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Nuclear fusion plants could provide a safe, clean, and efficient energy source that cannot be matched by the latest fission reactors. Still a decade away from implementation, this energy source — inspired by the processes that power the stars — just received a major boost with new research confirming its viability.
During a period of increasing energy demands and diminishing fossil fuel reserves, coupled with a need to reduce emissions and protect the environment, we have increasingly turned to the Sun for clean renewable energy.
What if, in addition to harvesting the energy produced by the Sun, we could take the process that creates that energy, bring it down to Earth, and replicate it here on the surface of our planet?
Fusion power — the generation of electricity from the heat created by nuclear fusion reactions similar to those which occur in the heart of the Sun —could significantly transform the planet’s energy landscape.
Scientists and engineers worldwide are developing the first commercially viable nuclear fusion plant. Unlike current nuclear fission plants, fusion is clean , with reduced radioactivity, no danger of chain reaction accidents, little waste production, and virtually no greenhouse emissions.
Fusion power plants could provide emissions-free energy and the fuel they require — the isotope of hydrogen-deuterium — is abundantly available. However, such fusion plants are not yet a reality. Before this holy grail of energy can be realized, further research is needed to provide experimental evidence of the fusion concept.
Ahead of the first fully operational emission-free fusion power plant — the International Thermonuclear Experimental Reactor (ITER) expected to begin operations within the next decade or so — researchers at Massachusetts Institute of Technology (MIT) have released a series of papers detailing another pioneering next-generation fusion experiment.
The result of a two and half year collaboration with Commonwealth Fusion Systems (CFS) — a start-up that branched out from MIT’s Plasma Science and Fusion Centre — the experiment, known as SPARC, shows the viability of fusion power and could lay the groundwork for practical, emissions-free fusion power plants.
The seven papers authored by 47 researchers from 12 separate institutes are published in a special edition of the Journal of Plasma Physics. Collectively, they lay out a roadmap that will guide the SPARC experiment, proving its viability and fusion power future.
Building a Reactor with Star Power
The SPARC reactor is based on fusing the nuclei of hydrogen atoms to form a heavier atomic nucleus. During this process, not all the mass from the initial particles go towards the daughter particle's mass. A tiny proportion of the initial mass is converted to energy . However, as the simplified version of Einstein’s mass/energy relationship E=mc² demonstrates , even a tiny amount of mass yields a lot of energy.
Fusion reactors are designed to exploit this energy. The technique used is similar to that which powers the stars, with the fusion process occurring in a plasma — the fourth state of matter consisting of a gas of atoms stripped of electrons.
Stars constrain this plasma thanks to intense gravity. The fact that fusion plants cannot rely on this extreme gravitational force to constrain plasma has been one of the hindrances to implementing such technology. Researchers have found a way around this problem by using intense magnetic fields to constrain plasma and hold it within reactors. A device that contains plasma with a strong magnetic field is generally known as a tokamak.
However, this magnetic field is not just for containment. In a star, fusion is driven by the intense pressure and temperatures caused by tremendous gravitational force. Therefore, the magnetic fields employed in tokamak have to be strong enough to cause the hydrogen ions to overcome their natural repulsion and force them together, kick-starting the fusion process.
This ‘plasma’ burning has never been seen on Earth and will allow the team to study the behavior of plasma as fusion occurs closely. Mapping this behavior is crucial for implementing emission-free nuclear fusion plants and the vital next step in the journey to fusion power — a working prototype.
Come Together: Why Fusion Beats Fission
Both fusion and fission power involves the liberation of energy locked within atoms. In contrast, fusion brings lighter atoms together to form heavier elements. Fission — the process upon which existing nuclear power plants are based — takes heavier elements and tears them apart.
Fission occurs when a neutron is slammed into a heavy atom — usually uranium or plutonium in nuclear reactors — exciting it and forcing it to split into two smaller atoms. This releases more neutrons, sparking a chain reaction. In fission plants, the energy released by this process heats water into steam, which can then drive a turbine to produce electricity without burning carbon.
While practically emission-free and much cleaner than burning fossil fuels, the fission process does not proceed without producing potentially dangerous waste products. The daughter products left over after energy liberated from the fission process are often radioactive and chemically active and pose a very real disposal problem.
The chain reaction that occurs during the fission reactions needs to be carefully controlled. The consequences of a runaway chain reaction can be devastating, as seen in Chernobyl's accidents and, more recently, Fukushima.
The main product created by nuclear fusion plants is helium which is not chemically active or radioactive. Fusion power is not dependant on a chain reaction meaning that a Fukushima-type nuclear accident is not possible in a fusion device. If a disturbance occurs in the magnetic field that constrains the plasma in a fusion plant, the plasma will cool almost immediately, ceasing reactions.
However, the advantages of fusion power over fission are not all to do with cleanliness and safety. There is also a significant economic advantage. Fission power beats fission because the hydrogen or deuterium needed to begin fusion reactions is far more abundant than the uranium or plutonium needed by fission plants. It is also much safer to handle.
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Can SPARC Improve on the Fusion Concept?
The SPARC experiment began life in 2018, and on completion, it is expected to be around twice the size of its fusion predecessor, MIT’s now-defunct Alcator C-Mod experiment.
Its power will vastly outstrip that of this previous experiment. However, SPARC’s capabilities are more in line with the much larger ITER reactor currently in construction in France. This boost in power without a corresponding increase in size is thanks to the team's superconducting magnets currently being developed. The strength of the field generated by these magnets means SPARC should constrain the plasma much more efficiently than ITER’s tokamak.
The benefits of hot plasma constraint in a more compact device can be conferred to other generators by adopting the superconducting magnets and their further development.
To conduct their research, MIT and CFS performed simulations on supercomputers developed and used by the international team of researchers and engineers working on ITER's design. These simulations allowed the development of a consensus of the best tools and techniques to maximize SPARC’s capabilities.
The computer simulations seem to show that SPARC's design specifications should allow it to meet, and even exceed, its planned fusion energy output. The efficiency of fusion plasma is measured by ‘Q-Factor’. The team’s series of studies indicated that SPARC should have a Q-Factor of 2 , meaning that it will produce twice as much fusion energy as input energy used to drive the nuclear reactions . However, the team calculates that this could reach an efficiency as high as Q 10.
This is significant as it would be the first time a fusion generator has produced more energy than it consumed. Therefore, SPARC could be somewhat unique amongst its fusion competitors. The first experimental device is likely to achieve a self-sustaining fusion reaction, fusing different isotopes of hydrogen with the need for further energy input.
Though SPARC's progress has been slowed by COVID-19, the MIT team still hopes that construction can start in June 2021. In the meantime, the researchers will now work on fine details of the machine, expecting some minor modifications, including a projected increase of the reactor’s diameter by about 12%.
Though a great deal is still to be learned about the burning of plasma, the seven papers in the MIT/ CFS team’s series represent a significant step forward for fusion power, confirming the viability of this emission-free, safe, energy revolution.
References and Further Reading
Dorland. W., Schekochihin. A., et al, [2020], JPP Special Issue: Status of the SPARC Physics Basis, [https://www.cambridge.org/core/journals/journal-of-plasma-physics/collections/status-of-the-sparc-physics-basis]
Advantages of Fusion, ITER Science, [https://www.iter.org/sci/Fusion].
SPARC, PSFC, MIT, [https://www.psfc.mit.edu/sparc]
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