Can fusion answer our energy needs?
For some nuclear fusion is an impossible dream, but there are signs it is turning the corner, writes Rachel Courtland.
In a lab in Cambridge in the early 1930s, two physicists are locked in an ethical battle. They have just achieved nuclear fusion. The younger is excited that it might provide a useful source of power. The elder fears the evil that could be unleashed.
The place is the University of Cambridge’s Cavendish Laboratory, the most famous physics laboratory in the world, awash with Nobel Laureates past and future. Founded by James Clerk Maxwell, the father of electromagnetism, it is where J.J. Thompson discovered the electron and James Chadwick the neutron.
The youngster, an Australian by the name of Mark Oliphant, has been at the laboratory since winning a scholarship in 1927. The elder is New Zealander Ernest Rutherford, the lab’s long-time director who has rocked the world with his investigations of the nucleus at the centre of the atom. Years before, it was a talk Rutherford gave in Oliphant’s home town of Adelaide that “electrified” the youngster and set him on the path to Cambridge.
By the summer of 1933, Oliphant who is something of a technological wizard, is working in a grungy cellar, slamming atoms together using a particle accelerator he has built and entertaining Rutherford, who has a knack for spilling ash and bits of flaming pipe tobacco over Oliphant’s data, on frequent visits.
It is in this setting that the first observation of nuclear fusion is made. Oliphant and Rutherford find that firing atoms of deuterium – a heavy version of hydrogen – at a target containing other deuterium atoms, creates helium. The process also releases energy. It is the same reaction that has powered the Sun for 4.5 billion years.
Oliphant begins to think about what might be done to harness this energy. While Rutherford is away on a trip to South Africa, he devises a new experiment to see if he can boost the fusion rate. On his return, Rutherford is furious. “He was dead scared, you see, that this or some equivalent result would enable nuclear energy to be set free … that some silly fool would blow the world to pieces,” Oliphant later recalled.
Rutherford was not the only scientist to be alarmed. A decade earlier, in 1920, astrophysicist Arthur Eddington had warned the British Association for the Advancement of Science: “If indeed the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling this latent power for the wellbeing of the human race – or for its suicide.”
Within a generation Oliphant had been recruited to develop the atomic bombs that obliterated Hiroshima and Nagasaki. Even more lethal hydrogen bombs were later tested in the South Pacific
Eddington and Rutherford’s fears were well-founded. In the bid to develop an atomic weapon before Nazi Germany, Oliphant was pressed into service of the United States’ Manhattan project, which produced the fission bombs dropped on Hiroshima and Nagasaki. In 1952, less than 20 years after Oliphant’s experiment, the US tested the world’s first fusion-based hydrogen bomb. Arsenals of fusion-powered thermonuclear devices followed, compact enough to be carried on a missile and destructive enough to wipe out an entire city.
Recoiling from the horror of Hiroshima, Oliphant denounced the use of nuclear weapons but never gave up on the dream of the “peaceful atom”.
For three generations of dreamers, this has been nothing less than the holy grail.
Self-sustaining fusion reactions could end our dependence on fossil fuels and drastically cut the carbon dioxide emissions that threaten our climate. And they could do it without the highly toxic radioactive waste we get from fission-based nuclear power plants, waste that may need to be buried for at least 100,000 years. In place of all this: an elegant system that takes in deuterium and tritium – two forms of hydrogen – and spits out low-level radioactive waste, helium and lots of heat. Just as in a conventional power plant, that heat can be used to turn water into steam to drive a turbine. But fusion fuel packs a hefty punch. A few kilograms of deuterium and tritium could create as much energy as thousands of tons of coal. In a fusion-powered world, according to an oft-cited estimate, 45 litres of water would be enough to supply the energy needs of an average European for 30 years.
Like most epic quests, the path to fusion has been strewn with daunting obstacles and dashed hopes. Not long after World War II, fusion power looked like it might be just around the corner. But fusion-based weapons (triggered by fission) proved far easier to create. “The history of abandoned concepts in fusion is enormous,” says David Gates, a principal research physicist at the Princeton Plasma Physics Laboratory in New Jersey.
