Author: Graham Brennan, transport programme manager, SEAI

Introduction

The European Union has set itself the target of reducing emissions from the electricity sector to near zero by the year 2050. The expectation is that this will be achieved through a combination of energy efficiency, renewables, carbon capture and storage and nuclear fission power plants. Nuclear fission power has been with us since the 1950s and numerous catastrophes over the years have shaken the public’s confidence and support for this option. Whereas fission is the act of splitting heavy unstable atoms apart, nuclear fusion is the process of joining lighter nuclei together. The extremely high temperatures needed to join the two nuclei together have been demonstrated in short bursts in test reactors. While these reactors consume more energy than they produce no reactor has yet been made which can contain a continuously operating reaction while delivering a net gain in energy. But now, work is underway to bridge this gap. Part 1 of this article explores the theory, safety and environmental benefits of fusion compared to its rival fission. Part 2 discusses the progress made with experimental reactors, the current move to commercial scale and finally derives a timeline for the first commercial fusion power station to begin operation.

Fusion and fission

In the early days of atomic research, scientists noticed something very odd. When the mass of an atom was measured, this mass was lower than the combined mass of all of the individual protons, neutrons and electrons when measured separately. Somehow, when all of the particles joined together in a nucleus, they lost some of their mass. This difference was referred to as the mass defect problem. Only when Einstein’s famous E = mc2 equation was applied and the fission process of uranium was observed and measured was it finally understood that the mass was being converted into energy. [caption id="attachment_26487" align="alignright" width="300"]Fusion 1 Fig. 1 Binding Energy Curve showing when Fusion and Fission can occur (click to enlarge)[/caption] Using the mass defect and Einstein’s equation, scientists established how much energy is required to remove a nucleon (i.e. either a proton or a neutron) from the nucleus of an atom. This is called binding energy and when it was plotted for all of the elements a peak appeared where iron was reached (see Fig. 1). Iron has one of the most tightly bound nuclei and highest mass defect per nucleon. Therefore atoms that are lighter than iron, such as hydrogen, tend to fuse together to form more stable nuclei (i.e. fusion) tending up in size towards iron. Atoms that are heavier than iron tend to split apart to form individual fragments which are more stable tending down in size towards iron (i.e. fission). In both cases a small portion of mass is converted into energy as the nucleus becomes more optimally bound which can be calculated by subtracting the binding energy of the starting elements from the resulting products. In order to fuse two nuclei together, great kinetic energy is required to overcome the positive repulsion forces of the protons which would cause the particles to deflect away from each other. This implies enormous heat must be supplied to the reactants, so much so that the atoms lose their electrons and become ions whizzing about in a state referred to as plasma. For example, when two hydrogen nuclei get close enough together, the strong nuclear force takes control binding the two ions together to form a new helium nucleus and ejecting a neutron particle (Fig. 2). The energy from fusion is contained in the kinetic energy of the resulting products which is split 20 per cent for the helium nucleus particle (called an alpha particle or alpha radiation) and 80 per cent for the neutron. In a fusion reactor power station, the neutron’s energy would be captured by water and thus generate steam for a turbine. One of the easiest combinations of fuels used by researchers to achieve fusion is Deuterium (an isotope of hydrogen with 1 proton and 1 neutron) and Tritium (1 proton and 2 neutrons). This is referred to as a D-T reaction. [caption id="attachment_26481" align="alignright" width="269"]fusion-1 Fig. 2 The D-T fusion reaction – fusion energy is stored in the form of kinetic energy of the products[/caption] The wonderful prize offered by nuclear fusion becomes apparent when we compare the phenomenal energy yield with those of electronic reactions such as those which occur in a combustion process:
  • D-T Hydrogen Fusion yields 17.6 MeV per reaction => 339,000,000 MJ/kg of hydrogen
  • U235 Uranium Fission yields 215 MeV per reaction => 88,000,000 MJ/kg of uranium
  • Methane = 56 MJ/kg
  • Diesel = 48 MJ/kg
In 2014, Ireland used 13.3 million tonnes of oil equivalent primary energy. This fuel energy would be equivalent to the fusion energy available from 1.6 tonnes of hydrogen D-T fuel! (For reference, an equivalent fission reactor would require 6.3 tonnes of Uranium 235 per annum) Deuterium (D or 2H) is freely available in the waters of the world. Semi-heavy water (i.e. a water molecule containing one D atom) exists naturally in water at the rate of 1 molecule in 3,200. It is readily extracted via a distillation and electrolysis process. Tritium (T or 3H) however is extremely rare on earth. It is used to cause the dials on your watch glow green at night time and is mildly radioactive with a half-life of just over 12 years. The radiation it emits is called a beta particle which can only travel a short distance in air and cannot pass through the skin. It is generally manufactured in nuclear reactors where neutrons are used to bombard lithium. A fusion reactor would therefore produce its own tritium fuel once the reactor starts off. The uranium fission reaction is initiated by a neutron, which leads to the release of two more neutrons and so on thus creating a chain reaction. If this happens in an uncontrolled way, the energy release becomes unstoppable leading to catastrophic failure of the reactor core such as happened in the Chernobyl disaster. Fusion reactors however operate with vacuum pressures near the wall increasing to atmospheric pressures at the core of the plasma. Fusion can only be maintained if the conditions of temperature and density can be maintained in a stable manner therefore it cannot run away in an uncontrolled manner like a chain reaction process. If a reactor wall fractured, air would likely enter the core and cause the finely balanced fusion process to stop rapidly. A uranium fission reactor sized to match all of Ireland’s primary energy demand would produce 158 tonnes of radioactive spent fuel rods per annum. Much of the fissioned products remain highly radioactive for 100,000 to one million years later requiring expensive storage methods. Fusion on the other hand produces small quantities of helium nuclei (alpha particles) and neutrons. As for a fission reactor, the neutrons will irradiate the walls of the reactor over time making them dangerous areas to work in should a repair be required. However, this radiation is deemed to be short lived with a half-life of 50 years and can be stored relatively easily once the reactor is decommissioned at the end of its life. Note that water which is used to capture the kinetic energy of the neutron is very difficult to activate. The alpha particles themselves, if allowed to escape, would be a concern if ingested by animals, however, they quickly cool when leaving the reactor and become helium atoms which forms an inert gas. A D-T fusion reactor meeting the total primary energy demand for Ireland as discussed earlier would produce an estimated 1.3 tonnes of helium gas per annum.

