# The Path to Fusion Power

## Chris Llewellyn Smith,Chair ITER Council, Chair Consultative Committee for Euratom on Fusion, Theoretical Physics, Oxford

Сhoose language: РУС (Russian) / ENG (English)

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### Introduction

In reply to the question “When will fusion be ready?”, Lev Artsimovitch — who is generally regarded as the father of modern fusion research — replied «Термоядерная энергия будет получена тогда, когда она станет необходима человечеству» (“Fusion will be ready when society needs it”)1. In this talk I will address the questions: Why is fusion needed? When will it be needed? When will it be ready?

### The energy challenge and the need for fusion

The ‘energy challenge’ is a consequence of three facts:

1) The world uses a lot of energy.

The world is using energy at a rate of 15.7 Tera Watts (TW). Dividing by the world’s population, this is equivalent to 2,400 Watts per person, a number which is relatively easy to grasp. 15.7 TW is therefore the power consumed by twenty four 100 W light bulbs burning continuously for every man, woman and child on the planet.

This energy use is very uneven. For a few selected countries, the figures for use per person are: the USA (near, but not quite at, the top of the scale) — 10.3 kW, the Russian Federation — 6.3 kW, the United Kingdom — 5.1 kW, Bangladesh — 0.21 kW, only 2% of the figure for the USA!

2) World energy use is increasing dramatically.

The International Energy Agency’s 2006 World Energy Outlook predicted that world energy use will increase 50% by 2030. The developed world could survive perfectly well with less energy, but an increase is needed to lift billions of people out of poverty in the developing world, where 1.5 billion people still lack electricity.

3) 80% of the world’s primary energy is currently generated by burning fossil fuels (oil, coal, and gas), which is i) causing potentially catastrophic climate change, and ii) unsustainable as they won’t last for ever.

What are the timescales for actions to prepare for the end of the fossil fuel era and to avoid or mitigate climate change? What actions should we take?

There is a Saudi saying: “My father rode a camel. I drive a car. My son flies a plane. His son will ride a camel”. Is this true? Very likely: yes. Oil will probably be largely exhausted in 50 years. Even using the US Geological Survey’s estimate of the amount of remaining oil (which is significantly larger than all others), the peak of oil production cannot be much more than 20 years away (and many predict that production will peak in 5-10 years or even sooner, and then fall ~3% p.a.). Gas is expected to last a little longer. It is often said that there is enough Coal for over 200 years — but that is with current use. Use of coal is currently growing 4.5%/year, which turns 200 years into 50 years!

It is therefore clear that we need to start preparing for the post fossil fuel era now.

The timescale for climate change is set by the fact that CO2 stays in the atmosphere for hundreds of years. We should therefore have taken action yesterday! It seems very likely that most of the world’s remaining fossil fuels will be burned in the next hundred years. If this is so, then the only action that can moderate climate change is Carbon Capture and Storage (CCS), i.e. the separation and burial of CO2 from power stations and large industrial plants, which should be developed as a matter of urgency and (if feasible) rolled out on the largest possible scale. This is easy to say, but implementation will be difficult as CCS will increase costs significantly.

The actions that need to be taken in preparation for the end of the fossil fuel era are:

i) Reduction of energy use.

Improving efficiency, in energy production, transmission and use, must be a priority. This requires initial investments, but could save a lot of money over time. There are some easy targets, especially in road transport and the construction of buildings. But I think it unlikely that greater efficiency can do no more than slow down the growth in world energy use, unless we are prepared to tolerate continued and growing differences in the standards of living in developed and developing countries, where more energy is needed to provide a decent standard of living for billions of people (and the population is set to grow nearly 30% by 2030).

ii) Development and expansion of low carbon energy sources.

Today, in rounded numbers, the world’s primary energy comes from2:

• Fossil fuels — 80%
• Burning waste & bio-mass — 10%
• Nuclear — 5%
• Hydro — 5%
• The rest (wind, solar, geo, marine, ...) — 0.5%

Given the relatively small amounts of energy supplied by the alternatives, it is obvious that replacing the 13 TW currently provided by burning fossil fuels will not be easy. In fact:

i) The maximum additional (thermal equivalent2) amounts that I can imagine being provided by wind (up to 3 TW), hydro (up to 1 TW), bio (up to 1 TW), geothermal (up to 100 GW), and marine energy (up to 100 GW), add up to less than 6 TW. These energy sources should be expanded to the maximum extent reasonably possible, although this will not be easy as they will mostly remain relatively expensive compared to burning fossil fuels until coal and gas become scarce. There is clearly an enormous gap between the optimistic maximum that these sources can provide (6 TW) and the energy supplied by fossil fuels (13 TW, and growing). The world will therefore need something else.

ii) The only energy sources that could fill the gap are solar and nuclear (fission and/or fusion).