Part of the problem is the fundamental physics of fusion. Existing nuclear power plants rely on the fission of large atoms like uranium or plutonium that fall apart easily. Creating energy from fusion requires forcing matter to do something it would rather not. It must be heated and squeezed to create conditions that mimic those in the Sun, without the help of the Sun’s enormous gravity.
But there are reasons, physicists say, to think that, after years of fits and false starts and paltry outputs, fusion experiments are now getting quite close to the critical break-even point where the total amount of energy streaming out of a machine matches the amount put in.
Key experiments are ramping up. A number of start-up companies have also joined the game, generating quiet buzz in a field that has historically been almost the exclusive domain of governments and academic institutions. The road ahead for fusion may still be long. But it might not be interminable.
For most of human history the Sun has provided our energy needs. The wood, coal, oil and natural gas we burn are all derived from organisms that transformed solar energy into organic fuel. Even water wheels, dams and windmills wouldn’t run without the Sun’s ability to drive the weather.
The movement toward an entirely different source of energy started in the late 19th century, with a steady parade of experiments that slowly uncovered the true nature of the atom. It began with the discovery that atoms emit a panoply of different sorts of radiation: X-rays, gamma rays, electrons and alpha particles. Gradually, like putting together the strewn pieces of a puzzle, these radiated components allowed physicists to get a sense of the structure of the atom. The epicentre for this atomic revolution was the Cavendish Laboratory. It was there, in 1909, that a team supervised by Rutherford had fired alpha particles into gold foil. The prevailing view at the time was that the atom had a “plum pudding” structure, consisting of a cloud of positive charge folded through with electron “plums”. The positively charged alpha particles should have just shot through this airy pudding. Quite unexpectedly, some bounced straight back, a phenomenon that could only be explained if an atom concentrated its positive charge (which was later found to be made up of protons) in a central nucleus.
In 1932, James Chadwick found protons were nestled in with close companions called neutrons. Not long after, physicists showed that when neutrons were fired at heavy atoms like uranium or plutonium, the atoms would fall apart, releasing more neutrons in the process.
Splitting atoms, or fission, was relatively simple to achieve with massive and unstable atoms containing an excess of neutrons.
Splitting atoms, or fission, was relatively simple to achieve with massive and unstable atoms containing an excess of neutrons. When these atoms are packed close enough together, one need only provide a few neutrons to trigger a chain reaction. Neutrons ricocheting from one split nucleus zoom out, splitting nearby nuclei, which in turn release more neutrons. The result is an explosive release of energy.
But Rutherford and others were also interested in the other side of the periodic table, where light atoms such as hydrogen, helium and lithium reside. The thought was that the centres of such atoms could “fuse” together, creating a heavier atom more tightly bound than the atoms it was built from. This fusion process would release energy, just as it does in the Sun and other stars. Fusion has been responsible for creating all the elements heavier than hydrogen.
Both fission and fusion release energy because an atom’s nucleus is, in a sense, less than the sum of its parts. If you were to add up the masses of the protons and neutrons inside a nucleus, you would overestimate the total mass contained there. The mass isn’t really missing, however. As Albert Einstein found, mass and energy are equivalent. The difference between the mass of an atom’s nucleus and its component parts is accounted for by the energy needed to bind those protons and neutrons together.
Atoms from iron upwards on the periodic table need more energy to bind them together, given the larger numbers of proton. Splitting such an atom into smaller, more stable ones releases the extra energy needed to hold the larger atom together. Fusion works in much the same way, only in reverse. Atoms less massive than iron generally grow more stable as they increase their mass, so fusing lightweight helium atoms releases energy, as not so much is needed to hold the fewer protons together. Of course the positively charged nuclei of two atoms will repel one another but that can be overcome. If the atoms collide with enough speed, their nuclei can fuse.
Oliphant and Rutherford could get the odd fusion reaction to happen in their lab, but to get the self-sustaining reaction that might power the planet, reactors need to achieve three conditions. The atoms inside them must be raised to a very high temperature and density, and the energy produced when the atoms fuse must stay in the system long enough to help trigger their close neighbours to do the same thing. Just as it happens in the Sun.
