Thinking ahead – fusion energy for the 21st century?

Key text

At our present rate of use, experts predict that fossil fuels will become limiting within 50 years. To limit global warming, many researchers believe a two phase plan to reduce CO₂ emissions is required. The first phase consists of a reduction in CO₂ emissions until about the middle of the century by making modifications to existing technologies.

Related site: Past and future CO2 concentrations Provides a graph of atmospheric CO2 concentrations versus time. (United Nations Environment Program GRID-Arendal)

The objective of the second phase, over the second half of the century, is to stabilise CO₂ concentration to about 450 to 550 parts per million. This is likely to limit the global average temperature rise to about 2 degrees Celsius, which should result in noticeable but tolerable changes to climate systems.

To limit CO₂ emissions and meet the increasing energy requirements of developed and developing countries a new generation of energy production technologies will be required. Energy produced from nuclear fusion may be one of them.

The power of the atom: Fission and fusion

There are two types of nuclear reactions: fission and fusion.

Related site: E=mc2 explained Provides short audio quotes from 10 top scientists about the famous equation. (NOVA, Public Broadcasting Station, USA)

In nuclear fission, a heavy atom such as uranium is bombarded with neutrons, causing the atom to split. When an atom is split, the difference in mass is released as energy, defined by Einstein's equation E=mc².

Nuclear fission currently provides about 17 per cent of the global electricity requirement, but in France it provides 75 per cent of electricity.

The other form of nuclear reaction is fusion, which is the source of energy from the sun and stars. In fusion reactions, two light atoms are brought together and fused, creating a new element and releasing energy. The most common reaction for fusion reactors is the D–T reaction:

deuterium + tritium helium + neutron + energy

An equal mix of deuterium and tritium is the easiest to use, since it fuses at the lowest temperature and the yield of energy is the greatest of any fusion reaction.

Although the reactions look simple on paper, in practice the conditions required to initiate fusion reactions are difficult to achieve. To understand why, we need to take a closer look at the D-T reaction.

Deuterium and tritium are types of hydrogen

Deuterium and tritium are both types of hydrogen, which differ in the number of neutrons they possess. Deuterium is found naturally in water and is not radioactive. Tritium is radioactive with a half life of 12 years and is made by combining lithium with a neutron. The raw materials for a working fusion reactor are a few grams each of deuterium and lithium, both of which are abundant on Earth.

The small amount of waste generated from the D–T reaction is less radioactive than fission waste, with shorter decay times, and is easier to dispose of. (Box 1: Comparison of amounts of fuel and waste). The helium produced in the reaction is not radioactive.

Plasma – the fourth state of matter

The most common states of matter on Earth are solid, liquid and gas. The D–T reaction occurs in the most abundant form of matter in the universe – plasma. In plasma, all electrons are removed from the atoms, creating positively charged nuclei. Plasma is created by supplying lots of heat. The D–T reaction occurs at temperatures above 100 million degrees Celsius.

Large amounts of energy are required to get atoms to fuse

Having the same charge, the nuclei in plasma repel each other. Only large amounts of heat and pressure can force them close enough together to fuse.

Related site: Heating the plasma Describes methods used to heat plasma. (European Fusion Development Agreement – Joint European Torus, UK)

Methods used to heat the fuel include:

• compressing the fuel;
• applying an internal electric current;
• bombarding the fuel with neutral particles; or
• absorbing power from microwaves or lasers.

Even larger amounts of energy are released when atoms fuse

Even though it takes considerable energy to force nuclei to fuse, when they do fuse more energy will be released than it took to force them together. For example, the energy barrier for the D–T reaction is about 0.1 mega electron volts, but the total energy given off is 17.6 mega electron volts.

By heating the nuclei, they gain energy and can eventually overcome the 0.1 mega electron volt barrier required to start the reaction. Once above the limit, the fusion reaction can sustain itself if the energy produced goes into keeping the plasma hot. In the D–T reaction, only 20 per cent of the input energy is used to sustain the reaction: the rest can be used to generate electricity.

