This week, we look at how the nuclear physics that takes place inside stars can be applied to create fusion technologies here on Earth. We’re joined throughout the week by researchers from the Fusion Centre for Doctoral Training. This is a collaboration between the Universities of Durham, Liverpool, Manchester, Oxford and York and includes plasma physicists, engineers, and materials scientists - all crucial if fusion energy is going to be a sustainable power source for the future.
4.1 Fusion Power in Stars (Questions)
4.2 Fusion Power on Earth (Questions)
4.3 Magnetic Confinement Fusion (Questions)
4.4 Virtual Tokamak (Online Simulation)
4.5 Virtual Tokamak (Questions)
4.6 Materials Science (Questions)
In this module, we will look at the technical details of how fusion, similar to that taking place in the Sun, can be developed into a viable energy resource for generating electricity. But why is this necessary? Watch the short introduction to fusion below, produced by members of the Fusion CDT:
To understand how energy is released in nuclear reactions, consider the graph of nuclear binding energy per nucleon in figure 1.
Figure 1: A graph of the average binding energy per nucleon.
For nuclei heavier than iron (Fe-56), binding energy per nucleon decreases as nucleon number increases. By splitting a heavier nucleus into two smaller nuclei, you have increased binding energy per nucleon and energy is released. This is known as nuclear fission and is the process used in current nuclear power stations. You can find out more about current nuclear technologies in the ‘Find Out More’ section at the bottom of the page.
For nuclei lighter than iron, the binding energy per nucleon increases as nucleon number increases. By fusing two lighter nuclei to create a heavier nucleus, you have increased binding energy per nucleon and released energy. This is known as nuclear fusion.
Notice the difference in the gradients of the graph. The steeper gradient in the low-mass region indicates that larger amounts of energy can be released per nucleon through fusion than through fission (where the gradient is less steep). You can also see this in the Nuclide Chart.
In both cases, the energy released originates from mass. This decreases the overall mass of the nuclei. You can calculate how much energy is released using the famous equation E = mc2 where E is the energy released, m is the decrease in mass from the reactant nuclei to the product nuclei, and c = 3.00 x 108 ms–1 is the speed of light.
In the Sun, a three-step process called the proton-proton chain reaction generates energy:
Two protons fuse to form deuterium;
The deuterium captures a third proton to form helium-3;
Two helium-3 nuclei fuse to create helium-4 (freeing two protons).
The process is shown in figure 2.
Figure 2: Proton-proton chain reaction
Doctor C, CC BY-SA 4.0, via Wikimedia Commons
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Fusion Power in Stars
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Although there are lots of different fusion reactions in nature, only a few can be used to produce a useful amount of energy here on Earth. One way to compare different fusion reactions is to measure their cross-section, σ. This is a measure of how easy it is for the two reactants to hit each other and for the required reaction to take place. This can be expressed as the 'area' you have to hit for the reaction to happen.
For a real life example, the cross-section for hitting a barn with a ball would be the area of the side of the barn. On the other hand, if you require a specific reaction to take place, such as 'the ball goes into the barn', the cross-section will depend on the specific reaction and the properties of the target and projectile. For example, if you require the ball to get into the barn, the cross-section will be the size of the barn door if the door is open. If the door is closed, perhaps the cross-section is only the size of the barn window, and this might additionally require that you throw the ball hard enough to break the window - i.e. the cross-section will depend on the energy of your projectile.
It is exactly the same in nuclear physics: when deuterium (D) hits a target, the cross-section for a fusion reaction will depend on whether the target is deuterium (D) or tritium (T), as shown in figure 3. Remember that deuterium (D) is an isotope of hydrogen with one neutron (H-2) and tritium (T) is an isotope of hydrogen with two neutrons (H-3).
The cross-section also depends on the energy of your projectile, since you have to overcome (i.e. 'break through') the repulsive force between the two positively charged particles. In the graph in figure 3, you can see how the cross-section, σ, changes with energy (here shown as the centre-of-mass energy between the two reactants) for a range of fusion processes. This energy comes from the thermal movement of the particles. Therefore, the energy is directly related to the temperature of the plasma. In plasma physics, it is therefore also common to refer to temperatures in terms of the typical particle energy (typically in keV), rather than the temperature (in kelvin).