Achieving fusion on Earth

Fusion happens within the core of our Sun where temperatures reach 15 million K and pressures reach 250 billion atmospheres. The Sun uses gravity to compress and confine the plasma. While we have no materials which could operate at these conditions, we can use magnetic confinement to contain the charged plasma such that it does not touch the walls of the reactor vessel. To compensate for the lower pressures and densities, we must increase the temperatures to 150million K. Following a fusion reaction, the kinetic energy of the helium ion is transferred through collision with the surrounding plasma particles causing it to heat up. If all of the helium energy can be captured by the plasma such that all external heating systems can be turned off, this moment is referred to as plasma 'ignition'. It has never been achieved in any reactor. However, when we consider that the fleeing neutrons can be used to generate steam to create electricity, we will see later on that achieving even “partial” ignition is more than sufficient for our energy goals. The criterion for ignition is therefore described as: Plasma fusion heating rate (from helium ions only) >= Plasma heat loss rate [caption id="attachment_26411" align="alignright" width="300"]aaafusion2 Fig. 3 Fusion triple product curves showing the locus of plasma ignition points for various fusion fuel combinations and temperatures. An entirely self-sustaining plasma will exist when conditions exceed these curved lines[/caption] Following from this equation, the minimum conditions for ion density (n), temperature (T (measured in electron volts)) and energy confinement time (te) to ensure ignition can be determined. Referring to Fig. 3, it can be seen therefore that at about 150 million K, the D-T fuel combination presents one of the least onerous set of conditions to achieve in a reactor making it the main focus of researchers around the world. The power of the reactor is related to the pressure which can be achieved. Given that magnets are used to confine the plasma, the Beta ratio measures the ratio of Plasma Pressure to Magnetic Field Pressure. For a typical test reactor the plasma pressure at the core of the plasma is about 1 atmosphere. The density of the plasma is very low in the region of 20x10-5 g/m3. In this case a Beta of 0.01 to 0.05 (or one per cent to five per cent) is typical. Much higher Beta values are possible, however, the plasma quickly becomes unstable and quenches itself against the walls of the reactor. One of the most common electromagnetic confinement reactor designs is referred to as the tokamak shown in Fig. 4. These reactors use two sets of magnets: One in the polar direction (poloidal or north-south) and the other in the toroidal (east-west) direction. The first set causes the charged plasma particles to move in a vertical plane with a circular motion and the second set moves the plasma around the torus resulting in a helical path on each revolution. This provides good confinement of the particles and prevents the ions and electrons from 'mixing out'. To give some comparisons of the fields involved, the magnetic flux density of a permanent magnet is approximately 0.5 T, experimental fusion reactor electromagnets can generate 3.5 T and the superconducting magnets for the planned large scale fusion plant (ITER) will generate 12.5 Tesla. A lot of electrical current is required to create these powerful fields over such large areas enclosed by the reactor. This in fact becomes a limiting factor on operation of the test reactors using standard electromagnets as the coils need to be cooled to protect them from damage. [caption id="attachment_26478" align="alignright" width="300"]Fusion 2 Fig. 4 Tokamak reactor schematic showing the poloidal and toroidal magnets creating a helical flow pattern in the plasma[/caption] Heating is performed in three stages. On start-up, the magnets are turned on and the reactor is pumped down so that a vacuum is formed in the chamber. The tokamak reactor is designed like a shell transformer. The D-T fuels are injected and ohmic inductive heating is initially applied to heat the reactants to 20 million K. In the second phase of heating, Neutral Beam Injection is used to fire fuel atoms into the reactor at one per cent of the speed of light. This method can heat the plasma to the necessary 150 million degrees depending on the amount of fuel which is ultimately required. Microwave heating is the third method and by varying the frequency the plasma can be heated locally and at various depths which improves plasma stability. A plasma fluid is like a trapped lightning bolt except far hotter. It is highly turbulent and once up and running, the plasma is vulnerable to the formation of small eddies which grow in size until it kinks over on itself becoming unstable and hitting the walls of the reactor. This has a number of consequences, firstly it cools the plasma and stops the fusion process, secondly it burns the walls of the chamber and introduces impurities into the reaction impeding the fusion process and lastly it throws enormous amounts of energy into the walls of the reactor leading to possible damage. These are known as disruptions and must be kept to a minimum for any commercial reactor.

Conclusion

So we should now have an understanding of the enormous promise held by fusion and the significant challenges involved in controlling and containing a fusion reaction here on earth. In Part II we will look at what has been achieved to date with fusion reactors and try to make a realistic estimate of when fusion could enter commercial service.