Controlling plasma

Although physically surrounded by a vessel, the plasma in reactors is confined in space by magnetic fields, gravity or inertia.

Gravity
In the sun, gravity forces the nuclei together to react.

Inertial confinement
Laser or ion beams are used to squeeze and heat the hydrogen plasma.

Magnetic confinement
Since plasma particles are charged, and plasma conducts electricity, it can be contained by magnetic fields. In fusion reactors, magnetic fields are used to contain the plasma, usually within a large doughnut shape called a torus. Reactors of this shape are known as tokamaks.

In tokamaks, charged particles orbit around the magnetic field lines. They generally travel parallel to those field lines, so with the correct placement of magnets, the particles don't touch the wall of the reactor vessel. The plasma forms a continuous – although turbulent – circuit in the shape of a doughnut.

Looking ahead – the International Thermonuclear Reactor

Because of the cost and complexity of fusion research, projects are usually collaborative efforts. The past 20 years have seen dramatic improvements in tokamak design and energy output – the rates of improvements parallel those of Moore's Law.

So far, researchers at the Joint European Torus, located in Oxfordshire, UK have been able to achieve 'break even' point experimentally, where the power input is equal to the power output. 

If advancements continue at the present rate, energy break even could be routinely achieved by 2010. Commercial power plants would then be the next goal of fusion researchers.

The International Thermonuclear Reactor (ITER) is the next major experimental reactor to be built in France, with the support of Europe, Japan, USA, Russia, China and Korea. The objective of ITER is to demonstrate the scientific and technological feasibility of fusion power. Although Australia has a history of involvement in fusion research, it is not currently part of the ITER consortium (Box 2: Fusion science in Australia).

The ITER facility should be able to achieve a self-sustaining fusion reaction called a burning plasma, where energy output is about 5 times the energy input. When plasma temperature, density and confinement exceed a certain limit, the reaction has a greater output than input and is capable of generating electricity. ITER is designed to release more energy than it takes in: about 500 megawatts, or the same as a medium-scale coal power plant. ITER will be the largest fusion reactor built so far, but there are already plans for others to follow.

Fusion energy for the future?

It's true that there are still unknowns in fusion research, as there are in any field of research (Box 3: There's work to be done). But, many experts believe that renewable energy sources such as sun and wind alone may not be able to supply enough energy to meet the needs of a growing world population. With so many favourable features and environmental credentials, fusion energy may offer a long term solution to our energy needs.

Box 1: Comparison of amounts of fuel and waste

Comparison of amounts of fuel for fission and fusion reactors

The table below provides a comparison of the fuel requirements for power stations continuously producing one gigawatt of power for one year.

Power station	Amount of fuel
coal-fired	4.4 million tonnes of black coal*
coal-fired	10.8 million tonnes of brown coal
fission reactor	1.3 tonnes of uranium-235 (from 35 tonnes of uranium oxide or 210 tonnes of uranium ore)
fusion reactor	150 kilograms of deuterium (obtained from two Olympic-sized pools of water) and 500 kilograms of lithium

*energy density 24 MJ/kg

Control of nuclear reactors

The fission of uranium in a power plant is a self sustaining process, and requires neutron absorbing control rods to prevent uncontrolled meltdown.

In contrast, magnetic fields are used to confine the plasma in a fusion reactor and to keep it hot. Collapse of the magnetic field causes the plasma to cool, so fusion reactions can be stopped with the flick of a switch. There are no chain reactions to control.

Safety concerns

In 1992, the European Safety and Environmental Assessment of Fusion Power concluded that fusion has the potential to be a safe and clean method of generating electricity.

Fusion power plants are said to be intrinsically safe, however there are some safety concerns that need to be addressed:

• Although they produce no 'high level' radioactive waste, one of the most important safety concerns of ITER is to ensure that tritium is not released into the environment. Analysis of the risk of tritium escape in a 'worst case scenario' indicates that if such an event did occur, the public need not be evacuated from the area because the exposure to radioactivity would be below internationally accepted limits.

