Fusion Power Plant as a Clean Energy Source

(Transcript of the video commentary.)

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 development is underway. In the future, it could become a source of clean, emission-free and safe energy.

All stars use fusion reactions. They shine and heat because of the fusion of atoms within them. Even our sun generates light and heat by fusing hydrogen atoms.

For terrestrial applications, the reaction of deuterium with tritium is most suitable. It has a high energy yield and a relatively low ignition temperature of “only” 160 million degrees 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. After their fusion, a helium atom nucleus and a neutron are produced. 17.53 megaelectron volts of energy are released.

This means that the fusion of one gram of a mixture of deuterium and tritium releases around 300 GJ of energy. Two orders of magnitude more than is released by burning a ton of coal.

This energy can be transferred to a cooling medium, such as water, in a power plant. The heat in a steam generator turns the water into steam, which drives a turbine that runs an electric generator. In a fusion power plant, the energy of the stars will thus be converted into electricity by a 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 flux could be used to produce electricity directly by induction or by changing magnetic fields. The efficiency of such a conversion could exceed sixty percent.

But first, a fusion power plant using the fusion of hydrogen isotopes of deuterium and tritium will be built. 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. Deuterium can be easily separated from water. So there is enough easily and environmentally available deuterium on Earth to run fusion power plants.

However, the other isotope, tritium, is very scarce on Earth. It is a radioactive element with a short half-life of twelve years. It is produced in the upper atmosphere by interaction with cosmic rays. In small quantities, it is generated by CANDU-type nuclear reactors. The world's supply of tritium is estimated to be only 30 kilograms.

As it is assumed that an average fusion power plant would need approximately 200 kg of tritium per year, these supplies 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 energy are released.

Since deuterium-tritium fusion generates neutrons, it is possible to produce tritium directly in the reactor of a fusion power plant in this way.

There are approximately 50 million tonnes of proven lithium reserves worldwide and an estimated 250 billion tonnes of lithium dissolved in seawater. An average fusion power plant would need about 500 kilograms of lithium per year to produce tritium.

In order to have enough tritium fuel, we need more tritium atoms to be produced from one fusion neutron. This is achieved by using a neutron multiplier, which generates a shower of neutrons after a collision with a neutron, thus multiplying the number of neutrons. A suitable neutron multiplier is, for example, lead or beryllium.

Worldwide reserves of beryllium are estimated at 400,000 tonnes and lead at 85 million tonnes. A fusion power plant should consume several hundred kilograms of beryllium or lead per year.

The fuel source for the fusion plant will therefore be deuterium, extracted from water, and lithium, lead or beryllium, which are extracted from the ground. All elements are relatively easily and widely available. Figuratively speaking, the plant will thus produce energy from water and rocks.

If we could master the more difficult fusion reactions, such as the fusion of boron and protons, hydrogen, of which there is a virtually inexhaustible supply in water, and boron, of which there are more than a trillion tonnes in the world, could be used as fuel.

The waste from a 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 as well as party balloons.

The output of a fusion plant could be approximately 200 kg of helium per year.

The fusion power plant therefore produces no harmful emissions, no carbon dioxide and no hazardous waste. Its operation is ecological and without any environmental impact.

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 important work. The others hit the reactor’s interior. 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. In addition, the reactor chamber gradually becomes radioactive.

Some parts of the fusion plant will therefore gradually become radioactive. Be it the blanket of the 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.

Well-proven technology from nuclear power plants will be used to handle them. It will be a process very similar to the handling of spent nuclear fuel. The radioactive parts will be removed from the fusion reactor using remote manipulators and replaced with new ones. It will then be placed in a storage pool. A layer of water will both cool it and provide protection 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. They will be safely disposed of decades after the plant has ceased operation. A fusion power plant will therefore not have to deal with any long-term storage of waste.

Mastering aneutronic fusion would completely eliminate this problem, since no neutrons are produced in this reaction. The plant would not have to deal with secondary activation at all.

The heart of a fusion power plant will be the reactor in which fusion reactions take place. There are a number of methods for achieving fusion. Tokamaks, stellarators, laser-driven inertial fusion, electrostatic confinement fusion and others. A fusion power plant can be based on any of these.

The most serious candidate for a fusion power plant reactor is the tokamak. These devices 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. Massive 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 containing the hot plasma.

The current flowing through the plasma is generated in the tokamak by a transformer pulse. A change in the current in the primary winding of the transformer induces a current in the secondary winding, which in the case of a tokamak is the plasma itself. For this reason, however, the tokamak is unable to operate in a steady state. It is a pulse device.

The stellarator uses the principle of magnetic confinement, just like the tokamak. However, it generates a suitable magnetic field only by means of specially shaped magnetic coils. It is therefore not a pulse device and is capable of maintaining hot plasma for as long as desired. This would predispose the stellarator to become a suitable reactor for a fusion power plant, but research on stellarators is not yet as far along 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. Huge 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 together. For example, magneto-inertial fusion creates a magnetically confined plasma that is further compressed by lasers. These approaches often try to achieve aneutronic fusion straight away.

An essential part of any fusion reactor will be the tritium breeders located in the walls of the vacuum chamber. In them, tritium, an important component of the fuel, will be produced when neutrons hit the lithium.

Heating and compressing the fuel to ignition values will only be necessary in the start-up phase of the reactor. The fuel will be heated to 160 million degrees in the chamber using microwaves or neutral particle injection and fusion will occur. Some of the energy generated will heat the surrounding fuel and provide the heat necessary for fusion. The reaction will run itself and no further external heating will be required. 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 the chamber walls. This heat will be used in the steam cycle to generate electricity.

Water transfers its energy in a steam generator to the water of the secondary circuit, which converts to steam. The steam drives a turbine, which turns a generator. This non-nuclear part will be exactly the same as all thermal power plants — nuclear, coal or biomass. So the fusion plant will use proven electricity generation technologies.

The fusion power plant will also take safety procedures from technologies proven in nuclear power plants 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. At any one time, there is less than a gram of fuel in the chamber.

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. This makes operating a fusion reactor even safer than operating a nuclear reactor.

Tokamak and laser-driven fusion are not yet able to operate continuously. Pulsed operation could be a serious obstacle to continuous power generation.

The solution is to install a thermal storage tank between the reactor and the steam generator, containing e.g. molten salts. At times when the fusion device would be operating, the tank would be heated. The heat would then be continuously withdrawn 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 demonstration fusion power plants, 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 demonstration fusion plant. The main purpose of these projects is to prove the commercial viability of fusion and to finalise the last technical details. However, all of them assume that they could already supply electricity to the grid around 2050.

The list of energy sources will be joined by clean, safe and essentially inexhaustible fusion energy alongside nuclear and renewable sources.