What is Nuclear Fusion and how close are we?

The ultimate solution to all our energy problems is to harness the science behind the power of the sun. And yes, we’re talking way beyond solar panels.

One of mankind’s greatest discoveries was the secret of harnessing the energy inside the atomic nucleus, allowing us to split the heaviest elements. This process is known as nuclear fission. In the US today, there are over 100 active nuclear fission power plants that supply over 20% of the country’s electricity.

While nuclear fission powers our current nuclear power plants, it poses three massive problems:

1. The resources required for nuclear fission – fissile Uranium or Thorium –  are both finite, and incredibly rare.

2. Both the reactants and waste products are highly radioactive.

3. And finally, the risk of an environmental disaster due to an accident (called a nuclear meltdown) – carries with it Chernobyl-level consequences.

Nuclear fusion, however is a perfect solution. It’s simply the exact opposite of nuclear fission. Instead of splitting heavy atoms, light ones are fused together to release energy. As opposed to fission, there are no chain reactions, removing the risk of nuclear meltdowns, and no radioactive waste is produced. It’s almost too good to be true.

So how exactly does fusion work? And why do we have fission reactors but not yet a fusion reactor?

First things first – Where does the fusion energy come from?

Nuclear fusion is the process of combining the nuclei of two atoms together. Think back to those dark days of middle school chemistry – and you’ll remember that an atom consists of a nucleus made up of protons and neutrons surrounded by a cloud of electrons zipping about around them.

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The source of fusion energy is the binding energy that holds the protons and neutrons together in the core of the atom. The larger this energy, the stronger the bonds within the nucleus and the harder it is to split.

Which elements do we need for nuclear fusion?

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The proton number determines which element the atom belongs to and therefore its chemistry.

The element with the largest binding energy is Iron, which has 56 protons in its core (see image above). Iron is roughly midway in the periodic table. Elements with more protons than iron (heavy elements), are used for nuclear fission in current nuclear power plants, such as Uranium.

In a fusion reactor, however, we don’t need to deal with such heavy elements. The opposite process is used, wherein light elements such as Hydrogen or Helium are fused together to form a heavier element that has a larger binding energy.

The closer the final element is to Iron, the more favourable the arrangement, i.e. the more energy is released.

So how does nuclear fusion work?

Since nuclei are positively charged, they repel each other. However, when forced close enough, the nuclear binding interaction takes over from the electrostatic repulsion and they fuse into one.

This means the nuclei first have to come very close together,  and this can only be achieved at very large temperatures and pressures (Keep this point in mind). At such high temperatures, the electrons are no longer attached to their nuclei, like it is the case in solids, liquids and gases, but instead float around freely. This is called a plasma, and is sometimes referred to as the fourth state of matter.

The least well-known state of matter is, paradoxically, also the most prevalent: 99.99% of the visible Universe, including stars and intergalactic matter, is in a state of plasma.

This is why the study of fusion is often called Plasma Physics.

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So is this how the sun produces its energy?

The Sun, like all stars, is able to create energy because it is essentially a massive fusion reaction. Its radiative energy is produced in a rather complex cycle that combines protons to form Helium-4, which consists of 2 neutrons and 2 protons. The neutrons are produced by a radioactive decay of the protons. So now you know how stars get their energy. Go on, have a look at the rather complicated diagram we made below.

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How can this work on Earth?

If you think the Earth is big, consider this: The sun is the size of 1,300,000 earths. As a result, the pressure at the sun’s core is very large, far larger than any pressure we could possibly achieve on Earth. Since we can’t artificially create this kind of pressure, we have to rely on making our fusion reactors very hot, up to 100 million degrees, which is six times as hot as the centre of the sun.

To maximise the energy output whilst keeping the pressures and temperatures as low as possible, a slightly different reaction to the one found in the sun is used.

In fusion reactors, two isotopes of Hydrogen are combined to produce Helium-4 and a neutron. The two isotopes are called Deuterium (with one extra neutron as compared to standard Hydrogen) and Tritium (two extra neutrons).

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One question that had scientists scratching their heads and stroking their beards in the early days of fusion research was how to confine plasma at such a high temperature. Any contact between a material cage and the plasma would cause the plasma to lower its temperature and douse the fusion reaction. This is easily solved by exploiting the electromagnetic properties of the plasma. Since the plasma particles are all charged, their motion is affected by magnetic fields.

It is therefore possible to construct a cage of magnetic field lines, around which the plasma particles move in a spiral motion, keeping them confined to the fusion domain. This is known as magnetic confinement.

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The neutron that is produced carries about 80% of the energy of the reaction. Since neutrons carry no electric charge, they can escape the magnetic cage and hit a blanket of material surrounding the plasma, converting their energy to heat, which can be used to produce electricity. There you have it – we’ve got our energy source.

What are the advantages of fusion?

As we’ve shown above, the only waste product of the reaction is Helium-4, an inert gas that is a natural constituent of our atmosphere. No life-threatening radioactive substances here, folks.

A fusion reactor also physically cannot explode: as soon as the tiniest bit of air enters the vessel, the plasma will instantly break down and the reaction will stop.

Furthermore, the fuel required could not be simpler: Deuterium can be obtained from the Hydrogen contained in water molecules (no problems there), whilst Tritium can be produced from Lithium, a commonly occurring metal on Earth.

Nuclear energy is one of the most powerful energy sources known to mankind – according to the Max-Planck-Institute for Plasma Physics, one bathtub of water and the amount of Lithium contained in a typical laptop battery could supply a family with electricity for 50 years. No need for rare elements like Uranium that have to be mined, as is the case for nuclear fission.

Doesn’t all this sound too good to be true?

Theoretically, nuclear fusion should solve all our energy problems today. The reality however is more complicated. Since the 1940s there has been active research into fusion reactors, but we are still far away from a working industrial reactor.

Firstly, a lot of energy is required to sustain the high temperatures required, as well as to create the magnetic cage to hold the plasma in. So far no reactor was able to produce more energy than was put in – and due to energy limitations no fusion experiment has lasted for longer than 20 seconds.

Another large challenge is to find a material for the blanket, which can withstand the wear and tear due to the constant bombardment by high-energy neutrons for a long period of time.

A third and final problem is the instability of the plasma itself – the larger the plasma available, the more stable it is. This however requires enormous reactors, something which is currently too expensive to be feasible.

Is there hope? – ITER (“The Way” in Latin) is one of the most ambitious energy projects in the world today.

Located in the South of France, ITER is expected to be the first fusion reactor that can run in a self-sustaining manner. 35 nations are collaborating to build it using the so-called tokamak architecture, where the plasma is enclosed in a doughnut-shaped magnetic cage.

ITER plans to fulfill the three conditions that are required to finally achieve nuclear fusion in the laboratory, very high temperature (on the order of 150 million degrees); sufficient plasma particle density (to increase the likelihood that collisions do occur); and sufficient confinement time (to hold the plasma, which has a propensity to expand, within a defined volume). If it succeeds, it promises to solve all the energy problems of today – giving us the supreme form of clean, green, and renewable energy.

Why stop at mineral power or nuclear fission when we can aim for the energy source of the stars – literally.

Resources

1. ITER website

2. IoP report on plasma physics and fusion research

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