Clean, Limitless Energy, as Nature Intended
Nuclear fusion is a process by which light-weight atoms combine to form heavier atoms.
The biggest fusion reactor in our vicinity is the Sun, where unimaginable amounts of hydrogen are converted to helium every second.
It gives us the potential to produce plentiful, clean energy if only we can master its physics
There are several scientific endeavours in this direction, from small startups to massive international organisations.
The previous articles in our Going Nuclear series featured applications of the release of energy from splitting large, heavy atoms. From the Manhattan Project to nuclear power plants, man’s first hesitant steps into the nuclear world all involved nuclear fission. Less well known but more ubiquitous on a stellar scale is fission’s big brother - fusion.
Nuclear fusion is a process by which light-weight atoms combine to form heavier atoms. Depending on the weight of the reactant atoms and the product atom, the process can generate immense amounts of energy. The most energetic object in our solar system - the very sun that produces all solar, wind and hydroelectric power on Earth - is powered by the fusion process. The stars that wink at us at night from many light years away are powered by fusion. Fusion is responsible for the creation of the heavier atoms that make up most of the matter on planets, including living beings such as ourselves. The very ground we stand on only exists because of the fusion process.
The physics of fusion
The nucleus of atoms contains two types of particles, protons and neutrons, collectively nucleons. Protons are positively charged while neutrons have no charge. As any high school physics student knows, particles of the same charge repel each other and that repulsion decreases with the square of the distance between the particles. It follows then that very small distances - such as in the atomic nucleus - would result in incredibly high repulsive forces between the positively charged protons. This force between two electrically charged particles is known as the Coulomb force, named after French physicist Charles-Augustin de Coulomb. His discovery of the force and its variation with distance was integral to the study of electromagnetism and the development of electromagnetic devices.
The effect of the inverse square law on the force of gravity. A similar effect is observed for the Coulomb force (Encyclopaedia Brittanica)
How then do atoms keep their nuclei from disintegrating? The answer lies in the nuclear force - a short-range attractive force that acts between nucleons and is powerful enough to overcome the Coulomb force. The Coulomb force makes it difficult to force together two or more protons but, once they are brought together, the short-range nuclear force takes over and the resultant atomic nucleus is stable.
How short is this range and how small are these atoms? This video provides an idea of the scales we are talking about.
What does this process look like? Perhaps the easiest example to envision is one we see every day - the sun. Every second, the sun converts approximately 620 million tons of hydrogen to helium. In less than 400,000 years, approximately 1 Earth’s weight of hydrogen atoms is consumed. These atoms are squeezed together by the immense pressure at the Sun’s core and fusion converts them to helium. The mass at the end of this process is slightly less than that at the beginning; as we saw with fission, the missing mass is converted to energy via a familiar equation, E=mc2.
Fusion process in the sun (from Energy Education)
Nuclear fusion on Earth
Hydrogen is plentiful on Earth, found mostly in the water that covers 70% of the planet. It can be produced in several different ways - from natural gas via the gasification process, from water via hydrolysis or from sugar-rich feedstocks via fermentation, to name a few. When it burns, the only product is water. The ubiquity of hydrogen and the non-toxicity of water make hydrogen an interesting fuel that is being explored for several uses such as transportation, heating and electricity generation. Of perhaps even greater interest, however, is the possibility of using hydrogen for fusion process on Earth.
Helium, the product of this fusion reaction, is also non-toxic which makes fusion an attractive energy source compared to fission, where waste materials may be radioactive. Helium also has several uses such as in superconductors (think MRI machines) and quantum computing (a topic we have discussed here), so the “waste” products of fusion are not necessarily wasted. More importantly, fusion reactors are not vulnerable to meltdowns such as the accident at Chernobyl - the reactor would contain insufficient fuel for a runaway reaction and the peak temperature would be well under that of the melting point of the reactor walls.
Why, then, do we not already have fusion reactors producing gigantic amounts of clean, cheap energy to replace our current energy sources? Simply, we haven’t found a way to make fusion happen at anywhere near the scale we need. The main difficulty is in controlling the behaviour and pressure of a dense cloud of ‘plasma’ - hydrogen nuclei - well enough for it to undergo fusion.
The most promising method for controlling plasma is via magnetic confinement. Plasma’s positive charge means any moving plasma in a magnetic field is subject to the Lorentz force, which enables the plasma to be accelerated in a desired direction by the application of a magnetic field of the correct orientation. One popular application of magnetic confinement is the tokamak, a doughnut-shaped chamber with a magnetic coil.
Schematic of a tokamak (from ITER)
The frontrunner for fusion research is currently ITER (International Thermonuclear Experimental Reactor). ITER is a truly global effort to harness fusion for energy generation using a tokamak. Involving 35 different countries and estimated to cost USD 65 billion, it is the most expensive science experiment in history - twice as expensive as the famous Large Hadron Collider at CERN. The construction of ITER at its site in France is expected to be complete by 2025.
But, as we explored in a previous article, Goliath doesn’t always come out on top. As with most scientific advances in recent years, the presence of a vibrant ecosystem of small companies and startups bodes well for the prospects of us cracking the fusion enigma. Several companies have ambitious plans for demonstrating practical, workable fusion prototypes in the next decade.
First Light Fusion, a company spun out of the University of Oxford, uses a principle somewhat different to that of magnetic confinement, called inertial confinement. Instead of using magnetic fields to control the plasma, it is confined by its own inertia when it is highly compressed. One application of this (known as the “direct drive” method) shoots a laser at a spherical pellet containing isotopes of hydrogen. The outer shell explodes; momentum is transferred to the contents which are highly compressed and may undergo fusion if conditions are right.
Oxford is not the only university getting into the fusion game. Commonwealth Fusion Systems, spun out of MIT, is using a compact tokamak and high temperature superconducting magnets in their fusion experiments. They expect to start commercialising fusion in 2025.
One of the largest private players in this industry is General Fusion, which is collaborating with big names such as Microsoft, McGill University and the Princeton Plasma Physics Laboratory. Among their unusual innovations is a liquid metal reactor wall to help absorb energy from the fusion reaction and protect the outer reactor walls.
Our previous article on nuclear energy shone a light on the dark side of nuclear fission. While its safety record is undeniably impressive, the spectre of accidents at power plants looms large in the public’s mind whenever nuclear energy is brought up. Several accidents in storage and disposal of waste have not helped its case.
Fusion process poses none of these risks. It remains a safe, clean and plentiful source of energy - all that fission was promised to be in the early post-war years. While fusion is still very much a phenomenon of research laboratories, if there’s one thing history has taught us, it’s that humans find a way. While research on fusion continues apace, who would bet against it?
Our next article in this series on energy focuses on some rather more familiar technology - renewables.
Disclaimer: This article is based on our personal opinion and does not reflect or represent the views of any organisation that we might be associated with.