Solar energy has enormous potential in principle. The sunlight that falls on 0.5% of the world’s land surface converted into useable energy with 15% efficiency would produce 19 TW. However, exploitation of this potential will require big breakthroughs, which should be sought as a matter of urgency, in i) cost and ii) energy storage and transmission, in order to provide energy at night and in relatively sunless areas. “Concentrated solar” using parabolic mirrors or heliostats to focus sunlight and produce heat, which could be stored in molten salts, looks promising, although dealing with fatigue produced by day/night temperature changes will be challenging. If suitable materials can be developed, heliostat systems could reach the temperatures required for “catalytic cracking” of water to produce hydrogen as an energy storage medium. Photovoltaic systems with over 15% efficiency are already available commercially, albeit currently at a high cost, and could be used to produce hydrogen through electrolysis, as well as producing electricity.

Nuclear (fission) energy is of course widely available now, and should be expanded rapidly in my opinion. If, however, there is a major nuclear renaissance, cheaper uranium will probably be exhausted sometime in the second half of the century. The era of cheap uranium can — and should — be pushed back by employing more efficient fuel cycles, which could allow up to twice as much energy to be extracted from a given quantity of uranium. To prolong the nuclear age significantly, however, it will be necessary to develop thorium breeder reactors (more thorium is available than uranium, and about forty times as much energy can be extracted from a given quantity) and/or plutonium fast breeders (which can provide about sixty times more energy from a given quantity of uranium than a conventional nuclear reactor: this could make the use of large scale “unconventional” uranium sources, such as sea water, practical).

The only other option is nuclear fusion, which needs to be developed as a matter of urgency, even if success is not certain.

### Fusion

Fusion powers the sun and stars, and is potentially an environmentally responsible and intrinsically safe source of essentially limitless energy on earth. Experiments at the Joint European Torus (JET) in the UK, which is currently the world’s leading fusion research facility and has produced 16 MW of fusion power, and at other facilities, have shown that fusion can be mastered on earth.

So fusion works. The big question is: when will it be made to work reliably and economically on the scale of a power station? Before attempting to answer this question, I consider the questions: What is fusion? What will a fusion power station look like? Given that mastering fusion has turned out to be very difficult: why bother? And finally: why is it taking so long?

### What is fusion?

Reactions between light atomic nuclei in which they fuse to from a heavier nucleus, with the release of energy, are called fusion reactions. The reaction of primary interest as a source of power on earth involves two isotopes of hydrogen (Deuterium and Tritium) fusing to form helium and a neutron:

D + T → 4He + N + energy (17.6 MeV)        (1)

Energy is liberated because Helium-4 is very tightly bound: it takes the form of kinetic energy, shared 14.1 MeV/3.5 MeV between the neutron and the Helium-4 nucleus3 .

To initiate the fusion reaction (1), a gas of deuterium and tritium must be heated to over 100 million°C (henceforth: M°C) — ten times hotter than the core of the sun. At a few thousand degrees, inter-atomic collisions knock the electrons out of the atoms to form a mixture of separated nuclei and electrons, known as a plasma. Being positively electrically charged, the rapidly moving deuterons and tritons (the nuclei of deuterium and tritium) suffer a mutual electric repulsion when they approach one another. However, as the temperature — and hence their speeds — rises, they come closer together before being pushed apart. When the temperature exceeds 100M°C, the more energetic deuterons and tritons in the plasma approach within the range of each other’s nuclear force and fusion can occur copiously.

There are three major challenges. The first is to heat a large volume of D and T gas to over 100M°C, while preventing the very hot gas from being cooled (and polluted) by touching the walls. This is done using a ‘magnetic bottle’, known as a tokamak4 , which holds the plasma away from the wall. The plasma is heated to around 3M°C by an electric current which is driven around the torus. To reach the somewhat higher temperature needed for fusion, additional heating is required: it is supplied by pumping radio-frequency electromagnetic waves into the plasma, rather as in a microwave oven, and injecting beams of energetic neutral particles, which transfer their energy to the plasma as they are slowed down by collisions. Heat is also provided by the fusion reaction itself, as described below, and the plasma can actually ‘burn’ in a large enough fusion device. ITER (the International Tokamak Experimental Reactor, described below, which is now under construction in France) will be the first tokamak that is able to produce a burning plasma.