• A fusion reactor operates by the continual addition of fuel. The amount of fuel in the reactor at any one time is enough for about one minute of operation.

• Most of the radioactive materials produced by a fusion reactor are relatively short lived, with the level of radioactivity decreasing 10,000 fold within 100 years. Radioactive material may be kept on site until it has decayed to a point where it is no longer considered radioactive, it may be stored in a permanent repository or it may be recycled.

• The neutrons released by the fusion reaction can cause the material in the vessel walls to become radioactive, creating an occupational safety risk. The vessel cannot be handled directly, so maintenance of the vessel needs to be done using robots.

• Researchers are designing and selecting new materials for the reactor vessel. Ideally, the vessel materials will have half lives of tens of years, minimising the burden of waste disposal on future generations.

Related sites:

• European Commission
  • Safety and environment
  • Understanding radiation

• Radioactivity (Georgia State University, USA)

Box 2: Fusion science in Australia

Timeline of fusion research in Australia

Australia has significant expertise in fusion. The fusion of light nuclei was first observed by Australian Sir Mark Oliphant in 1932.

1932	Sir Mark Oliphant discovers He3+, T and D–D reaction
1946	Peter Thonemann (Australia) and Sir GeorgeThomson (UK) pioneer toroidal confinement research
1958	Sir Mark Oliphant commences plasma physics research at the Australian National University (ANU)
1964–1978	LT1, LT2 and LT3 tokamaks operational at the ANU. From 1964 to 1969 this was the only tokamak located outside of the former USSR
1970–1998	Flinders University rotomak research program
1975–present	Inertial confinement research at the University of NSW
1978–1984	LT4 tokamak operational at the ANU
1981–1992	University of Sydney TORTUS tokamak program; study of Alfven wave physics
1984–present	SHEILA (Small Heliac Experimental Apparatus) and H1 heliac research program, and helicon wave heating at ANU
1988	Australian Nuclear Science and Technology Organisation demonstrates the world’s first spherical torus, now a leading magnetic confinement concept
1997	H-1 National Plasma Fusion Research Facility established
1995–present	Research into electrostatic ion confinement at the University of Sydney

H1 National Plasma Research Facility

The Australian National University has a plasma confinement experiment called the H-1 heliac which is the centrepiece of the H-1 National Plasma Fusion Research Facility. It works much like a tokamak, but uses more complex magnet shapes, making it easier to confine the super hot plasma.

H-1 is a ‘flexible heliac’ which is basically a twisted doughnut shape. The twist of the plasma in the heliac is controlled by currents produced by a central circular conductor. This coil and 41 other electromagnet coils provide a high degree of control over the shape of the plasma, giving it good stability and confinement properties.

The H-1 facility is used to understand the behaviour of plasmas at temperatures approaching 1 million degrees Celsius. The plasma is generated using high powered radio waves, to turn low pressure gas into plasma. The plasma is then further heated by microwaves, similar to those used in a microwave oven, but with 250 times the power, at ten times the frequency.

The Facility provides a focus for national and international collaborative research in Australia, and makes significant contributions to the global fusion research effort. Researchers process and visualise the data provided by experiments on H-1 to understand the basic physics of hot plasma, and to measure the features of plasma behaviour under different conditions.

Possible Australian contributions to ITER

Australia is not part of the ITER partnership, but could supply scientific, technological and engineering expertise. Australia can contribute:

• knowledge and testing of advanced materials to withstand extreme heat and neutron bombardment;
• expertise in diagnostic systems; and
• computer modelling of plasma confinement and behaviour.

Australia currently supplies 70 per cent of the world's lithium, so it could also supply refined tritium fuel to ITER.

Related sites:

• Sir Mark Oliphant (Bright Sparcs Exhibition, Australia)

• The ITER project

• Fusion energy: Star power for a renewable future (Australian ITER Forum)

• H-1 National Plasma Research Facility (Australian National University)

Box 3: There's work to be done

Much of the technology used in ITER has been demonstrated to work using improved computer models of plasma behaviour, but there are a number of technical uncertainties about fusion reactors that can only be answered by doing some experiments. Planned ITER experiments include health, safety and waste management procedures.

