(Transcript of the lesson commentary.)
Fusion reactions
A fusion power plant is a facility that uses the process of fusing light atoms to create energy that can be used to generate electricity. For now, no such plant exists, but intensive work is being done to develop one. In the future, it could become a source of clean, emission-free and safe energy.
All stars harness fusion reactions. They glow and heat because of the fusion of atoms within them. Even our sun gives off light and heat due to the fusion of hydrogen atoms.
The reaction of deuterium with tritium is most suitable for terrestrial applications. It has a high energy yield and a relatively low ignition temperature of “only” 160 million kelvin.
There are other reactions that can be used. However, these are more difficult to ignite because they require even higher temperatures.
In deuterium-tritium fusion, two isotopes of hydrogen fuse. When they fuse, a helium atom nucleus and a neutron are produced. 17.53 megaelectron volts of energy are released.
This energy can be transferred to a cooling medium, such as water, in the power plant. We have wide experience in the energy generation from hot water. In fact, all thermal power plants, whether they are nuclear, coal or biomass-fired, use the steam cycle. The heat in the steam generator turns the water into steam, which drives a turbine that rotates an electric generator. In a fusion power plant, the energy of the stars will thus be converted into electricity by the steam turbine.
If we could master other types of fusion reactions, such as the fusion of protons with boron, we would have the possibility of more efficient power generation than the steam cycle. The proton-boron fusion reaction produces only charged alpha particles. Their flow could be used to produce electricity directly by induction or by changing magnetic fields. The efficiency of such a conversion could exceed sixty percent.
Inputs
But first, a deuterium-tritium fusion plant will be developed. Both elements are isotopes of hydrogen. Deuterium contains one proton and one neutron in its nucleus. Tritium has one proton and two neutrons.
Deuterium is quite common on Earth. It makes up approximately 0.0156 percent of hydrogen atoms — in other words, one atom out of 6,420 hydrogen atoms is deuterium. There are about forty trillion tons of deuterium in the oceans alone.
On Earth, there is enough readily and environmentally available deuterium to run fusion power plants. An average power plant would require around 200 kg of deuterium per year.
However, the other isotope, tritium, is very scarce on Earth. It is a radioactive element with a short half-life of twelve years. The world’s supply of tritium is estimated to be only thirty kilograms. Since it is assumed that an average fusion power plant would need approximately 200 kg of tritium per year, these reserves will not be sufficient.
Fortunately, there is a transmutation that allows us to produce the required tritium. Transmutation is the process by which the nucleus of an atom captures a neutron and is transformed into the nucleus of another atom. When a lithium atom captures a neutron, it changes into the desired tritium. In addition, the nucleus of the helium atom and the energy are released. Since neutrons are produced in deuterium-tritium fusion, tritium can be produced directly in the reactor of a fusion plant in this way.
Lithium is a light, soft metal. Lithium ores are widely distributed. There are approximately 50 million tonnes of proven reserves of lithium in the world and an estimated 250 billion tonnes of lithium dissolved in seawater. The average fusion power plant would need about 500 kilograms of lithium per year to produce tritium.
Waste
The waste from the fusion plant will be helium. The reaction of deuterium with tritium produces alpha particles, which are just the nuclei of the helium atom.
Helium is an inert, widely used industrial gas. It serves as a cooling medium for superconducting magnets in MRI machines or research facilities such as the LHC. It also cools the coils in fusion reactors. Helium is used as a shielding gas in arc welding, for leak detection, as a component of breathing mixtures such as trimix, for gas chromatography or as a coolant in fibre optics and semiconductor manufacturing. Last but not least, helium can be filled into weather balloons and party balloons.
The fusion plant could produce around 200 kg of helium per year. It can sell this desirable industrial gas or use it itself, because its superconducting coils require cooling with liquid helium.
The only problem remains the neutrons produced in the fusion reaction. While they are extremely useful in making tritium from lithium, not all of them do this valuable task. The others hit the reactor’s internals. The collisions of neutrons with the atoms of the vacuum chamber cause them to shift off the crystal lattice or produce radioactive isotopes. The chamber material can change its properties. This process is called radiation embrittlement. In addition, the reactor chamber gradually becomes radioactive.
Some parts of the fusion plant will therefore gradually become radioactive. Be it the blanket vacuum chamber, the divertor or the tritium breeders. Some of these parts will remain in place for the life of the plant, others will be replaced on an ongoing basis.
We will use well-proven technology from nuclear power plants to handle them. It will be a process very similar to the handling of spent nuclear fuel. The part will be removed from the fusion reactor using remote manipulators and replaced with a new one. It will then be placed in a storage pool. A layer of water will both cool it and shield it from dangerous radiation. Once the radioactivity of the components has decreased sufficiently, they can be safely disposed of or recycled.
All potentially hazardous parts will thus remain on the fusion plant site and no long-term waste disposal will be required. The fusion power plant therefore produces no harmful emissions, emits no carbon dioxide and produces no hazardous waste. Its operation is ecological and does not pollute the environment.
Mastering aneutronic fusion would completely eliminate this problem, since no neutrons are produced during this reaction. The plant would not have to deal with secondary activation at all.
Fusion reactor
The heart of the fusion power plant will be the reactor in which the fusion reactions will take place. There are a number of principles for achieving fusion. Tokamaks, stellarators, laser-controlled inertial fusion, electrostatically confined fusion and others. A fusion power plant can be based on any of these.