The world’s leading tokamaks routinely reach the temperature (of order 150M°C) and create the conditions needed in a fusion power plant, but ITER will be the first to do so on the scale of a power plant and for long periods. There is still plenty of scope for improving the conditions, in particular by increasing the plasma pressure, which is of key importance because the rate at which fusion occurs is proportional to (pressure)2 at fixed temperature. The challenge is to increase the pressure without provoking disastrous instabilities.

The helium nuclei that are produced by fusion (being electrically charged) remain in the ‘magnetic bottle’; they are slowed down by collisions with the other particles in the plasma, and the energy that they lose serves to help keep the plasma hot. The neutrons (being electrically neutral) escape and are captured by and heat up the walls: this heat is then used to drive turbines and generate electricity. The huge flux of very energetic neutrons and heat (in the form of electromagnetic radiation and plasma particles) can damage the container. The second major challenge is to make a container with walls that are sufficiently robust to stand up, day-in day-out for several years, to this neutron bombardment and heat flux.

Fusion power stations will be very complex, and the third, and perhaps greatest, challenge will be to make them work reliably. This will require extensive further development of the large range of sophisticated technologies that are involved.

### Fusion Power Stations

Figure 1 shows the conceptual layout (not to scale) of a fusion power station. At the centre is a D-T plasma with a volume ~2000 m3, contained in a ‘toroidal’ (doughnut shaped) chamber. D and T are fed into the core and heated to over 100M°C. The neutrons produced by the fusion reaction (1) escape the magnetic bottle and penetrate the surrounding structure, known as the blanket, which will be about 1 metre thick.

 Fig 1. Fusion power stations will be similar to existing thermal power stations, but with a different furnace and fuel

In the blanket, the neutrons encounter lithium and produce tritium through the reaction:

Neutron + Lithium → Helium + Tritium        (2).

There are various competing reactions which do not produce tritium, but there are also many reactions that produce additional neutrons, which in turn can produce tritium (and the production of additional neutrons can be enhanced, e.g. by adding beryllium or lead). The upshot is that, on paper at least, it is possible to design fusion reactors that would produce enough tritium for their own use, plus a small surplus to start up new plants: this will be tested at ITER, as described below.

The neutrons will also heat up the blanket, to around 400°C in so-called ‘near-term’ power plants that would use relatively ordinary materials, and conceivably to above 1,000°C in more efficient advanced models that would use materials such as silicon carbide composites. The heat will be extracted through a primary cooling circuit, which could contain water or helium, which in turn will heat water in a secondary circuit that will provide the steam to drive turbines.

### Why bother?

The major attraction of fusion is that it requires only tiny amounts of very abundant fuels (other attractions are listed below). The release of energy from a fusion reaction is ten million times greater than from a typical chemical reaction, such as occurs in burning a fossil fuel. Correspondingly, while a 1 GW coal power station burns ten thousand tonnes (ten train loads) of coal a day, a 1 GW fusion power station would burn only about 1 kg of D + T per day.

Deuterium is stable, and in one in every 3,350 molecules of ordinary water, one of the hydrogen atoms is replaced by a deuterium atom (left over from the big bang). Deuterium can be easily, and cheaply, extracted from water. Tritium, which is unstable and decays with a half-life ~12 years, occurs only in tiny quantities naturally. But, as described above, it can be generated in-situ in a fusion reactor by using neutrons from the fusion reaction impacting on lithium to produce tritium through reaction (2).

The raw fuels of a fusion reactor would therefore be lithium and water. Lithium is a common metal, which is in daily use in mobile phone and laptop batteries. Used to fuel a fusion power station, the lithium in one laptop battery, complemented by deuterium extracted from 45 litres of water, would produce 200,000 kW-hours of electricity (allowing for inefficiencies) — the same as 70 tonnes of coal. This is equal to the current electricity production per capita in the EU for 30 years. The fact that such a tiny amount of lithium can produce so much electricity, without any production of CO2 or air pollution, is sufficient reason to develop fusion urgently (unless or until a barrier is found) even if success is not 100% certain.

There is enough deuterium for millions of years, and easily mined lithium for several hundreds of years. When lithium becomes scare on land, it could be extracted from water, which contains enough to power the world for a few million years, at a high enough concentration (100 times that of uranium) to make extraction economical.

In addition to being capable (in principle) of powering the world for the indefinite future, without producing any CO2 or air pollution, the attractions of fusion are:

1) Intrinsic safety.

The plasmas in fusion power plants will be very dilute (the density will about one millionth of the density of the atmosphere). They will not contain enough energy to drive a major accident, and there will be no possibility whatsoever of a dangerous runaway reaction. Furthermore, fusion must be continuously fuelled and is easily stopped — indeed, if anything unusual occurred that changed the conditions appreciably, the fusion ‘fire’ would go out.

