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Nuclear Fusion – ’Bringing a Star to Earth’

First published in InFocus: Nuclear, a supplement to Cleantech magazine January/February 2008. Copyright Cleantech Investor 2008

by Denis Gross

 

In February 2008 the US National Academy of Engineering (NAE) revealed what its committee of experts believes are the grand challenges for engineering in the 21st century. One of them is providing energy from fusion.  Fusion energy is the process that powers the sun, in the interior of which immense heat and gravitational pressure compress nuclei of certain elements into heavier nuclei, releasing binding energy.  While reactors on earth are incapable of recreating the enormously high pressures of the sun’s interior, this can be compensated to a large extent by the ability to achieve far higher temperatures. For the past five decades fusion has been seen as one of the most promising inexhaustible clean energy sources for the future, with the fuel abundantly available and no carbon dioxide or long-lived radioactive decay products being produced. However, mainstream research has yet to identify a route to a commercial fusion power plant.

Since research on fusion got under way in the early 1950s, the problems with which international researchers have grappled are containing the fusion reaction and getting more energy out than is put in. The Burning Plasma Assessment Committee, formed by the US National Research Council, backed one approach that has consumed a significant proportion of fusion research money through the years. This approach – magnetic confinement – was outlined in its 2004 report ’Burning Plasma: Bringing a Star to Earth’. A burning plasma is a plasma (an ionised gas) in which at least 50% of the energy to drive the fusion reaction is generated internally. As a result, Washington was persuaded to commit the US to becoming a significant partner in a project that is expected to provide a breakthrough step in fusion energy, ITER.

Even at this early stage of the project, however, the US role in ITER has been drastically reduced by Congress in its fiscal 2008 budget, which cut back the US$160 million due for the current year to US$10.7 million. The impact on the project, which is scheduled to last for two decades, may be overcome. It does raise questions, however:

Is fusion a distant prospect, or are there signs of breakthroughs in the field?

And, has the focus of research, and a major part of research expenditure, on one particular approach to fusion energy, the tokamak, led to a valuable research tool, but not a viable route to the world’s energy markets?

When compared to nuclear fission, it is noticeable that for fusion the step from weapon of mass destruction to a major source of generated electricity is a long and costly one. The world’s first artificial nuclear reactor went live in a racquets court at the University of Chicago in December 1942, and under the auspices of the Manhattan project three nuclear weapons were built and detonated in 1945. These were: a plutonium bomb, detonated in a test explosion in New Mexico in July 1945; then an enriched uranium bomb detonated over Hiroshima on 6 August 1945, and another plutonium bomb detonated over Nagasaki on 9 August 1945.

In a little over a decade after the end of World War II, the world's first commercial nuclear power station, Calder Hall in Sellafield, England, was opened in 1956. Currently, nuclear fission provides more than 6% of the world’s energy, and almost 16% of global electricity, through over 400 reactors worldwide.

The first fusion weapon, the hydrogen bomb, was detonated by the US in 1952. This thermonuclear device used a fission explosion to compress the fuel sufficiently to trigger a fusion reaction. The early phase of peaceful fusion devices in the 1950s saw modest progress, leading to the recognition that international collaboration was essential to fund the growing research budgets. This was accompanied by the declassification of scientific information on fusion, and the establishment of the International Fusion Research Council of the International Atomic Energy Agency. By 1968, tokamak reactors achieved a ten-fold improvement in plasma temperature, and in the 1980s several large tokamaks became operational, including the Joint European Torus (JET) in the UK, the Tokamak Fusion Test Reactor in the US and the JT-60 Tokamak in Japan. Construction on the latest tokamak project, ITER, is due to begin in 2008, and over its 20 years of operation ITER is expected to demonstrate the ability to sustain fusion reactions with an energy gain over input power of ten. ITER’s estimated cost for 20 years of operation is US$15 billion, with the EU contributing 50%. The US, China, India, Japan, Russia and the Republic of Korea will jointly pay for the other 50%.

While other approaches to fusion are being followed, most mainstream nuclear researchers, including nuclear fusion’s staunchest advocates, believe a power-producing fusion plant is still decades away at best, despite fifty years and many tens of billions of dollars already spent on research.

Why fusion?  