The most likely candidate for a fusion power plant reactor is the tokamak. These facilities have already worked with D-T mixtures and achieved fusion. Research on them has made considerable progress and the largest experimental fusion reactor that is close to completion, ITER, will also be a tokamak.
The tokamak works on the principle of magnetic confinement. Strong toroidal and poloidal coils create a magnetic field in the shape of a torus. Charged particles are held inside it and can be heated to ignition temperatures without risking damage to the chamber walls from the high temperature. For the magnetic cage to work properly, it is necessary to supplement the magnetic field of the coils with a magnetic field generated by the current flowing through the plasma. This creates a helical magnetic field that is ideal for keeping the hot plasma. Because the tokamak generates the plasma current using a transformer pulse, it is a pulse device.
The stellarator uses the principle of magnetic confinement, just like the tokamak. However, it generates a appropriate magnetic field only by means of specially shaped magnetic coils. It is therefore not a pulse device and is capable of maintaining a hot plasma for any length of time. This would predispose the stellarator to become a suitable reactor for a fusion power plant, but research on stellarators is not as advanced as tokamaks. Their time may come later.
Laser fusion has not yet reached the stage of development where a power plant could be considered. This technology creates the conditions for fusion by compressing the fuel target with powerful lasers. Tremendous temperatures and densities are briefly reached inside the target. A fusion reaction is ignited, which further heats and compresses the rest of the target. Within a fraction of a second, a large amount of energy is released. However, current research facilities based on the principle of inertial fusion are not able to repeat this process more than once a day. The production of fuel targets is also extremely demanding. Therefore, this principle is not yet considered as a source of energy for a fusion power plant.
Hybrid approaches try to combine several ways of creating and igniting the plasma. For example, magneto-inertial fusion creates a magnetically confined plasma that is further compressed by lasers. Field-reversed configuration and spheromaks work with stable rings of plasma that can be collided or further compressed to achieve fusion conditions. These approaches often try to achieve aneutronic fusion. This difficult to ignite reaction produces only high-energy charged particles. From these, electricity could be directly extracted without the need to use the inefficient steam cycle.
Operation
The chamber walls in the fusion plant will contain tritium breeders. These will produce the important fuel component tritium after neutrons strike lithium. The breeders are likely to be solid panels filled with pebbles of a mixture of lithium and neutron multiplier. The breeders will be replaced once in a while and tritium will be extracted from the used ones. A variant in which a liquid mixture of lithium and lead will flow through the breeders is also an option. The tritium can then be separated continuously.
The fuel is heated to 160 million degrees in the chamber by microwaves or neutral particle injection and fusion occurs. Some of the energy will heat the surrounding fuel and provide the necessary energy for fusion. The reaction will run itself and no further external heating will be necessary. It is the same as a fire in a fireplace, which you need to light with a match, but continues to burn on its own. In the case of fusion, the match is 160 million degrees hot.
Some of the heat generated will heat the walls of the chamber and then be dissipated by the coolant flowing through them. This is likely to be high-pressure water, making this part of the plant similar to the widespread PWR nuclear plants. The heat will be used in the steam cycle to generate electricity.
The water transfers its energy in the steam generator to the water of the secondary circuit, which turns into steam. The steam spins the turbine, which spins the generator. This non-nuclear part will be exactly the same as in all thermal power plants — nuclear, coal or biomass. Using proven and tested technology will be very beneficial for the fusion power plant.
The fusion plant will also take from the technology proven in nuclear power plants safety procedures that will allow it to safely deal with various emergencies, such as loss of cooling.
The fusion reactor itself, whether it be a tokamak, a stellarator or some other principle, works quite differently from a fission reactor and has a number of advantages over it. It contains only a minimal amount of fuel. Less than a gram at a time. It also needs to achieve very specific conditions for the fusion reaction to start and proceed. If these conditions are not perfectly met, the fusion reaction stops. In fact, immediately after the fusion reaction is terminated, the reactor can be considered shut down. A chamber heated to 2,000 °C does not generate any heat of its own, so it cools down quickly. This makes the operation of a fusion reactor even safer than that of a nuclear reactor.
Tokamak and laser-driven fusion are not yet able to operate continuously. Pulsed operation could be a serious obstacle to smooth power generation. In the case of the tokamak, a solution is already being worked on. Technologies are being developed to allow it to enter steady-state mode. The current in the plasma will be generated by current drive method or bootstrap current technology.
However, the plant could produce electricity even if the reactor operated only in pulses. Between the reactor and the steam generator, a thermal storage tank would be inserted, containing e.g. molten salts. At times when the fusion plant is operating, the tank would be heated. The heat would then be continuously extracted by the steam generator and used to produce steam. This process is used, for example, by solar thermal power plants.
The countries planning to build the demonstration fusion power plant, DEMO for short, are China, South Korea, Japan, the European Union and the United Kingdom. Other countries, such as the USA and India, are also considering building a fusion power plant. The main purpose of these projects is to demonstrate the commercial viability of fusion and to finalise the last technical details.
However, all of them assume that they could be supplying electricity to the grid by around 2050.
Clean, safe and essentially inexhaustible fusion energy will thus be added to the list of energy sources alongside nuclear and renewables.
