Plasmas and nuclear fusion
Fusion is the process occurring within the plasma core of our Sun in which the nuclei of lighter atoms link to form a heavier atom. For example, when hydrogen nuclei collide, they can fuse into heavier helium nuclei and release tremendous amounts of energy in the process. What we see as light and feel as warmth coming from the Sun is the result of this fusion reaction process.
Since the Sun is a huge plasma ball made up primarily of hydrogen, it is the fusion of hydrogen into helium that is responsible for 85% of the Sun’s energy output. The fusion of hydrogen to form helium is a proton–proton chain reaction.
Proton–proton chain reaction
A proton–proton chain reaction is one of the ways by which stars fuse hydrogen into helium. It is the reaction that dominates in stars the size of the Sun.
Where does the energy come from?
Careful analysis of these reactions shows that the mass of the resulting helium-4 nucleus is very slightly less than the sum of the masses of the reactant particles involved.
This lost mass reveals itself as energy released, conforming to Einstein’s formula E=mc². The tiny bit of lost mass (m), multiplied by the square of the speed of light (c²), results in a very large amount of energy (E) created by a fusion reaction.
What fuels the Sun?
In this video, Associate Professor Bob Lloyd states that it is nuclear fusion that fuels the Sun. He then goes on to explain in simple terms how this process works by fusing lighter elements into heavier elements. By using Einstein’s famous equation E=mc2, he then explains why it is that so much energy is released.
The Sun converts about 600 million tonnes of hydrogen into helium every second, releasing an enormous amount of energy. Of this, it has been calculated that the ‘lost mass’ is 0.7%.
Using these figures, the energy production per second can be calculated as follows: E = mc² E = [0.007 x 600 x 106 x 103] x [3 x 108]2 E = 3.78 x 1026 J = 3.78 x 1023 kJ
Given that this is occurring every second, it means that the power output of the Sun is close to 4 x 1023 kW.
Fusion on Earth
Fusion reactions release colossal amounts of energy. Over the last 50 years, scientists have thoroughly investigated these reactions in the hope of developing nuclear fusion reactors that can transform fusion energy into electrical energy for public consumption.
Progress has been slow, and the main reasons for this are linked to making, controlling and containing extremely high-temperature plasma within which hydrogen nuclei can fuse to form heavier helium nuclei.
One of the largest nuclear fusion plasma physics experimental facilities (established in 1960) is located in Oxfordshire in the UK. Known as JET, which stands for Joint European Torus, it is a tokamak reactor within which fusion reactions are made to occur. Tokamak is a Russian word meaning ‘toroidal chamber with a magnetic field’.
The reactor has a doughnut-ring shape and is designed to heat hydrogen gas to a very high-temperature plasma state (150,000,000°C). Very large magnetic fields are also generated to control and contain the plasma.
D-T fusion
In D-T fusion, two isotopes of hydrogen – deuterium (2H) and tritium (3H) – are brought together. The fusion reaction generates helium (4He), a neutron and large amounts of energy.
The most efficient fusion reaction to reproduce in the reactor is that between two hydrogen isotopes – deuterium (D) and tritium (T). This D-T fusion reaction produces the highest energy gain at the lowest operating temperatures.
Since the helium nucleus carries an electric charge, it remains within the plasma. However, the neutron is not charged and can be absorbed by the walls of the tokamak if they are coated with neutron-absorbing materials. Since the neutrons carry 80% of the energy produced in this reaction as kinetic energy, when the walls absorb them, this energy is transformed into heat.
This can then be used to boil water into steam, which in turn is used to drive a turbine that then generates electrical energy.
Many of the scientific obstacles in fusion have now been overcome, and the experiments conducted with JET have proved the technical feasibility of fusion using deuterium and tritium.
Future projects
ITER
ITER is the next major international fusion project currently being built in Cadarache, France. It will be a 500 MW tokamak aiming to confirm the commercial feasibility of fusion power.
The challenge now is to prove that fusion can work on a power plant scale. International fusion research has embarked upon a plan to achieve power generation within 30 years. It will focus on three main projects:
ITER – a multinational project being built in the south of France. It will be a 500 MW tokamak aiming to confirm the commercial feasibility of fusion power.
IFMIF (International Fusion Materials Irradiation Facility) – a lab that will test the materials needed in a fusion power station.
DEMO – a demonstration power plant supplying fusion electricity to the grid.
Nature of science
Science involves the invention of explanations and theoretical entities, which require a great deal of creativity on behalf of the scientists. In the quest to produce, control and use nuclear fusion energy for electrical generation, it will be the inventiveness, creativity and perseverance of scientists that allows for a successful outcome.
Useful links
Find out more about nuclear fusion – one of the most promising options for generating large amounts of carbon-free energy in the future. Fusion scientists and engineers are developing the technology to use this process in tomorrow’s power stations. See the Culham Centre for Fusion Energy, part of the UK Atomic Energy Authority website.
Read this 2014 Stuff news article about how US company Lockheed Martin claims to have made a breakthrough in developing a power source based on nuclear fusion, with reactors small enough to fit on the back of a truck with the potential to be ready for use in 10 years. See here for a 2019 update on the progress they are making.
