Fusion Technology

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.

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. 

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).  

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. 

Virtual Tokamak

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. 

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.

• 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.