What are the hazards? First, although the products of fusion (helium and neutrons) are not radioactive, the blanket will become activated when struck by the neutrons. If, however, the walls are constructed with appropriately chosen materials, the radioactive products will decay with half-lives of order ten years, and all the components could be recycled within 100 years. Should the cooling circuit fail completely, radioactivity in the walls would continue to generate heat, but the temperature would peak well below the value at which the structure could melt.

Second, tritium is radioactive, with a relatively short half-life (12 years). But although the volume of plasma will be large, it will only contain a small amount of tritium (by weight, about the same as ten postage stamps) because the density will be so low. So not much fuel will be available to be released into the environment should an accident occur. Even in the almost inconceivable case that all the tritium in the blanket was released, the potential hazard would not be enormous, and in any case it will be easy to design reactors so that even in the worst imaginable accidents or incidents (such as earthquakes or aircraft crashes) only a small percentage of the tritium inventory could be released, and evacuation of the neighbouring population would not be necessary.

2) The cost.

The ‘internal’ cost (i.e. the cost of generation) is expected to be acceptable provided reasonable availability can be achieved (e.g. 75%), which admittedly will be a big challenge (estimates of costs are given later). ‘Acceptable’ means competitive with the cost of electricity from most other low carbon sources. The ‘external’ costs — impact on health, climate and the environment — will be essentially zero.

### Why is it taking so long?

Given the attractions of fusion, which have been known for over half a century, why has it not already been developed? There are three reasons — the first is extrinsic, the second and third are intrinsic:

1) Fusion has not been funded with any sense of urgency (for what probably appeared to be good reasons in the 1980s and earlier, when fossil fuels seemed abundant and few people worried about climate change). A 1976 study by the US Department of Energy’s Fusion Energy Advisory Committee estimated the time needed to complete the necessary R&D before a Demonstrator Fusion Power Plant could be built under various different hypotheses about funding. The study found that with annual funding below a certain level the necessary R&D would never be completed: actual funding has been significantly below this critical level.

2) Fusion cannot be demonstrated on a small scale. For reasons explained later, the fusion power that is generated in a magnetically confined plasma divided by the power needed to heat the plasma grows at least like the square of the linear dimensions. The result is that something on the scale of ITER is necessary to demonstrate the scientific and technical feasibility of fusion power. Society was not willing to fund such a device until the chance of success looked high.

3) Developing fusion is very challenging. Nevertheless, despite inadequate funding and delays caused by difficulties in choosing the sites for both JET and ITER, there has been enormous progress (even if not yet enough to be able to build a power plant).

### Progress in Fusion

The modern era of fusion research started in 1969 when it was convincingly shown that the ~1 m3 plasma in the Russian tokamak T3 had reached 3M°C. This persuaded the world that the tokamak is the most promising candidate configuration for magnetic confinement of plasmas. Within a few years, the bold decision was taken to construct the Joint European Torus (JET), with the much greater plasma volume of ~100 m3. JET, which came into operation in 1983 and is still the world’s largest tokamak, routinely heats plasmas to 150M°C.

Three parameters control the fusion reaction rate:

1. The plasma temperature (T), which as already stated must be above 100M°C.

2. The plasma pressure (P). The reaction rate is approximately proportional to P2.

3. The ‘energy confinement time’ (τE) defined by

τE = (energy in the plasma)/(power that must be supplied to keep the plasma hot)

τE measures how well the magnetic field insulates the plasma. It is obvious that the larger τE, the more effective a fusion reactor will be as a net source of power.

It turns out that the ‘fusion product’ P × τE determines the energy gain of the fusion device. With P measured in atmospheres and τE in seconds, the fusion product must be ten or more in a fusion power station. The ‘fusion performance plot’ (fig 2) of PτE vs. T, which shows data points from many different tokamaks, indicates the substantial progress towards power station conditions that has been achieved.

 Fig 2. Selected results from different tokamaks demonstrate substantial progress over recent decades from the low temperature, low energy gain points at the bottom left. Temperatures above 100M°C are now routinely achieved and an energy gain of around one has been reached. A power plant needs an energy gain above ten, which should be achieved in ITER

We can be rather confident that ITER will reach the region indicated in Figure 2. ITER is twice a big as JET in every dimension. The energy in an ITER plasma (other things being equal) will therefore be eight times that in a JET plasma, but the surface area through which heat can be lost will only be four times as big, while the heat will on average have twice as far to travel to the surface. This will almost automatically provide an improvement in confinement time of a factor of four. In fact the situation should be better because the magnetic fields in ITER will be bigger than in JET, providing a stronger ‘magnetic bottle’ which should be able to confine plasmas with higher pressures5.

