Fusion at a glance: clean, sustainable energy
Fusion is the process that powers the Sun. It is the energy that makes life on Earth possible. Unlike nuclear fission, which releases energy when a heavy atom splits into two lighter elements, fusion releases energy when the nuclei of two light atoms combine, such as when two hydrogen nuclei fuse to form a helium atomic nucleus.
The Sun: fusion at work
Inside the Sun, fusion reactions take place at very high temperatures (around 15 million °C) and under enormous gravitational pressures. At such high temperatures gas becomes plasma. Plasma is the fourth state of matter (solid, liquid and gas being the other three) and is best described as an electrically-charged gas. In plasma the negatively charged electrons in the gas atoms are completely split off from the positively charged atomic nuclei (or ions).
The Sun is a massive fusion power station. It produces around 300 billion billion watts (3 x 1026) of power, consuming 600 million tonnes of hydrogen fuel every second. The Sun burns hydrogen gas - the simplest atomic nuclei consisting of a single proton (a positively charged atomic particle).
On Earth: fusion research
On Earth scientists and engineers are working to reproduce fusion on a smaller scale. However, this is a significant scientific and technical challenge. The Earth does not have the gravitational pressure of the Sun so plasma must be confined and heated to temperatures ten times higher than those in the Sun in order to get a sufficient number of fusion reactions.
Making fusion on Earth requires two heavier types (or isotopes) of hydrogen: deuterium - with a nucleus of one proton and one neutron (an atomic particle with similar mass to the proton but no electrical charge) and tritium (with one proton and two neutrons).
When these two nuclei fuse together they produce a new helium nucleus (also known as an alpha particle) and a high-energy neutron. In a future fusion power plant, the energy of that neutron will be captured and used to heat steam to generate electricity as in a normal power station, while the electrically charged alpha particle will transfer its energy to the plasma, keeping it hot.
Why is fusion important?
Fusion energy has the potential to provide a sustainable solution to global energy needs. In particular it can provide a continuous baseload power supply which is sustainable, large-scale and environmentally responsible, using fuels that are universally available.
Limitless fuel - The raw fuels for fusion are water and lithium. There is around 0.033 grams of deuterium in every litre of water. Tritium is not found on Earth but can be easily made from lithium - an abundant metal found in batteries that power mobile phones and laptops. Tritium can be made in situ in a fusion reactor by using the neutron released by the fusion reaction. If the neutron is absorbed by a surrounding 'blanket' of lithium then tritium is produced.
Inherent safety - The volume of gas in a fusion reactor will always be low, at around 1 gram of fuel in 1000 cubic metres. Any problem will always cool the plasma and stop reactions - so a runaway situation is impossible. Also the raw fuels for the reactor (deuterium and lithium) are not radioactive. Tritium is mildly radioactive but will be produced and used within the reactor. Consequently, no transport of radioactive fuels will be needed for a fusion power plant - and even the worst possible case accidents would not require evacuation of neighbouring populations.
Environmental impact - Fusion power will not create greenhouse gases, produce other harmful pollutants or result in long-lasting radioactive waste. Its fuel consumption will be extremely low. A 1000 megawatt electric fusion power station would consume 100 kg of deuterium and three tonnes of lithium a year to generate 7 billion kilowatt-hours of power. To do the same a coal-fired power station would need 1.5 million tonnes of coal.
The fusion fuels are not radioactive. The neutrons generated by fusion will interact with the materials close to the reactor, but careful choice of these materials will ensure that no long-term legacy of radioactive waste is produced by fusion power. For further information on safety and environment please click here