With Cadarache finally selected as the site for ITER (International Tokamak Experimental Reactor), three leading members of the fusion research community reflect on progress to date and the steps needed to get to a working fusion based power station.
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 (pictured above), which has produced 16 MW of fusion power, and at other facilities, have shown that fusion can be mastered on earth.
However, controlled fusion power will not be available as soon as we would like. Current projections show that building a fusion power station is entirely feasible, and it looks as if the cost of fusion power will be reasonable. But time is needed to further develop the technology in order to ensure that it would be reliable and economic, and to test in power station conditions the materials that would be used in its construction.
Assuming no major surprises, a well-paced 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 10 years later.
A fusion reaction is a nuclear reaction between light atomic nuclei, producing a heavier nucleus with the release of considerable energy. The reaction of primary interest as a source of power on earth involves two isotopes of hydrogen, deuterium (D) and tritium (T), fusing to form
helium (4He) and a neutron:
D + T ? 4He + neutron + energy (1)
Energy is liberated because helium-4 is very tightly bound: it takes the form of kinetic energy, shared 4:1 between the neutron and the helium-4 nucleus.
To initiate the fusion reaction (1), a gas of deuterium and tritium must be heated to over 100 million °C (M°C) – ten times hotter than the core of the sun. Above a few thousand degrees, the energy imparted by inter-atomic collisions strips the electrons off 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 suffer a mutual electrostatic repulsion when they approach one another. However, as the temperature – and hence their velocities – rises, they come closer together before being pushed apart. When the temperature exceeds 100 M°C, the more energetic deuterons and tritons can approach each other sufficiently closely that the strong nuclear force takes over and fusion occurs.
There are two challenges. The first is to heat a large volume of D and T gas to over 100 M°C, while preventing the very hot gas from being cooled (and polluted) by touching the walls: as described below, this has been achieved using a ‘magnetic bottle’ known as a tokamak. The helium nuclei that are produced by fusion (being electrically charged) remain in the ‘bottle’, where their energy serves to keep the plasma hot. The neutrons, however, are electrically neutral and escape into, and heat up, the walls: this heat is then used to drive turbines and generate electricity.
The huge flux of very energetic neutrons can damage the container as can the large heat loads produced. This leads to the main outstanding challenge, which is to make a container with walls sufficiently robust to stand up, day-in day-out for several years, to this neutron and heat bombardment.
The tiny amount of fuel that is needed is one of the attractions of fusion. The release of energy from a fusion reaction is ten million times greater than from a typical chemical reaction, such as burning a fossil fuel. Correspondingly, while a 1 GW coal power station burns around ten thousand tonnes per day of coal, a 1 GW fusion power station would burn only about 1kg of D+T per day.
Deuterium is stable and can be easily, and cheaply, extracted from water. Tritium, which is unstable and decays with a half-life of ~12 years, occurs only in tiny quantities naturally. But, as described below, it can be generated in situ in a fusion reactor by using neutrons from the fusion reaction impacting on lithium to produce tritium in the reaction:
neutron + lithium ? helium + tritium. (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. The lithium from one lap-top battery, combined with the deuterium from 45 litres of ordinary water, will – allowing for inefficiencies – provide 200 000 kWh, which is the UK’s current per capita electricity production for 30 years.
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 of around 1000 m3 (actually contained in a toroidal chamber – see later). D and T are fed into the core and heated to over 100 M°C, a temperature routinely achieved at JET, as described below. 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.
In the blanket, the neutrons encounter lithium and produce tritium through reaction (2). There are various competing reaction channels, which do not produce tritium directly, but some of them produce
additional neutrons that can then produce tritium (the production of additional neutrons can be enhanced, eg 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 (the International Tokamak Experimental Reactor), as described below.
The neutrons will also heat up the blanket, to around 400 °C in so-called ‘near-term’ power plant models that would use relatively ordinary materials, and up to perhaps 1100 °C in models that use advanced materials such as silicon carbide. These more advanced materials will allow higher temperature operation and hence higher thermodynamic efficiency in generating electricity from fusion. The heat will be extracted through a primary cooling circuit, which could contain water or helium. This primary circuit in turn will transfer heat to a secondary circuit to drive turbines.
Detailed studies of the design of possible fusion power plants are carried out around the world with broadly similar outcomes. These incorporate a range of possible materials and different assumptions about the detailed operation and maintenance strategies. The studies show that economically acceptable fusion power stations, with major safety and environmental advantages, can be derived based on present devices, assuming that the existing candidate materials are successfully tested (more on this is described later).
Advantages and disadvantages
The advantages of fusion are
• essentially unlimited fuel;
• no CO2 or air pollution arising from operation;
• major accidents are impossible;
• ‘internal’ costs (ie costs of generation) look reasonable and ‘external’ costs – the impact on health, climate and the environment – will be essentially zero;
• it will meet a vital need.
There is enough deuterium to support current projections of world energy demand for millions of years, and easily mined lithium for several thousand years (after which it could be extracted from sea water).