Understanding of fusion plasmas has made steady progress the last two decades. There have been two especially important positive developments:

• The discovery (following a prediction made at Culham in the UK) of a self generated (‘bootstrap’) electrical current in the hot plasma in suitable conditions. For example, it is expected that 80% of the 15 MA current needed to confine plasmas in ITER will be self generated. The consequences are that i) much less external power will be needed than previously thought to keep the electric current in the plasma flowing, and ii) achieving steady state operation will be less of a challenge.
• The serendipitous discovery (in a fusion experiment at Garching in Germany) of a ‘high confinement’ plasma mode that allows higher pressure, and hence higher fusion power, with a given magnetic field.

Negative developments are of course not excluded in the future. There could be new instabilities in the burning plasmas that will be studied for the first time at ITER, although theoretical and experimental simulations suggest that this is unlikely. Despite the help provided by the bootstrap current, it could prove impossible to drive currents indefinitely. If so, the fall-back would be to build reactors that operates with long pulses (e.g. of eight hours), with a means to store heat and keep the electrical output going between pulses, or base power stations on stellarators (see footnote 4) rather than tokmaks.

### Next Steps for Fusion

The next major step is to construct ITER, which is designed to a generate a burning plasma and produce at least ten times more fusion power than the power that must be supplied to keep the plasma hot. ITER will be an experimental device: it will not be equipped with turbines to produce electricity, nor will it operate round the clock. It should, however, produce the conditions that will prevail in power stations, perhaps for an hour or more, although in order to be economical real power stations will have to be something like 30% larger in every dimension. Before the construction of a prototype fusion power station, fully equipped with turbines etc., it will be essential to i) continue development of materials that can survive in the harsh conditions in a fusion power station and test them in a special device called IFMIF (International Fusion Irradiation Facility), as described below, and ii) further develop a range of technologies (remote handling, heating systems, blankets, fuel cycle, etc.).

### ITER (the International Tokamak Experimental Reactor)

 Fig 3. The ITER project, which is beginning to be built, with a target date of 2018 for first plasma, is designed to produce at least 500 MW of fusion power. The person pictured at the front right sets the scale

ITER, which is shown in Figure 3, will be approximately twice the size of JET in linear dimensions, and operate with a higher magnetic field and current flowing through the plasma. The aim of ITER is to demonstrate integrated physics and engineering on the scale of a power station. The design goal is to produce at least 500 MW of fusion power, with an input ~50 MW.

ITER is being built by a consortium of the European Union, Japan, Russia, USA, China, S. Korea, and India. These countries are together home to over half the world’s population, so ITER is a global response to a global problem. Prototypes of key ITER components have been fabricated by industry and tested. The site, at Cadarache in France, has been cleared and construction of components is beginning. The first plasma is planned for 2018 and the first DT plasma for 2026 (extensive testing will first be needed with hydrogen and then deuterium plasmas).

Unlike JET, which can only operate for up to one minute, because the toroidal coils that produce the major component of the magnetic field are made of copper and get hot, ITER will be equipped with super-conducting coils, allowing indefinite operation (assuming the plasma current can be kept flowing — the design goal is above ten minutes). Super-conducting magnets will obviously be necessary in power stations, which must operate round the clock. Superconducting tokamaks exist, but super-conducting coils have not so far been used in really large tokamaks capable of using tritium. ITER will also contain test blanket modules that, for the first time, will test features that will be necessary in power stations, such as for example the in situ generation and recovery of tritium.

A major goal of ITER is to show that plasma performance that has already been achieved can be reproduced with much higher fusion power than can be produced in existing devices. Developments with the potential to improve the economic competitiveness of fusion power will also be sought (in experiments at existing machines as well as ITER). The main goals are:

1) Demonstrating that large amounts of fusion power (10 times the input power) can be produced in a controlled way, without provoking uncontrolled instabilities, over-heating the surrounding materials or compromising the purity of the fusion fuel. These issues are successfully managed in existing devices but will become much harder at higher power levels produced for longer times. ITER is designed to tolerate this but it remains a big challenge.

2) Finding ways of pushing the plasma pressure to higher values (recall that the fusion rate is proportional to the square of the pressure, at fixed temperature) without provoking uncontrollable instabilities. This would allow a power plant to operate either at higher power density or with reduced strength magnets, in either case lowering the expected cost of fusion generated electricity.