From figure 3, we can see that the DT fusion cross-section is the largest at low temperatures (energies). As this ideal temperature (corresponding to the peak on the graph) is lower than for the other curves, it is easier for us to achieve. Only having to be at 150 million K (still ten times the temperature at the centre of the Sun) makes reactors cheaper and easier to build. In addition, the D-T curve sits above the other curves (and so has a higher cross-section) over the majority of the temperature range shown. Only at around 400 keV does it dip below other curves, and with current engineering constraints it is unrealistic to achieve temperatures this high anyway (equivalent to 4600 million K).
Figure 3: Fusion cross sections for various fusion reactions versus ion temperature.
Deuterium is relatively abundant, accounting for 0.02% (by number) of all hydrogen in the Earth’s oceans (in the form of H2O). A supply chain already exists for extracting deuterium from sea-water. Tritium is very rare, however. It is radioactive, decaying into helium-3 with a half-life of approximately 12 years. However, it can be ‘bred’ (made) from lithium. Therefore, the reactions utilised in fusion reactions on Earth are:
Deuterium (H-2) fuses with tritium (H-3) to produce helium-4 and a neutron;
Lithium-6 is bombarded with a neutron (produced in the first reaction). This creates an excited form of lithium-7, which decays into helium-4 and tritium (H-3).
It is important that the particles produced in the reaction are stable: producing radioactive material would leave fusion with similar environmental concerns to fission (which produces radioactive waste with very long half-lives that therefore needs storing securely). It is also notable that chain reactions cannot occur in fusion reactors, so there is no risk of a ‘melt-down’ (which can occur in fission reactors when a chain reaction becomes uncontrolled).
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Comparing Fission and Fusion
Fuel for Fusion
Deuterium-Tritium Fusion
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There are two main approaches to achieving nuclear fusion: magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). Both methods heat the reactants to approximately 150 million K, (ten times hotter than the centre of the Sun). At these temperatures, the particles are completely ionised (stripped of their electrons). This means that all the particles have a charge, apart from any neutrons produced in the reaction. When this happens, we are in a fourth state of matter called a plasma.
Inertial Confinement Fusion uses lasers to implode a small pellet of fuel, reaching incredibly high densities (1 million kgm-3, or 100 times that of solid lead) and temperatures which last for a fraction of a second. However, we are going to focus on magnetic confinement fusion in this masterclass.
Magnetic Confinement Fusion uses magnetic fields to control the plasma. It is shaped into a doughnut shape using a device called a tokamak.
The largest tokamak in the world is currently JET (Joint European Torus) at Culham Centre for Fusion Energy in Oxfordshire. In this ‘Born to Engineer’ video, Control Engineer Kim Cave-Ayland takes us inside this tokamak to find out more.
Using MCF, the fusion reaction can be sustained for tens of seconds at the moment before the temperature of the plasma becomes too low and fusion stops. The aim is to achieve steady state fusion and to produce net energy (where the total power produced is greater than the thermal power input to heat the plasma) and construction of a new fusion reactor to achieve this is underway in the South of France.
ITER will be the world’s largest tokamak and is a collaboration between 35 nations. Experiments are scheduled to begin here in December 2025. The aim is to achieve steady state fusion and to produce net energy, (where the total power produced is greater than the thermal power input to heat the plasma).
ITER has lots of information on its website: iter.org/. We particularly like exploring the ITER site in virtual reality (or just on a phone/computer screen). You can even choose what date to view the site - why not see how the building work has been progressing?
3D Virtual Tour of ITER (Optional)
Figure 5: ITER
The UK have also recently started a new programme called STEP: Spherical Tokamak for Energy Production. Find out more about STEP, ITER, and how much progress have we made towards using fusion to generate electricity in the video below. Professor Howard Wilson gives a brief history of fusion experiments to date, and gives some exciting insights as to what the future may hold.
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Born to Engineer
Breeding Tritium
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There are dozens of nuclear fusion reactors around the world. Fusion energy is being produced all the time. So why don’t we yet have power plants? Partly because we haven’t fully understood every part of the design of the plasma fuel and reactor yet. Partly because optimising one part often compromises another, and we still need to find the ideal combination in one design to generate more electricity than a reactor consumes. The equations that describe how different aspects of the design interact have been written into the IPPEX Virtual Tokamak. The activity below will allow you to investigate different tokamak designs and explore how to operate a fusion reactor.
Use this Virtual Tokamak, made by Princeton Plasma Physics Laboratory, to explore different tokamak designs for magnetic confinement fusion.
