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Viable nuclear fusion

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A viable nuclear fusion reactor — one that spits out more energy than it consumes — could be here as soon as 2025.

That’s the takeaway of seven new studies, published Sept. 29 in the Journal of Plasma Physics.

If a fusion reactor reaches that milestone, it could pave the way for massive generation of clean energy.

During fusion, atomic nuclei are forced together to form heavier atoms. When the mass of the resulting atoms is less than the mass of the atoms that went into their creation, the excess mass is converted to energy, liberating an extraordinary amount of light and heat. Fusion powers the sun and stars, as the mighty gravity at their hearts fuse hydrogen to create helium.

But an enormous amount of energy is needed to force atoms to fuse together, which occurs at temperatures of at least 180 million degrees Fahrenheit (100 million degrees Celsius). However, such reactions can generate far more energy than they require. At the same time, fusion doesn’t produce greenhouse gases such as carbon dioxide, which drive global warming, nor does it generate other pollutants. And the fuel for fusion — such as the element hydrogen — is plentiful enough on Earth to meet all of humanity’s energy needs for millions of years.

"Virtually all of us got into this research because we’re trying to solve a really serious global problem," said study author Martin Greenwald, a plasma physicist at MIT and one of the lead scientists developing the new reactor. "We want to have an impact on society. We need a solution for global warming — otherwise, civilization is in trouble. This looks like it might help fix that."

Most experimental fusion reactors employ a donut-shaped Russian design called a tokamak. These designs use powerful magnetic fields to confine a cloud of plasma, or ionized gas, at extreme temperatures, high enough for atoms to fuse together. The new experimental device, called the SPARC (Soonest/Smallest Private-Funded Affordable Robust Compact) reactor, is being developed by scientists at MIT and a spinoff company, Commonwealth Fusion Systems.

If it succeeds, SPARC would be the first device to ever achieve a "burning plasma," in which the heat from all the fusion reactions keeps fusion going without the need to pump in extra energy. But no one has ever been able to harness the power of burning plasma in a controlled reaction here on Earth, and more research is needed before SPARC can do so. The SPARC project, which launched in 2018, is scheduled to begin construction next June, with the reactor starting operations in 2025. This is far faster than the world’s largest fusion power project, known as the International Thermonuclear Experimental Reactor (ITER), which was conceived in 1985 but not launched until 2007; and although construction began in 2013, the project is not expected to generate a fusion reaction until 2035.

One advantage that SPARC may have over ITER is that SPARC’s magnets are designed to confine its plasma. SPARC will use so-called high-temperature superconducting magnets that only became commercially available in the past three to five years, long after ITER was first designed. These new magnets can produce far more powerful magnetic fields than ITER’s — a maximum of 21 teslas, compared with ITER’s maximum of 12 teslas. (In comparison, Earth’s magnetic field ranges in strength from 30 millionths to 60 millionths of a tesla.)

These powerful magnets suggest the core of SPARC can be about three times smaller in diameter, and 60 to 70 times smaller in volume than the heart of ITER, which is slated to be 6 meters wide. "That dramatic reduction in size is accompanied by a reduction in weight and cost," Greenwald , told LiveScience. "That’s really the game-changer."

In seven new studies, researchers outlined the calculations and supercomputer simulations underlying SPARC’s design. SPARC is expected to generate at least twice as much as 10 times more energy as is pumped in, the studies found.

The heat from a fusion reactor would generate steam. This steam would then drive a turbine and electrical generator, the same way most electricity is produced nowadays.

"Fusion power plants could be one-to-one replacements for fossil fuel plants, and you wouldn’t have to restructure electrical grids for them," Greenwald said. In contrast, renewable energy sources such as solar and wind "are not accommodated well by the current design of electric grids."

The researchers ultimately hope SPARC-inspired fusion power plants would generate between 250 to 1,000 megawatts of electricity. "In the current power market of the United States, power plants typically generate between 100 to 500 megawatts," Greenwald said.

SPARC would only produce heat, not electricity. Once researchers have built and tested SPARC, they plan to construct the ARC (Affordable Robust Compact) reactor, which would generate electricity from that heat by 2035.

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nuclear fusion

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Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. This difference in mass arises due to the difference in atomic binding energy between the nuclei before and after the reaction. Fusion is the process that powers active or main sequence stars and other high-magnitude stars, where large amounts of energy are released.
The Sun is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 500 million metric tons of hydrogen each second.

The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above.
A fusion process that produces nuclei lighter than iron-56 or nickel-62 will generally release energy. These elements have relatively small mass per nucleon and large binding energy per nucleon. Fusion of nuclei lighter than these releases energy (an exothermic process), while fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is endothermic. The opposite is true for the reverse process, nuclear fission. This means that the lighter elements, such as hydrogen and helium, are in general more fusible; while the heavier elements, such as uranium, thorium and plutonium, are more fissionable. The extreme astrophysical event of a supernova can produce enough energy to fuse nuclei into elements heavier than iron.

In 1920, Arthur Eddington suggested hydrogen-helium fusion could be the primary source of stellar energy. Quantum tunneling was discovered by Friedrich Hund in 1929, and shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in nuclear transmutation by Ernest Rutherford, laboratory fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars was worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on 1 November 1952, in the Ivy Mike hydrogen bomb test.

Research into developing controlled fusion inside fusion reactors has been ongoing since the 1940s, but the technology is still in its development phase.

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