A key safety point is that, although it will occupy a large volume, the amount of tritium and deuterium in a fusion reactor will be tiny: the weight of the hot fuel in the core will be about the same as ten postage stamps. Because the gas will be so dilute, there will be no possibility whatsoever of a dangerous runaway reaction. Furthermore, there is not enough energy inside the plant to drive a major accident and (especially in the reactor itself) not much fuel available to be released to the environment if an accident did occur.
What are the hazards? First, although the main product of fusion (helium) is not radioactive, the blanket will become activated when struck by the neutrons also produced in the reactions. However, with the use of first wall materials selected for low activation properties, the
radioactivity decays away with half-lives of the order of 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 analyses show that melting would be impossible.
Second, tritium is radioactive, but again the half-live is relatively short (12 years) and the possible hazard is not very great. In any case it will be easy to design reactors and the tritium recycling plant 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.
Status of fusion research
The tokamak (a contraction of a Russian phrase meaning toroidal chamber with a magnetic coil) is the most promising magnetic configuration for ‘bottling’ fusion. The basic layout of a tokamak is shown in Figure 2. Three parameters control the fusion reaction rate:
• The plasma temperature (T), which must be in the 100-300 M°C range.
• The plasma pressure (P). The fusion power increases strongly with pressure.
• The ‘energy confinement time’ (tE) defined by:
tE = energy in the plasma divided by power supplied to heat the plasma
tE measures how well the magnetic field insulates the plasma. It is obvious that the larger the tE, the more effective a fusion reactor will be as a net source of power.
It turns out that the ‘fusion product’ P (in atmospheres) x tE (in seconds) determines the energy gain of the fusion device, and this must be ten or more for a fusion power station to be viable. The ‘fusion performance plot’ (Figure 3) of PtE versus T, which shows data points from different tokamaks, indicates the substantial progress towards power station conditions that has been achieved in recent decades.
How tokamaks work
The basic principles of the tokamak can be summarised as follows:
• A small amount of gas is injected into the vacuum chamber after the toroidal field has been turned on.
• The primary transformer circuit (Figure 2) generates an electric field in the gas, which breaks down to produce the single-turn secondary of the transformer.
• The resulting electric current (around 5 MA in JET) heats the plasma to about 30 M°C and produces a ‘poloidal’ magnetic field which, combined with the ‘toroidal’ magnetic field produced by the external coils, ‘confines’ the plasma, ie provides it with good thermal insulation.
• Additional heating power is supplied to the plasma to achieve fusion temperatures, by mechanisms that serve also to drive the current in the plasma when the flux swing of the transformer is exhausted.
• Plasma heating schemes, comprising microwaves (in the tens of MHz to hundreds of GHz range) and powerful beams of neutral fuel-species particles, provide tens of MW of heating power.
A number of special electrical power systems, some unique, others still under development, are required in order to satisfy a number of demanding conditions. For example:
• The magnets store extremely large magnetic energies (say 40 GJ for the main toroidal field) but the reactor assembly must be compact. It is therefore necessary to dump the stored energy arising in a
superconductor quench somewhere other than in the coils themselves. This requires a steady-state high-current capacity superconducting switch in each circuit, which can be triggered (by a quench-detecting circuit) to rapidly commutate the current (of some tens of kA) into a high voltage resistor capable of absorbing most of the quench energy.
• The plasma within the toroidal vacuum chamber is vertically stretched to produce an elliptical cross-section (which is found to greatly improve the plasma confinement) but this makes the vertical position strongly unstable. Although induced currents in the conducting structures around the plasma slow down the unstable movement, it is still necessary to provide high power, fast feedback amplifiers to achieve dynamic control. Various semiconductor
devices have been used to create these amplifiers (eg in JET a five-rail thyristor chopper). A new IGCT (insulated gate commutated thyristor) arrangement is currently proposed for JET.
• The shape and position of the plasma must be determined in real time in order to produce feedback signals for the power supplies. This requires solving a complex non-linear equation in a matter of milliseconds so as not to introduce destabilising delays into the feedback loop. Tokamaks have used hybrid analogue-digital neural
networks, fast look-up tables of approximate solutions, and as Moore’s Law has relentlessly progressed, real-time solutions of the actual magneto-hydrodynamic equilibrium equation.
JET is the closest existing tokamak to a power plant both in size and in performance (in many, if not all, parameters). It is also the only tokamak in the world that can be operated with tritium; although most of the time deuterium alone (which has essentially the same plasma properties as a D–T mixture, but a much lower fusion cross section) is used – as in other tokamaks – to keep activation to a minimum.
JET holds the world record for fusion energy and power production. The record energy pulse produced was 4 MW for five seconds. A record fusion power, of 16 MW, was also produced for a shorter time.
Next steps – ITER and IFMIF
Two intermediate facilities are necessary (which can and should be built in parallel) before the construction of a prototype fusion power station, fully equipped with turbines etc, that will supply power to the grid. These are:
ITER (the International Tokamak Experimental Reactor)
ITER, which is shown in Figure 4, will be about 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 a plasma heating input around 50 MW.