The main incentive is the acquisition of an energy source that is inexhaustible. For example, the amount of lithium present in the battery powering a laptop computer for a few hours could, in fusion, supply a household’s electricity requirements for fifteen years. Also, fusion can be considered environmentally friendly, producing no combustion products or greenhouse gases. Although fusion is a nuclear process, the products of the fusion reaction are not radioactive (but they are energetic, which creates other problems).

In fusion, two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process some of the mass of the hydrogen is converted into energy. The most readily achieved fusion reaction on earth is to fuse deuterium (or ‘heavy hydrogen’) with tritium (or ‘heavy-heavy hydrogen’) to form helium and a neutron. Thus lithium is seen as the primary fuel for the first-generation deuterium-tritium fusion reactors, while deuterium could be the ultimate fuel in second-generation deuterium-deuterium fusion reactors. Deuterium is abundantly available in ordinary water – theoretically the oceans contain enough deuterium to meet the world’s energy needs for billions of years – and tritium is produced by combining the fusion neutron with the readily available light metal lithium. Lithium is more abundant than tin or lead, and exists in significantly higher quantities than the primary fission fuels, uranium and thorium, in the earth’s crust. There are also vast quantities (trillions of tons) in seawater. It could be said that fusion is the only energy source indigenous to Earth that will last as long as the planet.

CONFINEMENT TECHNOLOGIES

To ensure fusion, the atoms of hydrogen or heavy-hydrogen must be heated to extremely high temperatures of the order of 100 million degrees. The atoms become ionised – the ions and electrons forming a plasma. Because the ions and electrons in the centre of the plasma are at very high temperatures, they have correspondingly large velocities. In order to maintain the fusion process, the energetic particles in the hot plasma must be confined in the central region, or the plasma will rapidly cool. It is this confinement that has proven to be an immense problem to overcome. Confinement in the sun and stars is provided by gravity.  On Earth, in the absence of this gravitational energy, other methods of confinement have to be developed. Traditionally, magnetic and inertial confinement methods have been the focus of research, particularly magnetic confinement. Magnetic confinement fusion devices exploit the fact that charged particles in a magnetic field follow helical paths along the field lines. A number of techniques are available for achieving high plasma temperatures – for a burning plasma temperatures of 100 million degrees are required, The principal techniques are: ohmic heating (passing a large current through the plasma), radio frequency (RF) and microwave heating, and neutral beam injection.

Confinement Technologies    

Magnetic Confinement

Spurred by Russian tokamak design in the 1960s and 1970s; has evolved to the international ITER megaproject, being built in France.  

Field-Reversed Configuration, Levitated Dipole, Reversed Field Pinch, Riggatron, Spheromak, Stellarator, Tokamak

Inertial Confinement Fusion (ICF)

Almost all ICF devices to date have used lasers; most work has been undertaken in the USA.    

Bubble Fusion, Fusor, Laser-driven, Magnetised Target, Z-pinch

Other approaches  

Aneutronic fusion, Cold fusion, Dense Plasma Focus, Migma, Muon-catalysed, Polywell, Pyroelectric


The table demonstrates the range of ways in which the topic of confinement has been addressed. In the early days of fusion research, the UK effort in the 1950s centred on ZETA (Zero Energy Toroidal Assembly) at Harwell, which used a toroidal pinch form of magnetic confinement. While early indications of successful fusion were false, ZETA provided a valuable research tool. However, the early fusion pinch devices produced plasmas that were prone to rapid instabilities and short-lived confinement. In the USA the Princeton Plasma Physics Laboratory worked on another form of magnetic confinement device based on a toroidal structure, the stellarator. On the other side of the USA, the Lawrence Livermore National Laboratory in California began to look into an alternative approach, inertial confinement.

MAGNETIC CONFINEMENT

Magnetic confinement assumed its dominance over the 1960s and 1970s, to a great extent spurred by the Russian tokamak design, which demonstrated an order of magnitude increase in temperatures and vastly improved confinement. This culminated in the Joint European Torus, JET, in Culham, UK, which in 1991 achieved a world first by producing 1.7MW from controlled nuclear fusion. It was followed by Princeton’s TFTR device in 1993, which produced 10MW of power from a plasma comprising a 50:50 mix of deuterium and tritium, and JET then established the current world record in 1997 of 16MW of fusion power.

The evolutionary path of magnetic confinement has reached the international megaproject, ITER, based on the tokamak device, to be built on a site in Cadarache, France. The programme is anticipated to last for 30 years, ten of which will be for construction and 20 for operation, at an estimated cost of around US$15 billion. It is technically ready to start construction in 2008, and the first plasma operation is expected in 2016.

