolant because of its good heat-transfer properties, its ability to operate at lower pressures than other coolants and its relative “transparency” to fast neutrons, which means it does not interfere in the fission process, says Dr Hill. According to the IAEA, Russia, South Korea and India are all currently operating versions of the SFR, and China is due to bring a prototype online in mid-2010.

The gas-cooled fast reactor (GFR), in contrast, has yet to be demonstrated on a commercial scale. But many see it as a better bet than the SFR due to its technical similarity to generation III gas-cooled designs. Like the VHTR, the GFR uses pressurised helium both to cool the reactor core and drive a turbine, yielding higher thermal efficiency than systems with a secondary heat-transfer loop. As with a VHTR, the other advantages of a gas coolant, says Tom Wei, a senior engineer at Argonne, include its non-corrosive characteristics and its capacity for use at high temperatures (the GFR would operate at around 850{degree}C). But, like the VHTR, the GFR will require new materials to enable its cladding and fuel assemblies to withstand such high temperatures.

The third fast-reactor concept uses molten lead as the coolant, an approach historically favoured by the Soviet military, which used early lead-bismuth cooled fast reactors to power its submarines. Since the late 1990s there has been renewed interest in the lead-cooled fast reactor (LFR), particularly in Europe. A distinctive advantage of the LFR concept is its potential to be adapted to smaller “battery” designs, which can be manufactured as self-contained systems with a “lifetime core”. Such reactors could provide a way to extend civilian nuclear power to new countries without giving them access to the sensitive parts of the nuclear-fuel cycle.

Although a commercial fleet of fast reactors would be attractive from a waste-management perspective, it presents its own set of proliferation-related problems. According to Charles Ferguson, a nuclear expert at the Council on Foreign Relations, a think-tank, the commercial adoption of fast reactors would require “near real-time monitoring capabilities” via secure video links to ensure that the reactors were not being used to make weapons. Getting countries to agree to such intrusive measures, he says, would be very difficult.

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In the belly of the beast

The sixth shortlisted design, the molten salt reactor (MSR), works by dissolving nuclear fuel in a fluoride solution, which acts as both the fuel and the coolant in the reactor core. The molten salt, which has good heat-transfer properties and can be heated to temperatures above 1,000{degree}C without boiling, is moderated using graphite. The circulation of the fuel in this way eliminates the need for fuel fabrication and allows for continuous online reprocessing. It also makes the design well suited to the use of existing fissile material, which can be easily blended into the fuel mixture. And like fast reactors, the MSR can be designed to burn up many of the longer-lived byproducts of the fission process, resulting in nuclear waste that is much less radioactive than that produced by the once-through cycle.

One form of MSR, the liquid fluoride thorium reactor (LFTR), has garnered particular enthusiasm among those who regard thorium as an attractive replacement for uranium and plutonium in the fuel cycle. (Thorium is both cheaper and more abundant than uranium.) According to Kirk Sorensen, an engineer at NASA who also runs a blog on the merits of the thorium cycle, natural thorium provides at least 250 times more energy per unit than natural uranium. However, unlike fissile uranium, natural thorium must be “seeded” with external neutrons in order to get it to fission. Another obstacle for the MSR is finding materials capable of withstanding hot, corrosive, radioactive salt.

Flicking the switch

Which of these designs will prevail in the coming decades? After all, not all the generation-IV reactor concepts are likely to make it to commercialisation. Ideally, the strongest approaches will win out through “natural selection”, says Thierry Dujardin at the OECD's Nuclear Energy Agency (NEA) in Paris. But with each of the designs closely connected to different national research programmes—and international variations within each of the categories—governments are unsurprisingly reluctant to see their particular projects sidelined.

Harold McFarlane at the Idaho National Laboratory reckons the VHTR and SFR are almost ready to move out of the research phase and into the design stage. Others share this view: the British government has identified the VHTR, GFR, and SFR as high-priority designs, and Japan, France and America agreed last year to work together on SFR prototypes.

Dr Ferguson thinks the prospects of the entire generation-IV programme are contingent on the level of investment allocated to nearer-term projects. “Do we commit to generation III or do we leapfrog to generation IV?” he asks. Two important considerations for answering his question are regulatory compliance and economic viability. With regard to the former, the NEA's Multinational Design Evaluation Programme is considering an international licensing scheme to standardise safety requirements for the new reactors. As for the latter, the success of generation IV reactors is likely to hinge on large amounts of government support.

In the near term this support should take the form of increased research-and-development funding, says Dr Stacey of Georgia Tech. In the longer term, governments have an important role to play in the provision of loan guarantees, which are vital for overcoming engineering and “first of a kind” risks, says Joe Turnage at Unistar, a commercial nuclear joint-venture between Constellation Energy, an American utility, and EDF, a French one. But whatever the next generation of nuclear power-stations looks like, it is clear that the research being done around the world to develop such a variety of new reactors, rather than new nuclear weapons, has fulfilled Eisenhower's wish, back in 1953, that “the miraculous inventiveness of man shall not be dedicated to his death, but consecrated to his life.”