3) Demonstrating that continuous (‘steady state’) operation, which is economically and technically highly desirable if not essential, can be achieved without expending too much power. There is optimism that the plasma current can be kept flowing indefinitely by ‘current drive’, from radio-frequency waves and particle beams, boosted by the self generated (‘bootstrap’) current, however this must be optimised to minimise the cost in terms of the power needed.

### Materials

The materials in fusion power stations that are close to the plasma will be subjected to many years of continuous bombardment by a ~2.5 MWm–2 flux of 14 MeV neutrons. This neutron bombardment will on average displace each atom in nearby parts of the blanket and supporting structures from its equilibrium position some 30 times a year. Displaced atoms normally return to their original configuration (when thermal vibrations bring displaced atoms together with vacancies). It is possible, however, that the vacancies and displaced atoms may migrate differently, in which case they could accumulate at grain boundaries, producing swelling or embrittlement, and weaken the material.

It had been thought that only exotic materials (such as silicon carbide composite ceramics) could survive fusion neutron damage for long periods. The discovery during the 1990s, in tests at fission reactors, that special (body centred cubic) steels can probably survive in fusion reactor conditions for around five years before they would have to be replaced was therefore a very positive and welcome surprise. In the long-term, however, development of silicon carbide composites that could operate at very high temperature (perhaps above 1000°C), and hence produce power with high thermodynamic efficiency, remains as important goal.

Fusion generated neutrons will initiate nuclear reactions that produce helium inside the structural materials about 100 times more copiously per atomic displacement than neutrons from fission, which have much lower energies. There is serious concern that the helium could accumulate and further weaken the structure. Furthermore, the so-called plasma-facing materials and a component called the divertor (though which impurities and the helium ‘ash’ produced in D–T fusion are exhausted) will be subjected to additional fluxes of plasma particles and electromagnetic radiation of 500 kWm–2 and 10 MWm–2 respectively. Special solutions are required and have been proposed for these areas, but they need further development and testing in reactor conditions.

Various materials are known that may be able to remain robust under such bombardments (it is in any case foreseen that the most strongly affected components will be replaced periodically). However, before a fusion reactor can be licensed and built, it will be necessary to test the materials for many years in power station conditions. The only way to produce neutrons at the same rate and with essentially the same distributions of energies and intensity as those that will be experienced in a fusion power station, is by constructing an accelerator-based test facility known as IFMIF (International Fusion Materials Irradiation Facility). Further modelling and proxy experiments (e.g. using neutrons produced by fission and by spallation sources) can help identification of suitable candidate materials. But they cannot substitute for IFMIF, and neither will testing in ITER be sufficient, because i) the neutron flux will only be ~30% that in an actual fusion power station, which will be larger in order to produce several GWs of fusion power, and ii) as an experimental device, ITER will only operate for at most a few hours a day, while — like a power station — IFMIF will operate round the clock day-in day-out.

IFMIF, which will cost ~€1 billion, will consist of two 5 MW accelerators that will accelerate deuterons to 40 MeV (very non-trivial devices). The two beams will hit a liquid lithium target that will produce neutrons, stripped out of the deuterons, with a spread of energies and an intensity close to that generated in a fusion reactor over a small volume in which samples of candidate materials will be placed.

### Power Plant Studies

The most recent, and comprehensive, power plant conceptual study was completed in 2005, in the framework of the European Fusion Development Agreement. This study provided important results on the viability of fusion power, and inputs to the critical path analysis of fusion development described below. The study assumed that the first fusion power stations will be based on a conventional (ITER/JET-like) tokamak (rather than stellarators — see footnote 4, or less conventional Spherical Tokamaks — see footnote 7). This will almost certainly be the case, unless ITER produces major adverse surprises.

Four models (A–D) were studied as examples of a spectrum of possibilities. Systems codes were used to vary the designs, subject to assigned plasma physics and technology rules and limitations, in order to produce an economic optimum. The resulting parameterisation of the cost of fusion generated electricity as a function of the design parameters should be used in future to prioritise research and development objectives.

The near-term models (A and B) are based on modest extrapolations of the relatively conservative design plasma performance of ITER. Models C and D assume progressive improvements in performance, especially in plasma shaping, stability and protection of the ‘divertor’, through which helium ‘ash’ and impurities will be exhausted . Likewise, while Model A is based on a conservative choice of materials, Models B–D would use increasingly advanced materials and operate at increasingly higher temperatures (which would improve the ‘thermodynamic efficiency’ with which they turn fusion power into electricity).