Instructions:
On the first page, you can set the design configuration for your tokamak. Several real world examples are available along the top of the screen (ITER, JET, TFTR, NSTX-U, ARC), or you can make your own decisions regarding the shape and size of your tokamak, the type of magnets used, and whether to add a lithium blanket.
Once you click CONTINUE, you will see a screen with a series of tabs across the top: VIEW REACTOR, VIEW HEAT EXCHANGE, VIEW CITY, VIEW INFORMATION. Investigate the information available on each of these tabs (which you will need later to answer questions 3.5). You will find that the VIEW HEAT EXCHANGE AND VIEW CITY tabs are greyed out and not available unless you have selected a blanket when designing your tokamak. To return to the tokamak design page, click EDIT REACTOR on the VIEW REACTOR tab.
Along the bottom of the screen, you will see three major active control levers that you can use to operate your reactor: Density (the amount of fuel filling the plasma volume), Auxiliary Power (the amount of heating energy that will be put in), and Magnetic Field (the strength of the magnetic field confining your plasma). There are also some metrics indicating the success of your reactor: Wall Health, Temperature, and Electric Power). These are combined in a single Score.
Your aim is to achieve the highest score by varying both your reactor design, and your use of the control levers.
Share your score in the poll at the top of the Question Forum and see if you can get the highest score! We will reveal our fusion expert's score at the end of the week!
Once you have investigated the virtual tokamak, test your knowledge by completing the questions below (4.5).
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Fusion Power Generation
Tokamak Performance
Tokamak Heating
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Materials Science bridges the gap between the three main sciences and engineering. Taking into account the different properties of a material, you can choose the best material for the problem. Engineering usually concerns itself with the macroscopic qualities of a material, for example how much load can it bear, whereas Materials Science looks at the microscopic scale of the material, such as the grain size. For example, if a material is strong, then what about the microstructure makes it strong? If a material is corrosion resistant under certain conditions, what are those conditions and how best can you improve the service life of that material?
Watch this video for a short introduction to the materials science needed to enable fusion technologies.
When it comes to fusion, there are a few key Materials Science properties that are very important:
Thermal properties: Within a fusion reaction, the hottest sections of the inner wall inside the reactor can reach 20MWm-2, the same heat output as the surface of the Sun! It is vital that the components which touch the plasma have a high melting point and high thermal conductivity so that heat can quickly be removed from the area.
Strength: A fusion reactor is a huge engineering project. ITER, the new fusion test reactor mentioned above, will weigh 23,000 tonnes when it is finished - as heavy as three Eiffel towers! Specialist steels which can carry such heavy loads have been created specifically for fusion reactors - your average stainless steel just won’t cut it!
Corrosion resistance: In ITER, there will be approximately one million parts integrated into the machine. Most components will be made from different materials which will be in contact. Depending on which materials are in contact, you can corrode a material faster or slower. A good example of corrosion is iron in water and air - the iron is oxidised to produce a brown rust. If not properly considered, corrosion could drastically reduce the service life of ITER.
Neutron irradiation resistance: The deuterium and tritium reaction produces high speed neutrons with energies of around 14 MeV (about 7 times higher than those produced in nuclear fission power stations). The inner wall material needs to withstand the neutron damage over many years, a huge challenge as no material has taken this high energy neutron flux before.
The neutrons collide elastically with metal atoms at high speeds and are able to knock them free from their regular lattice positions. The first atom they collide with receives the most energy, meaning it can recoil within the lattice and dislodge other atoms, disrupting the crystalline structure and causing it to become more disordered (similar to that of a glass). The typical lattice binding energy of an atom is less than 100 eV, meaning hundreds of these collisions can occur from a single neutron with an energy of MeV, resulting in a damage ‘cascade’.
The diagram in figure 7 shows a perfect crystalline lattice structure before (a) and after irradiation damage (b). The video below shows a collision cascade. A neutron hits the material and causes displacement of the atoms. At the end of the video, many atoms are displaced from their original positions.
In addition to scattering, the neutrons can also be absorbed to form highly unstable nuclei and induce fission, or they can be absorbed to change either the isotope number or increase the proton number of the incident nucleus. This process of forming new isotopes is known as ‘transmutation’ and often leads to the production of radioactive nuclei. A common example is the production of plutonium-239 when a U-238 nucleus absorbs a neutron; another is the ‘breeder’ reaction required for producing tritium from lithium as a fusion fuel.