Bigger is better

One of the advantages of ITER facility is its size. ITER is twice as big as other tokamaks, with an outer radius of 6.2 meters. Generally, the more room available for plasma to move, the better. A large plasma volume reduces the amount of heat lost to the walls of the reactor and reduces the severity of problems associated with high density neutron fluxes to the walls.

Learning to handle the heat

In the plasma core, the temperature will approach 100 million degrees, which is about 10 times hotter than the core of the sun. Until now, researchers have been able to control the plasma temperature by turning down the heat. But for commercial fusion reactors to become a reality, they need to learn how to hold a lot of energy in a small space and let it out in a controlled way.

Electricity production

Researchers also need to learn how to capture and use the power produced by the reactions. It is likely that liquid coolants will be used to cool the blanket and diverter, and the heated coolant then used to heat water, to drive steam turbines.

Development of advanced materials

A lot of energy generated by fusion reactions is in the form of fast moving neutrons, which will irradiate the beryllium-coated blanket surrounding the plasma. But beryllium may not be well suited to handle the heat and radiation. Japanese researchers are considering building a test facility to develop materials more suited to the task.

If ITER is successful, helium will accumulate in the reactor and will be captured by the diverter at the bottom of the reactor. Researchers need to learn how that will react with the plasma and surrounding materials. The walls may, for example, accumulate radioactive tritium.

Related sites:

• Fusion: The materials challenge (Engineering and Physical Sciences Research Council, UK)

• Technology R and D (European Fusion Development Agreement Joint European Torus, UK)

Activities

• General Atomics Fusion Education (USA)
  • Starpower – By manipulating magnetic field strength, power, fuel, density, pressure, shape and boundary, students attempt to produce at least 1000 MW of electricity from a fusion reactor.

• Internet Plasma Physics Education Experience (USA)
  Provides information about fusion and a number of applets, including:
  • The virtual tokamak
  • Magnetic confinement demonstration
  • Operate your own tokamak reactor
  • Analysis of data from the tokamak fusion test reactor
  Also includes interactive modules on matter, electricity, magnetism, energy and fusion.

• An Educator's Reference Desk Lesson Plan (USA)
  • Magnetic forces – an introduction to magnetic attraction, strength of force and the magnetic forces of the Earth.

• ScienceNetLinks (American Association for the Advancement of Science, USA)
  • Risks and benefits – students assess and weigh the risks and benefits associated with innovations in science and technology.
  • Radioactive decay: A sweet simulation of a half-life – demonstrates that the rates of decay of unstable nuclei can be measured, that the exact time that a certain nucleus will decay cannot be predicted, and that it takes a very large number of nuclei to find the rate of decay.
  • Isotopes of pennies – provides lessons about radioactivity and isotopes using coins to represent subatomic particles.
  • What do scientists do? – students develop an understanding of the diversity and nature of various science disciplines.

• ThinkQuest (Oracle Education Foundation, USA)
  
  • Basic fusion – provides background information on fusion reactions and then checks submitted answers to questions.
  • Binding energy – provides background information on binding energy and then checks submitted answers to questions.

• Energy Information Administration (Department of Energy, USA)
  • Energy source web quest – students learn about energy sources using web-based resources and create PowerPoint presentations to teach other students.

Further reading

Useful sites

Fusion science slide show (General Atomics Fusion Education)

An online slideshow that provides basic information about fusion, a comparison of fuel requirements for different sources of energy, an explanation of how fusion reactors work and current fusion energy research projects.
http://fusioned.gat.com/slideshow.html

European Commission

• Introduction to fusion
  Includes sections on how fusion works, the advantages of fusion and a brief history of fusion research.
  http://ec.europa.eu/research/energy/fu/fu_int/article_1120_en.htm

• A small dose of basic nuclear physics
  Answers the questions:
  • What are the basic building blocks of an atom?
  • What is the binding energy of a nucleon?
  • What is the mechanism of energy release during nuclear fission and fusion?
  • What are stable and unstable nuclei?
  • Why does the radioactive decay of nuclei take place?
  