The power plant study shows that the cost of fusion generated electricity decreases with the electrical power output (Pe) approximately as Pe–0.4. It was assumed that the maximum output acceptable to the grid would be 1.5 GW. Given the increase of temperature and hence thermodynamic efficiency, the size and gross fusion power needed to produce Pe = 1.5 GW decreases from model A (with fusion power 5.0 GW) to D (fusion power 2.5 GW). The cost of fusion generated electricity is dominated by the capital cost. It therefore depends very sensitively on i) the cost of borrowing money or discount rate (D), and ii) the availability of the plant (A), the dependence being ~D0.6 (for D in the range 5% to 10%) and A–0.6. The cost figures below assume 6% (in real terms) for D and A = 0.75. Achieving high availability is probably the greatest challenge that fusion will face in the future. If anything like 75% is going to be reached relatively quickly, intensified development of fusion technology will be essential, and a systems engineering approach (focussed on buildability, reliability, operability and maintainability, building on experience from fission) will have to be adopted very soon.

The generating costs estimated in the power plant study decreased from 9 Eurocents/kWhr for an early model A to 5 Eurocents/kWhr for an early model D (these costs would decrease as the technology matures). Even the model A result would be competitive with other generating costs if there was a significant carbon tax, which now effectively exists in Europe with the Emissions Trading Scheme. If acceptable and necessary, larger plants (with Pe > 1.5 GW) would be more cost effective, as discussed above.

These cost figures should not be taken too seriously in detail. The main point is that the order of magnitude is not unreasonable. The conclusion of the power plant study is that economically acceptable fusion power stations, with major safety and environmental advantages, seem to be accessible through ITER with material testing at IFMIF, and intensive development of fusion technologies.

### Fast Track development of fusion

Until quite recently it was generally assumed that intensive development of the materials and technology needed in a fusion power station would, and — prudently — should, wait until ITER has worked satisfactorily. The European Union, and the world’s fusion community, now takes the view that the probability of ITER working satisfactorily is sufficiently high, and the importance of developing fusion is so great, that it would be better to develop fusion materials and technology in parallel with building and exploiting ITER, in order that the first prototype fusion power station, which has become know as ‘DEMO’ (an abbreviation of Demonstrator), can be built as soon as reasonably possible. This has become known as the Fast Track approach (although, given the potential importance of fusion, it is disappointingly slow).

A detailed study of the time that will be needed to develop fusion was carried out at UKAEA Culham in 2004. This study assumed adequate funding for all the necessary steps, no delays in finalising negotiation of the ITER agreement and choosing a site for ITER, or in taking political and funding decisions, and that ITER will not produce any major bad surprises6 . The information that will be needed to finalise the design of DEMO was identified and estimates were made of when this information could be provided by ITER and IFMIF. With ‘just in time’ provision of the necessary information, the conclusion was that after ten years for construction of ITER and IFMIF, construction of DEMO could begin in 20 years from now, and DEMO could be delivering power to the grid in thirty years. Commercial fusion power stations would follow some ten or more years after DEMO comes into operation.

It should be stressed generally that this fast track timetable is a technically feasible proposal, not a prediction. Meeting the timetable will require a change of focus in the fusion community to a project orientated ‘industrial’ approach, as well as the necessary funding and political backing.

The Culham fast track timetable reflects an orderly, relatively low risk, approach7 . It could be speeded up if greater financial risks were taken, e.g. by starting DEMO construction before in situ tritium generation and recovery have been demonstrated at ITER. The risks could be reduced — and the timetable perhaps speeded up — by the parallel construction of multiple machines at each stage. In particular, with an Apollo project approach, it would be desirable to start building a low performance DEMO now, in parallel to proceeding in an orderly way via ITER and IFMIF to a superior DEMO; the lessons from actually constructing a DEMO, and confronting the systems engineering issues involved in building a real power station, would be invaluable.

### Concluding Remarks

The world faces a very serious energy challenge, or perhaps more accurately a ‘looming energy crisis’.

Fossil fuels will become scarce in the second half of this century. Meanwhile, they will be burned, so developing and deploying Carbon Capture and Storage will be essential if the world is serious about limiting climate change.

Solar power, nuclear fission (meaning fast breeder reactors in the long term) and fusion are the only options that can provide a large fraction of the world’s energy that is currently provided by fossil fuels. Improved efficiency in the use of energy will be vital, but it is unlikely to lower total demand given the rate at which the world’s population is growing, and the need for more energy to improve living standards in the developing world. Other renewables (wind etc.) can and should also play an important role, but on their own they will not be sufficient to satisfy future needs.