JET 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 heat up. While steady state cooling of copper coils is of course possible, the power demand for an ITER-sized machine would be in the range of hundreds of MW. This would not be acceptable in a fusion power station. ITER will therefore be equipped with superconducting coils that will allow indefinite operation if the plasma current can be kept flowing (the design goal is above ten minutes). Superconducting tokamaks already exist, and others are under construction, but superconducting coils have not so far been used in tokamaks as large as JET let alone ITER. ITER will also contain test blanket modules which, for the first time, will test features that will be necessary in power stations, such as the in situ generation and recovery of tritium.
A major goal of ITER is to show that existing plasma performance 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 on existing machines as well as ITER). The main goals are:
• Demonstrating that large amounts of fusion power (10 times the input power) can be produced in a controlled way, without over-heating the surrounding materials. ITER is designed to tolerate this but with the increased power levels it remains a big challenge.
• Finding ways of pushing the plasma pressure to higher values 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.
• Demonstrating that continuous (‘steady state’) operation, which is economically and technically highly desirable if not essential, can be achieved without expending too much power.
Prototypes of all key ITER components have been fabricated by industry and tested. Construction of ITER, which will cost r4.5 billion and be undertaken by a consortium of the EU, Japan, Russia, USA, China and South Korea, will begin as soon as administrative arrangements have been put in place, now that Cadarache in France has been selected as the construction site.
IFMIF (the International Fusion Materials Irradiation Facility)
Those ‘structural’ materials from which fusion power stations will be built that are close to the plasma will be subjected to many years of continuous bombardment by neutrons. On the basis of experience of neutron-induced damage in fast breeder reactors, it is known that there exist good candidate materials that may meet the target of a useful lifetime of around five years before the materials would have to be replaced. This must be confirmed in the higher energy neutron flux appropriate to fusion. The much higher energy fusion neutrons will, however, initiate nuclear reactions that can produce helium inside the structural materials, and there is a concern that the helium could accumulate and further weaken them: this also needs to be studied. 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). Coupled with modelling activities this materials test programme is an important part of making fusion electricity production a reality.
Timetable for a prototype
A detailed study of the time that will be needed to develop fusion has recently been completed at UKAEA Culham. It was assumed that construction of ITER and IFMIF both begin in the immediate future. The information that will be needed to finalise the design of the first prototype fusion power station, which has become known as DEMO (for Demonstrator), was identified and estimates were made of when this information will be provided by ITER and IFMIF. Assuming “just in time” provision of the necessary information, this led to the construction timetable for DEMO shown in Figure 5. Commercial fusion power stations would follow some ten or more years after DEMO comes into operation.
The Culham fast track timetable reflects an orderly, relatively low risk, approach. It could be speeded up if greater financial risks were taken, eg by starting DEMO construction before in situ tritium generation and recovery have been demonstrated. The risks could be reduced – and the timetable perhaps speeded up – by the parallel construction of multiple machines at each stage.
Figure 5 assumes that ITER and IFMIF are approved at the same time, which is highly desirable but may not be realistic (some delay in IFMIF construction might however be tolerable without comprising the end date). It should be stressed generally that the fast track model is a technically feasible plan, not a prediction. Meeting the timetable will require a change of focus in the fusion community to a project orientated ‘industrial’, approach, accompanied of course by the necessary political funding and backing.
In addition, alternative magnetic confinement configurations (eg ‘spherical tokamaks’ and ‘stellerators’) are under development and will form an important activity in parallel with the main strands of the Fast Track approach. At Culham, the MAST (Mega Amp Spherical Tokamak) – see Figure 6 – investigates the spherical tokamak concept which confines the plasma in a compact configuration around a narrow central column. Although spherical tokamaks are in their infancy compared to JET and ITER, this design offers potential advantages over conventional tokamaks (in terms of plasma performance and efficiency) and could form the basis for later prototypes and second generation fusion power stations. In the meantime, they provide additional insights into the behaviour of plasmas and fusion technology which feed into the mainstream, conventional tokamak line.
Fast track development justified
Given the remarkable progress that has been achieved in recent decades, we are confident that fusion will be used as a commercial power source in the long term. It requires greater optimism to believe that fusion will be available commercially on the time-scale outlined above, which would require appropriate funding and a properly
focussed and managed programme, with no major surprises. However, given the magnitude of the energy challenge and the relatively small investment that is needed when compared with the $3 trillion pa energy market, we are absolutely convinced that accelerated/fast track development of fusion would be fully justified.
Acknowledgement: This work was funded by the UK Engineering and Physical Sciences Research Council and Euratom.
For further information on the aspects of fusion covered in this article,
see: www.jet.efda.org, www.iter.org, www.fusion.org.uk, and Fusion, the
energy of the universe, G McCracken and P Stott, Elsevier (2005).