  http://ec.europa.eu/research/energy/fi/fi_bs/article_1172_en.htm

For students and educators (European Fusion Development Agreement)

Provides general information about fusion, the ITER project, a picture gallery, and answers to frequently asked questions.
http://www.efda.org/usercases/students_and_educators.htm

European Fusion Development Agreement Joint European Torus (UK)

• Fusion basics and glossary
  Looks at the conditions required for a fusion reaction to occur, magnetic plasma confinement and heating and measuring the plasma.
  http://www.jet.efda.org/pages/fusion-basics.html

• Focus on: computer modelling of fusion plasmas
  Looks at some of the advantages and drawbacks of computer modelling of plasmas.
  http://www.jet.efda.org/pages/focus/modelling/index.html

• Focus on: fusion technology
  Provides a diagram of the ITER site and describes some of the research of the Joint European Torus that is relevant to future research of ITER.
  http://www.jet.efda.org/pages/focus/fusion-tech/index.html

• History of JET and scientific achievements
  Provides a summary of JET, a timeline of fusion research at JET and a strategy to achieve a commercial fusion power plant.
  http://www.efda.org/multimedia/downloads/brochures/jet.pdf

How stuff works (USA)

• How nuclear fusion reactors work
  Provides an introduction to fusion including the conditions required for fusion, plasma confinement and fusion applications.
  http://science.howstuffworks.com/fusion-reactor.htm

• How nuclear radiation works
  Describes the process of nuclear radiation and radioactive decay.  
  http://science.howstuffworks.com/nuclear.htm

• How plasma displays work
  Explains how plasma TV screens work.
  http://www.howstuffworks.com/plasma-display.htm

• How fluorescent lamps work
  Explains how fluorescent lamps emit light and why fluorescent lamps are more efficient than incandescent lighting.
  http://www.howstuffworks.com/fluorescent-lamp.htm

• How electromagnets work
  Describes how electromagnets work and suggests some simple experiments with an electromagnet.
  http://science.howstuffworks.com/electromagnet.htm

Fusion (Catalyst, 27 April 2006, Australian Broadcasting Corporation)

Discusses what fusion is and shows an experimental fusion reactor in action.
http://www.abc.net.au/catalyst/stories/s1625306.htm

What are plasmas? (Perspectives on plasma, USA)

Provides basic information about the use of plasmas for energy production, manufacturing and business.
http://www.plasmas.org/rot-plasmas.htm

Glossary

break even. The point where the power input of a controlled nuclear fusion reaction (supplied by either external sources or the products of reaction) is equal to the power output. Three conditions need to be met for a sustained fusion reaction to occur. They are:

• plasma temperature of 100-200 million Kelvin;
• plasma density of one thousandth of a gram per cubic metre; and
• energy confinement time of 1 to 2 seconds.

The confinement time is a measure of the rate at which a system loses energy to its environment.

electromagnet. A device that produces a magnetic field using an electric current flowing through a coil of wire, generally wound on a soft iron core. Electromagnets are temporary magnets – when the current is turned off, the magnetism is gone.

electromagnetic force. One of the four forces – gravity, strong force, electromagnetic force, weak force – that act on particles. Electromagnetic force acts on charged particles and is made up of electric and magnetic forces (eg, moving magnets produce electric forces and moving electric charges produce magnetic forces).

The electric charge that is the source of electromagnetic force can either be positive or negative. Because there are two types of charge, the electromagnetic force can be either attractive or repulsive. Opposite charges attract, like charges repel. Physics theory explains that electromagnetic force is carried by photons (packets of electromagnetic radiation). For more information see The four fundamental forces (ThinkQuest, USA).

electronvolts (eV). A measure of energy used for convenience in atomic systems. It is the amount of kinetic energy gained by an electron when it passes through an electrostatic potential difference of one volt. It is equal to one volt (1 volt = 1 joule per coulomb) multiplied by the charge of a single electron (in coulombs). One electronvolt is equal to 1.602×10^-19 joule.