With so few options, development of fusion power is essential, even if success is not certain. According to the Financial Times (25/1/04):

Even if ITER runs well over budget, its spending level is unlikely to exceed $1 bn per year, a small price to pay for a reasonable chance to give the world another energy option for a time when it will no longer be possible to burn fossil fuels on the profligate scale of the earlier 21st century. I think this is right, but it leads to the questions what is the chance, and is it true, as claimed by Artsimovich, that «Термоядерная энергия будет получена тогда, когда она станет необходима человечеству» ? («Fusion will be ready when society needs it»). Assuming no major surprises, an orderly fusion development programme — properly organised and funded — could lead to a prototype fusion power station putting electricity into the grid within 30 years, with commercial fusion power following some ten or more years later. Fusion could therefore be ready when needed to replace fossil fuels, and could play an important role in the energy mix in the second part of the century. Success in developing fusion as an effective large scale source of power on earth is not guaranteed, although I think the probability of success is high. However, given the magnitude of the energy challenge, and the relatively small investment that is needed on the ($4 trillion per annum8 ) scale of the energy market, accelerated/fast track development of fusion is fully justified in view of its enormous potential. With so few other options available to provide the world’s power as the availability of (and willingness to use) fossil fuels decreases, we cannot afford not to develop fusion power.

1 Детская энциклопедия. М., Педагогика, 1973, т. 3, с. 381 (Children’s Encyclopaedia, Moscow, Pedagogica, 1973, Vol 3, page 381). Artsimovich lived from 1909 to 1973.

2 The figures for Hydro and ‘The rest (wind etc.)’ are ‘thermal equivalents’, i.e. they represent the amount of thermal energy (as produced by burning fossil fuels) that would be needed to generate an equal amount of electricity.

3 MeV is a unit of energy: 1 MeV is the energy that a proton or electron would acquire if accelerated through a million volts. A chemical reaction, such as occurs when coal is burned, typically liberates 1 eV, i.e. ten million times less that the fusion reaction (1).

4 Tokomak is an acronym derived from ТОроидальная КАмера с МАгнитными Катушками (Toroidal Chamber with a Magnetic field). Tokamaks, which were proposed by Igor Tamm and Andrei Sakharov and first developed by Lev Artsimovich, consist of a solenoid bent into the form of a torus (doughnut). The magnetic fields produced by i) the solenoid and ii) a large electric current which is driven through the plasma around the torus, combine to form a helical magnetic field, which spirals (slowly) around the torus. This field confines the plasma, i.e. holds it away from the walls. The current, which is needed to produce the helical magnetic field, is produced initially by transformer action. One of the big challenges for fusion will be to sustain the current for long periods. It is proposed to do this by driving the current with the electromagnetic and particle beam heating systems described in the text. An alternative is to build magnets that produce a helical field directly, thereby avoiding the need to drive a current through the plasma. This is the basis for devices called stellarators, which are less developed than tokamaks, and have proved much harder to build.

5 The result of a full analysis (which is supported by semi-empirical scaling laws that interpolate rather accurately between results from machines with very different sizes, magnetic fields and plasma currents) is that confinement time improves with the linear dimension L like LP with p closer to three than two.

6 The specific target dates given in the Culham study can no longer be met because the first two assumptions have not been fulfilled. More time was needed to complete the ITER negotiations and choose the site than anticipated in 2004, and setting up the ITER Organisation and reviewing the 2001 design (which was frozen when the site negotiations began) has also taken longer than expected. Furthermore, increases in the cost of ITER are constraining funding for other activities, and have jeopardised the prospects for proceeding as fast as technically possible. Nevertheless, the general conclusions of the study, as reported above, remain valid (with today as the starting date), subject however to the same assumptions and caveats.

7 The study assumed that it will be possible to obtain a nuclear operating license for DEMO on the basis of data from IFMIF and ITER. There is a question whether this will be possible without first building a relatively small ‘driven’ fusion device (which would consume more power that it generates, but otherwise produce power station conditions) in which whole components of future fusion power stations (not just samples of materials, as in IFMIF) could be tested in a full fusion power station environment (including heat and particle, as well as neutron, fluxes). Construction of such a Component Test Facility (CTF) would slow down fusion development if (which I doubt) it is needed to license DEMO. In any case a CTF will be highly desirable, if not absolutely essential, in parallel to and beyond DEMO in order to optimise the design of future power stations. The Spherical Tokamak, pioneered at Culham in the UK (which is a compact device, shaped more like a cored apple than a doughnut), is a particularly promising candidate on which to base a CEF, although further work is needed to show that this would be possible.

8 This is a rough estimate of the current size of the market. The amount that the world pays for energy is very dependent on the cost of oil, which fluctuates wildly. When the price of oil was $140/barrel in 2008, the total energy market was ~$7 trillion.

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