When physicists in the US, Britain and the Soviet Union set out to create reactors with these conditions, no one expected it was going to be so hard. “Coming off World War II, people thought they could do anything,” says Gerald Kulcinski at the University of Wisconsin-Madison. Instead it turned out to be an exceedingly tricky achievement to pull off. Fission-based reactors can use solid fuel. But fusion requires working with an exotic state of matter known as plasma, in which atoms are so hot they are stripped of electrons – just what happens to hydrogen and helium atoms in the Sun. The result is an electrically conductive soup that can act unpredictably and suddenly. Think solar flares. As one physicist puts it, plasma almost “has a mind of its own”.
The Sun’s immense gravity pulls plasma into a tight embrace, providing positively charged atomic nuclei with the heat, proximity and time needed to overcome their repulsion and fuse together. There is no way to replicate such gravitational force. Physicists had to find another way to wrangle plasma, one that would protect the walls of their experiments from temperatures of more than 10 million degrees Celsius.
Physics itself provided one possible solution: a magnetic field. When a charged particle moves in a magnetic field, it experiences a force. The right configuration of magnetic fields should be able to trap, squeeze and fuse charged plasma particles.
In the decades following the end of World War II, there was a flurry of research into magnetic traps or cages. Some developed linear cages with magnetic field lines that bounced plasma particles back and forth. Others pursued the idea of a torus: a hollow, donut-shaped ring surrounded by magnets, that could in principle push a plasma around and around, colliding and generating energy ad infinitum. In either case it was hard to trap plasma particles in a stable configuration.
In 1968 a team of Russian physicists revealed a potential solution. Dubbed the tokamak, a name derived from the Russian for “toroidal chamber and magnetic coils”, the machine had transformers that could induce a current in the plasma itself. This created a second set of magnetic fields – in addition to the loop going the long way around the torus, a field that pulled the plasma toward the centre.
In the years since the tokamak debuted, it has emerged as the frontrunner in the fusion game and a series of ever bigger tokamak machines were built. For example, the Joint European Torus (JET), which came online in 1984 and is the world’s largest operating tokamak, established a new world record in returning 70% of the amount of power fed into the machine. It could produce high enough temperatures and densities to get fusion going but could not achieve the third necessary condition: to keep the hot, dense plasma in place long enough to get self-sustaining fusion.
The problem: eddies diffusing heat out of the reactor. “The hot plasma swirls outwards, and the colder plasma swirls inwards,” explains Steven Cowley, who directs the Culham Centre for Fusion Energy in Britain where JET is located. The answer, he says, is to increase the distance these eddies must traverse. In other words, the machine needs to be bigger.
The answer is ITER (International Thermonuclear Experimental Reactor), an ambitious US$20 billion, 23,000-ton machine being built at the Cadarache nuclear research facility in southeastern France. ITER, the world’s most expensive piece of scientific machinery (more than double the price of the Large Hadron Collider used to find the Higgs boson) captures the knowledge and expertise painfully gained through decades of experimentation. Many expect it will be fusion’s proving ground. Set to begin operation in 2020, it is designed to contain a plasma volume of 840 cubic metres – eight times the volume that JET can hold – inside a stainless steel vessel weighing more than the Eiffel Tower.
The task of controlling ITER’s plasma, which will be heated to
150 million degrees Celsius, is no small feat.
ITER will face a number of challenges experienced on previous machines. The task of controlling ITER’s plasma, which will be heated to 150 million degrees Celsius, is no small feat: “It’s almost like taking a lump of jelly and trying to suspend it in mid-air with knitting wool,” Cowley says. One of the key concerns is the possibility of disruptions – instabilities that cause the plasma to unpredictably buck and slam into the walls of the reactor.
Nevertheless its designers believe ITER will be the one: the machine that smashes the break-even barrier. They predict that 50 megawatts of heat will deliver 500 megawatts of power, and that the reactor will be able to run for 400 seconds at a stretch. But that’s still far from an electric power plant. There are no plans to generate electricity from the energy ITER produces. Assuming all goes well, the first demonstration of an ITER-like reactor capable of generating energy for the electricity grid is still decades away: in Europe a follow-on project, called DEMO, isn’t expected to produce electricity until about 2050.