The energy of a fusion reactor is expressed as megaelectronvolts (MeV: 1,000,000 eV) or gigaelectronvolts (GeV: 1,000,000,000 eV). For more information see Energetic particles (National Aeronautics and Space Administration, USA).

E=mc². E stands for the energy released, m stands for the mass that is converted into energy, and c is the speed of light (300,000 kilometres per second).

ionising radiation. Any form of radiation that has sufficient energy to remove electrons from atoms, so producing charged particles called ions. It can consist of high energy particles (electrons, protons or alpha particles) or short wavelength electromagnetic radiation (ultraviolet, X-rays and gamma rays).

isotope. One of the different kinds of an atom of the same element. All atoms of an element have the same chemical properties, but the different isotopes have different weights. The different weights are because the isotopes have a different number of neutrons.

kilowatt hour. A unit of energy that is normally used to measure the consumption of domestic electricity. The joule (1 watt per second) could be used but the numbers become very large and it is common to use the kilowatt hour (1 kilowatt hour = 3,600,000 joules or 3.6 megajoules).

kilowatt, megawatt, gigawatt. The unit of energy is the joule (J) and the unit of power (the rate at which energy is used) in the metric system is the watt (W); a kilowatt is 1000 watts. A watt is a very small amount of power and in most mechanical applications we count power in kilowatts. A kilowatt is about equal to the heat energy put out by a single bar radiator, and is also about equal to the power expended by a person running up stairs. A car engine typically produces 50 to 100 kilowatts.

When we consider power generation, we use larger units. A megawatt is 1,000,000 watts or 1000 kilowatts. A typical coal-burning power station produces about 1 gigawatt (1000 megawatts) of power.

laser. Light amplification by stimulated emission of radiation. A device that produces a high-intensity, directional, monochromatic beam of light.

magnetic fields. Are created by electric currents in wires or electrons moving in orbit around a nucleus. Sources of magnetic fields have a north and south magnetic pole. The SI unit for magnetic field is the Tesla (T). For more information see Magnetism (School for Champions).

microwaves. The highest frequency radio waves, with wavelengths between about 1 millimetre and 30 centimetres and frequencies between about 300 gigahertz and 300 megahertz. Microwaves are a type of electromagnetic radiation.

Moore's Law. Refers to the advance in computing power per unit cost. Moore's law is based on the observation that the number of transistors on a computer chip, which is a rough measure of computer processing power, doubles every 18 months. A graph plotting the number of transistors on a chip versus time on a log scale is a straight line. The graph plotting transistor size versus time is also a straight line.

neutron. A particle having no charge that is a constituent of an atom. It has a mass similar to a proton.

nuclear fission. Also referred to as atomic fission. The process by which large nuclei are split into two parts, by bombarding them with neutrons, in order to release large amounts of energy.

parts per million. This is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, so parts per million or ppm means out of a million. Therefore 500,000 ppm is the same as 50 per cent, because 500,000 is half of a million. The concentration of oxygen in unpolluted fresh water is about 8 ppm – only 8 parts of oxygen for every 1 million parts of other substances.

superconducting magnet. A type of electromagnet (a temporary magnet formed when an electric current is conducted through a coil of wire). In superconducting magnets, the wire is cooled to a temperature close to absolute zero. At this temperature, there is virtually no resistance to the flow of electricity through the wire. For more information see How electromagnets work (How Stuff Works, USA).

superconductor. A substance that has no resistance to the flow of an electric current. Superconductors currently require very low temperatures to function. They can be used for energy storage, storing and retrieving digital information, medical imaging machines and friction free transport. For more information see What is superconductivity? (How Stuff Works, USA) and Superconductor information for the beginner (Superconductors.org).

tokamak.For more information see Tokamaks (National Space Research Institute, Brazil).

uranium. A radioactive heavy metal. The natural element is a mixture of different isotopes or atomic forms. The isotope uranium-235 is used in nuclear non-breeder reactors.