ITER will be designed to produce approximately 500MW of fusion power sustained for up to 400 seconds (compared to JET’s peak of 16MW which lasted for less than a second) by the fusion of about half a gram of deuterium/tritium mixture in its approximately 840m3 reactor chamber. Although ITER is expected to produce net power in the form of heat, the generated heat will not be used to produce any electricity.

Magnetic confinement, despite the dramatic advances made, is still a long way from practicality, and there are fears that the tokamak and stellarator may prove too complex and too expensive to commercialise. Simpler and potentially easier-to-engineer compact toroids like the spheromak, in which the stabilising magnetic field is largely self-generated through plasma currents, leading to a stable, typically toroidal plasma, may yet come to the fore.

INERTIAL CONFINEMENT FUSION (ICF)

Inertial confinement, or inertial confinement fusion (ICF), mentioned briefly earlier, has gathered a significant degree of momentum. Here the fusion fuel is present in a small target, and energy is delivered to the outer layer of the target using high-energy beams of lasers, ions or electrons to compress and heat the fuel in order to create fusion conditions.  Almost all ICF devices to date have used lasers, and most of this work has been done in the USA.

The typical target is a pin-head sized fuel pellet containing around 10mg of fuel, typically deuterium-tritium isotopes. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards and sending shockwaves into the centre. It is essential that the driver delivers a sufficiently powerful set of accurately timed shockwaves that can compress and heat the fuel at the centre to a degree that fusion reactions occur. The energy released by these reactions will then heat the surrounding fuel, which may also begin to undergo fusion. The aim of ICF is to produce a condition known as ’ignition’, where the heating process causes a chain reaction that burns a significant portion of the fuel. Edward Teller, the ’father of the H-bomb’ has described ICF as essentially the internal combustion engine approach to nuclear fusion energy.

For much of the 1980s and 90s ICF experiments focused primarily on nuclear weapons research. More recent advances suggest that major gains in performance are possible, once again making ICF attractive for commercial power generation. A number of new experiments are under way or being planned to test this new ’fast ignition’ approach.

The Lawrence Livermore Laboratory made most of the initial running with ICF, with the 20-beam Shiva laser system that began operations in 1978. Shiva was a ‘proof of concept’ design intended to demonstrate compression of fusion fuel capsules to many times the liquid density of hydrogen. While Shiva successfully compressed the fuel pellets to 100 times the liquid density of deuterium, fusion yields were low. A solution appeared to lie in the ability to triple the frequency of high intensity laser light efficiently. A discovery made by the Laboratory for Laser Energetics (LLE), University of Rochester, in 1980 enabled experimentation with this approach in the OMEGA laser at Rochester. Although funding for this sort of research was severely restricted in the 1980s,in recent years international interest has been increasing and a number of large ICF projects are now under way. The descendant of Shiva, OMEGA and NOVA, a high energy laser that attempted fusion experiments at Lawrence Livermore between 1984 and 1999, is the National Ignition Facility (NIF), also at Lawrence Livermore. NIF is scheduled for fusion experiments in 2009/2010 using a 192-beam laser array. (Lawrence Livermore and the Laser Megajoule (LMJ) in France, which will be undertaking similar research, both concentrate primarily on nuclear weapons research, and devote only between 10% and 20% of their time to research other areas of physics.) NIF’s array comprises two 96-beam laser bays, and is designed to deliver 1.8 million joules of ultra violet energy (after conversion from the lasers’ infra red output) in a ten nanosecond pulse. This has been equated to 1,000 times the electrical generating power of the US in the same time period.

FAST IGNITION (FI) – a variant of ICF

In the conventional ICF approach, the same lasers compress the fuel pellet and heat it. In a variant of this, called fast ignition (FI), different lasers are employed for the two stages which could use smaller lasers and less energy than the conventional approach, and hence be cheaper, while producing fusion power of the same magnitude. In this method, rather than use the timed shockwaves to heat the compressed fuel, a second, shorter, laser pulse is utilised to start the fusion reaction. Although the implosion can be less precise, there is a difficulty in that the short laser pulse can be deflected by the instabilities created in the plasma. A solution to this has been found by placing a polymer cone in the fuel pellet, which allows the beam to pass through with no obstruction or deflection.