Materials used in fusion reactors have a goal of not producing any long-lived (over 100-yrs) radioactive waste to aid in decommissioning and environmental safety. Each isotope has a different likelihood of absorbing a neutron of a given energy, known as a cross-section, which means that highly absorbing elements cannot be used.
Figure 7 (above): Left, perfect lattice structure (a). Right, irradiated lattice (b)
Figure 8 (below): Video showing a collision cascade
Ion irradiation resistance: Another product from the reaction is a helium ion (an alpha particle) which bombards the inner wall. This can cause helium bubbles within the top surface of the inner wall, producing ‘fuzz’ – hugely degrading the material surface and its properties. The image in figure 9 shows tungsten nanofuzz observed on a component from the PISCES-B tokamak. This fuzz can reduce the thermo-mechanical properties of the material and may compromise the structural integrity of the tokamak, as well as making it much less efficient.
Superconducting coils: In a magnetic confinement fusion reactor, a magnetic field is needed to contain the plasma and allow the fusion reaction to take place. A superconductor has zero resistance below a critical temperature, allowing high currents to flow to produce large fields for confinement. The selected material needs to generate a high field (11.8 T in ITER) and be commercially manufactured (100,000 km of superconducting wire has been produced for the toroidal field coils in ITER). The image in figure 10 shows a toroidal field coil for ITER, which is over 16m in length. The D-shape has superconducting coils within a steel casing.
Metals and alloys are among the key materials required to construct tokamaks to achieve the properties identified above. The atoms in metallic materials are typically arranged in crystal structures which are classified into 7 basic arrangements. Two of these structures are shown in figure 11 and lead to variations in properties in the overall component.
Part of the reason for these property variations is a result of differences in atomic stacking efficiency or ‘packing factors’, with higher efficiencies meaning a higher quantity of matter can occupy the unit cell volume and increases the solubility of alloying elements as a result.
Figure 11: Diagram showing the ‘hard spheres model’ and simplified unit cell structure of the BCC (a) and FCC (b) crystal types. Remember that the crystal lattice repeats in three-dimensions, meaning corner atoms are shared by each adjacent unit cell.
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Material Properties
Crystal Structures
Neutron Damage
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The UK Atomic Energy Authority (UKAEA) researches fusion energy and related technologies, with the aim of positioning the UK as a leader in sustainable nuclear energy, based at the Culham Centre for Fusion Energy, (in Oxfordshire). If you are interested in learning more about Fusion Energy, their YouTube Channel has a lot of content that you may want to watch!
Join Dr Kate Lancaster for her talk at the Royal Institution as she explains how the most powerful lasers in the world can be used to make some of the most extreme conditions possible on Earth, and are revolutionising science.
At the University of York, we have the York Plasma Institute Laboratories (YPI Labs) where a lot of our experimental research into plasma physics takes place. Explore the YPI Labs with this virtual tour.
Have a listen to A Glass of Seawater, a #FusionEnergy podcast. Join the team for lively conversations between PhD researchers (from the Fusion Centre for Doctoral Training) and experts from around the world!
Find out more about current nuclear power stations with this Nuclear Reactor Simulator, produced by the University of Manchester. It takes you through the process of nuclear power generation step-by-step before giving you the opportunity to control the power station yourself.
On Tuesday 13 December 2022, the US Department of Energy made the announcement that they had achieved gain in the National Ignition Facility (NIF). The NIF is a laboratory designed around inertial confinement fusion. A small pellet of cold hydrogen fuel is rapidly compressed by a set of high intensity lasers. The result is that the pellet reaches temperatures and densities high enough to cause nuclear fusion, releasing helium as an exhaust material.
The announcement of "gain" is no small news - it means that for the first time, a fusion experiment has released more energy from the reactions of the fuel than the equipment put into it.
This follows an experiment from late last year in which lasers delivered 1.9 MJ of laser light to the fuel and got 1.3 MJ back. Working on that, NIF directed a 2.05 MJ pulse at the fuel last week and it released 3.15 MJ of fusion energy. That's a gain (output divided by input) of 1.53, more than doubling the previous record and finally exceeding 1.
There's still work to be done before we get useful energy out. The lasers at NIF are using 40-year-old laser technology, with an efficiency of less than 1%, so it took 300 MJ to power them! Modern lasers can have efficiencies of 20% so this is one of many improvements to be made, and the result will be the next stepping stone towards getting more power from fusion than the entire facility needs. And that's the dream - to put that excess energy onto the grid.
Read more about this breakthrough and the further research needed to make this a reality.