About our writer
Naomi Foster is in her second year studying Engineering Science at St. Anne’s College, Oxford. Naomi was a Mentor for Cambridge Immerse in 2017.
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It’s the problem of the century – how do we power our energy-hungry lifestyles? With fossil fuels running out and none of the renewables we currently use really able to replace them, many are looking for a miracle. And nuclear fusion, with the power to provide virtually limitless energy, could be that miracle. But could it ever be a feasible power source?
What is nuclear fusion?
Nuclear fusion occurs when the nuclei of atoms, travelling at very high speeds, collide and combine to form a new single nucleus. This releases energy as there is a very slight change in mass, and as Einstein famously taught us with E=mc2, energy and mass can be exchanged.
The sun itself is a nuclear fusion reactor – it combines atoms of hydrogen, producing huge amounts of energy, which power our entire life on earth. If we could create a similar fusion reactor, we would have a long-term, sustainable energy source and our energy problems would be solved.
There is also no toxic nuclear waste produced by nuclear fusion, as there is from our current nuclear power stations which perform “fission” (splitting) of atoms. This is because the new atoms created by fusion are stable and will not decay.
Fission vs. Fusion
Nuclear fission and nuclear fusion are essentially opposite processes. Fission involves splitting atomic nuclei apart, creating energy, whereas fusion is joining nuclei together. Fission is already carried out on earth in many nuclear plants around the world.
To begin fission, neutrons are added to heavy nuclei, such as the nucleus of uranium or plutonium. This creates an unstable, radioactive isotope of the element, which is likely to decay by fission. The nucleus is likely to break into around two fragments, plus a small number of neutrons, which then go on to start the process again with other uranium nuclei, establishing a chain reaction. The breaking up of the nucleus causes a release of energy. The fission fragments cannot travel very far in the reactor, so their energy is converted into heat, which heats up the reaction chamber.
There are many drawbacks to fission power stations though – they produce toxic waste and are really expensive to set up and dismantle. Fusion, on the other hand, doesn’t produce this harmful waste, and on top of that creates more energy per gram of fuel!
How Fusion Works
The nucleus of an atom is made of protons and neutrons, so it may seem obvious that some force is needed to keep the nucleus together, as the protons will be repelling each other due to their charge, and pulling the nucleus apart. This force is called the strong nuclear force, and is felt by protons and neutrons. However, the electromagnetic force is acting against this strong force, pulling the nucleus apart – nuclear physics is all about the balance between these forces.
It may be useful to think about the nucleus as a drop of water. If the drop is too small, it wants to combine with other drops to become more stable. If the drop gets too big, it is likely to split apart into two or more smaller drops, in order to gain stability, meaning a medium-sized nucleus is the most stable.
If nuclei can get close enough to overcome the electrostatic repulsion, the strong nuclear force will bind them together into a new nucleus. As we can see from the binding energy graph, for small nuclei this larger nucleus will have a lower binding energy per nucleon. This leads to a release of energy, (either as kinetic energy given to the new nucleus or any emitted neutrons, or as gamma radiation) and a decrease in mass.
The graph also shows that for large atoms, the smaller they get, the lower the binding energy, showing why splitting big atoms can also cause energy to be released.
Problems with Fusion
Fusion had never been carried out on a large scale yet as it is very difficult to get a net gain of energy from fusion. It requires very high temperatures for fusion to take place, as the nuclei need to collide at such high speeds in order to have enough energy to overcome their repulsion and combine. This means the fuel has to become so hot it becomes something called plasma. This is a state where the element essentially becomes a stream of charged particles, as the energies in the plasma are so high, the electrons have been ripped from the nuclei, leaving charged ions. This happens at temperatures of around 40 million degrees Celsius, easily hot enough to vaporise any known substance.
This means that there is no container which can hold a fusion reaction. There are 2 main ways of getting past this problem; gravitational confinement and magnetic confinement. Gravitational confinement takes place in the sun, where the huge mass of matter creates intense pressures at the centre of stars, allowing nuclei to get close enough to fuse. However, there is not enough matter on earth to carry out this method here. Magnetic confinement works by containing the plasma within magnetic fields, and has been used at some large-scale projects.
If Fusion were Successful
If we found a way to successfully crack all the problems surrounding fusion, would it help our current energy crisis? The energy that would be released from fusing one gram of deuterium is a huge 100000kWh. This shows us that, as the mass of the oceans is 230 million tons per person, there is enough deuterium to supply every person, in a world population ten times its current size, with 100 times the average American consumption for 1 million years. This is a staggering amount of energy.
However, fusion projects come with a large price-tag. ITER, an international nuclear fusion research and engineering mega-project, is expected to cost around $12.8 billion in total, for 10 years of construction and only 20 years of operation. Many scientists predict that fusion will not be a viable source of energy until at least as late as 2050, meaning there is a long way to go before fusion is powering the world.