First demonstrated at the Gekko XII laser at Osaka University, Japan in 2001, FI has also been worked on at the Rutherford Appleton Laboratory, Imperial College and the University of York in the UK.   A £500 million FI project to be hosted in the EU is currently being set up, the High Power Laser Energy Research Facility (HiPER), which is expected to see construction commence in 2011 and be opened to the scientific community before the end of the next decade.

ALTERNATIVE APPROACHES

Considering the past and anticipated progress in both magnetic and inertial confinement,  a functioning power-producing fusion reactor is probably 40 - 50 years away: that is too far in the future for any reasonable conclusions to be drawn on its economic viability. There are a number of desktop alternative approaches whose proponents believe offer a far quicker, cheaper route to fusion power.

One of these alternative routes is bubble fusion, or sonofusion. Sonofusion is a fusion reaction that is hypothesised to occur during sonoluminescence, an effect first observed in 1934 at the University of Cologne as a result of work being carried out on sonar. An ultrasound transducer placed in a tank of photographic developer led to bubbles in the fluid emitting light when the transducer was active. This is attributed to a form of acoustic cavitation, and occurs whenever a sound wave of sufficient intensity induces a gaseous cavity within a liquid to collapse quickly. The collapsing bubble wall confines the energy, leading to a rise in temperature, possibly sufficient to achieve fusion. Shockwave simulations indicate that the temperatures inside the collapsing bubbles may reach up to 10 million degrees – as hot as the centre of the sun and thus theoretically capable of causing fusion. The reaction can be termed acoustic inertial confinement fusion (AICF).  

Attention was drawn to this field by the work of Rusi Taleyarkhan, who in 2002, while a senior scientist at Oak Ridge National Laboratories (ORNL), published a paper on fusion achieved by bombarding a container of liquid solvent with strong ultrasound vibrations. He moved from Oak Ridge to Purdue University in 2003, and published additional papers about the process, claiming to have observed energy and neutron emission consistent with fusion. However, there is no report of these results being reproduced by other researchers, and considerable controversy surrounds this issue.

Another form of inertial confinement, inertial electrostatic confinement, has a long pedigree (in the fusion field) in the shape of the Farnsworth-Hirsch Fusor, or fusor, which injects high temperature ions directly into a reaction chamber. Inside the reaction chamber, electrostatically charged electrodes confine the plasma.  Philo Farnsworth, the original inventor of the fusor, was a pioneer of television in the 1930s. As with fusion in general, the development of an energy source has been elusive to date, but the fusor is commercially available as a neutron source.

Fusion reactions emit energetic neutrons and, while there are no radioactive waste products as in fission reactions, neutron radiation produces ionisation damage. Thus fusion reactors have to be shielded with lithium and alloy blankets, which themselves are attacked by the neutron radiation. There are a number of fusion reactions where less than 1% of the energy evolved is carried by neutrons – aneutronic fusion. Another potential advantage lies in the potential of converting the energy of the charged fusion products directly to electricity – thus saving cost.

The drawback is that the aneutronic fusion reaction, even the most amenable of the range of possible reactions, the free proton-boron 11 one, is far more difficult to achieve than the deuterium-tritium reaction. It thus remains to be seen whether this approach will prove either scientifically feasible or economically practicable.

Mention should also be made of cold fusion. As with sonofusion, cold fusion is surrounded by controversy. As the name suggests, cold fusion is nuclear fusion at room temperature and atmospheric pressure – a far remove from recreating the conditions at the centre of the sun. The internationally respected electrochemist, Martin Fleischmann, and Stanley Pons at the University of Utah announced in 1989 that they had observed a nuclear reaction in their apparatus, which consisted of two electrodes immersed in heavy water. The possibility of cold fusion generated immense academic and government interest, and certainly raised hopes for a ready source of abundant, cheap power. There is still no clear scientific explanation of how this type of fusion could take place, despite in-depth investigation by the US Department of Energy and the US Navy, and the experiment has not been reproduced successfully. That said, there remain a number of vociferous proponents who continue to receive funding, and we await developments.

Thus in the field of fusion, ’big science’ is undertaking a lengthy journey that is unlikely to see a commercial outcome for many decades to come. There are possibilities of alternative approaches which may get there a lot more quickly, and these should certainly receive serious attention, although assigning a probability of success to many of these approaches is difficult.
 

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