Thinking ahead – fusion energy for the 21st century?
Key text
This topic is sponsored by the Research School of
Physical Sciences and Engineering at the Australian National University, the Australian Nuclear Science and Technology Organisation, the School of
Mathematical and Physical Sciences at the University of Newcastle and the School of Physics at the University of Sydney.
Fusion is the oldest, and newest, form of energy. What role will it play in our energy-hungry future?
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You will get more from this topic if you have mastered the basics of atoms and molecules – these links will take you to an annotated list of sites with helpful background information. |
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.
Boxes
1. Comparison of amounts of fuel and waste
2. Fusion science in Australia
3. There's work to be done
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Posted February 2007.