For some, ITER is the future of fusion. “I think it’s the most promising route,” says Simon Pinches, who is working on the computational infrastructure needed to run the machine. Although fusion is already accomplished on a daily basis, ITER will be where “fusion is demonstrated as a viable energy source from an engineering perspective,” he says.
Others doubt the practicality of this path. ITER’s costs have quadrupled, even as its designs have been scaled back. Physicists are not yet sure how to address some key questions. To successfully generate electricity, for example, the walls of DEMO and its successors will have to withstand a constant and heavy stream of high-energy neutrons, which will both weaken the material and render it radioactive. “Even if it’s successful, nobody’s going to want it,” says Robert Hirsch, an energy consultant and a former director of fusion energy research in the US. “It’s going to be extremely expensive and complicated.”
Hirsch was involved in arranging the first American tokamak experiments but has since had a change of heart about the approach. For too long, he says, physicists have run the show. “The question is, when are the engineers going to say the emperor has no clothes?”
ITER and other magnetic fusion experiments also face a rival approach. Dubbed inertial confinement fusion, the idea is to take a small target, pack it with deuterium and tritium, and crush it with fantastically powerful lasers. The current leader in this effort is the US$4.5 billion National Ignition Facility (NIF), based about an hour east of San Francisco at the Lawrence Livermore National Laboratory.
Where the ITER approach is slow and steady, NIF’s is explosive and brief. It involves 192 laser beams – the world’s largest laser array – being fired at a cylinder and generating X-rays to blast a peppercorn-sized pellet embedded within. The outer layer of the pellet blasts off in all directions and in reaction, the core is compressed and heated, hopefully to the point of igniting a self-sustaining fusion reaction. If this can be done multiple times a second, it could be a useful source of energy.
NIF, like ITER, has suffered its share of budget overruns and delays. Construction took 12 years, and the troubles didn’t stop when the facility went operational in 2009. Last year the US government expected it to achieve a self-sustaining fusion reaction. It didn’t. “It’s a long hard road,” says NIF’s former principal associate director, Edward Moses. “It’s difficult to do science on a schedule.”
Scientists at work in NIF’s fusion target chamber. The device focuses the beams from 192 lasers onto a target the size of a peppercorn.
A handful of small private firms attempting to tackle the fusion
problem on their own.
A few upstarts are also taking aim at commercial fusion power. Gerald Kulcinski says he has noticed something new in the past 15 years or so: a handful of small private firms attempting to tackle the fusion problem on their own. “You don’t hear about them,” he says, “but the amount of money going into them is much larger than people realise.”
Details are scant, but some firms are aiming to fuse bare protons with atoms of boron. This reaction is more difficult to achieve but creates very few damaging neutrons and carries the potential to generate electricity directly from the creation of charged particles. But there is no free lunch. The amount of energy released in a proton-boron fusion is “painfully small” compared to the amount of energy that must be put in, says Allen Boozer, a theorist at Columbia University in New York. Any machine based on this reaction would have to be ultra-efficient. “You can’t afford any of the loss processes.”
Still, research continues to progress in fits and starts and even within smaller, more controversial fields. Some are exploring the possibility of fusing deuterium inside bubbles that are made to collapse using sound waves. Others are still pursuing the contentious “cold fusion”, which made a brief splash in 1989 when two chemists claimed their experiment showed deuterium atoms fusing at room temperature, a phenomenon requiring new physics to explain.
To many researchers, a thicket of obstacles has stood in fusion’s way including poor funding and trickier-than-expected physics. Worst of all, they believe it has suffered from the lack of a sense of urgency. “Even though we know fossil fuels are doing nasty things to the planet, economics drives everything,” says Cowley. “I don’t imagine that we will replace fossil fuels until we have something that’s cheaper.”
One of the fathers of fusion, Russian scientist Lev Artsimovitch, predicted that: “Fusion will be ready when society needs it.” Some would argue that time is now; the latest report from the Intergovernmental Panel on Climate Change warns that at our current rate of carbon emissions global temperatures could be headed for an increase of more than 2 degrees over pre-industrial levels by the middle of the century. It will take hard work and likely decades yet before fusion becomes a serious source of power. But as a long-term solution to our energy needs, many would agree with ITER’s Simon Pinches: “I think it’s the best thing we could do for the planet.