Nuclear fusion occurs when two smaller nuclei combine to form a larger nucleus. Fusion is essentially the reverse process of nuclear fission, but it only releases useful energy in light nuclei, unlike fission. When light nuclei undergo fusion, the heavier product is actually less massive than the sum of the masses of the reactants, and energy is released. (It is possible for elements heavier than iron to undergo fusion, but such reactions require an input of energy because the resultant nucleus is more massive than the sum of the masses of the two original nuclei.)
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An example of nuclear fusion is the collision between deuterium and tritium to form helium-4 and a neutron (Figure 10):
The products are less massive than the reactants by 0.018883 u, so this is an exothermic reaction and releases energy.
All nuclei are positively charged, so they tend to be pushed apart by electric repulsion. Furthermore, this repulsion increases in strength as the nuclei become closer to one another. How then is it possible for two nuclei to fuse together? If the nuclei collide at extremely high speeds, they can become momentarily close enough that the nuclear force (which is attractive at extremely short distances) will overcome the electric repulsion. In order to do so, the nuclei must exist at temperatures at least 10^7 K. Fusion that results from rapidly moving nuclei at extremely high temperatures is called thermonuclear fusion.
Fusion Interactive on BBC
Stellar Fusion
All stars are naturally occurring fusion reactors that operate at temperatures of millions of degrees. The majority of stars, including the Sun, are powered by hydrogen fusion. Other, mostly older stars fuse heavier elements such as helium. The Sun is approximately 74% hydrogen and 25% helium by mass, with the remaining percent consisting of other trace elements. Every second, the Sun converts about 657 million tons of hydrogen into 653 million tons of helium through fusion reactions. The “lost” 4 million tons of particle mass is released as radiation energy, and eventually reaches us as light and the “solar wind”—a collection of low-mass particles.
All stars are naturally occurring fusion reactors that operate at temperatures of millions of degrees. The majority of stars, including the Sun, are powered by hydrogen fusion. Other, mostly older stars fuse heavier elements such as helium. The Sun is approximately 74% hydrogen and 25% helium by mass, with the remaining percent consisting of other trace elements. Every second, the Sun converts about 657 million tons of hydrogen into 653 million tons of helium through fusion reactions. The “lost” 4 million tons of particle mass is released as radiation energy, and eventually reaches us as light and the “solar wind”—a collection of low-mass particles.
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Stars are constantly collapsing inward due to the attractive force of gravity. The energy produced by nuclear fusion increases the outward pressure within the star, which opposes the tendency toward gravitational collapse. In order to maintain a sustainable rate of energy production, stars must be both hot enough and dense enough to ensure that particles will collide with a high enough frequency to cause fusion reactions.
The energy generated by the Sun is the result of a multi-step fusion reaction known as the proton-proton chain (Figure 11).
The net result of this series of reactions is the conversion of four protons into one helium-4 nucleus, two positrons, two neutrinos, and two gamma rays.
In 1920, Arthur Eddington was the first to propose that the energy produced by stars resulted from the fusion of hydrogen to form helium. However, the exact combination of reactions had not been worked out. In 1934, Mark Oliphant and Ernest Rutherford became the first to experimentally demonstrate nuclear fusion by colliding two deuterium nuclei to form a helium nucleus, lending support to Eddington’s hypothesis. In 1939, Hans Bethe proposed the proton-proton chain, which accurately predicted the temperature, density, and composition of stars, including the Sun. Bethe was awarded the 1967 Nobel Prize for his explanations of stellar energy production.
Fusion Weapons
Nuclear fusion can also be harnessed for destructive purposes in what is known as a thermonuclear weapon or hydrogen bomb (“H-bomb”). Thermonuclear weapons use a small fission reaction to heat and compress the fusion fuel. A series of fission charges surrounding a mixture of fission and fusion fuel (the primary bomb) compresses the fuel and induces fusion. The energy released from the primary stage heats and compresses a secondary stage, causing an additional fusion reaction and releasing even more energy.
Nuclear fusion can also be harnessed for destructive purposes in what is known as a thermonuclear weapon or hydrogen bomb (“H-bomb”). Thermonuclear weapons use a small fission reaction to heat and compress the fusion fuel. A series of fission charges surrounding a mixture of fission and fusion fuel (the primary bomb) compresses the fuel and induces fusion. The energy released from the primary stage heats and compresses a secondary stage, causing an additional fusion reaction and releasing even more energy.
Hungarian-American physicist Edward Teller conceived of the principle behind the hydrogen bomb during his service on the Manhattan Project. When it was discovered that the temperatures inside an atomic bomb were several times greater than the core of the Sun, Teller proposed that a nuclear fission explosion could initiate a potentially even more destructive fusion reaction. Based on calculations by Stanislaw Ulam and others, Teller subsequently developed the final design for a hydrogen bomb in 1951. Although Teller was not the sole contributor to the development of the bomb, his continued passion for the project led to his reputation as “the father of the hydrogen bomb.”
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The first hydrogen bomb, which utilized Teller’s multi-stage design, was detonated in 1952. The bomb produced a 10.4 megaton blast, which was more than 450 times more powerful than the fission bomb released over Nagasaki. The Tsar Bomba, the most powerful nuclear weapon ever detonated, was a hydrogen bomb. Its detonation on October 30, 1961 in the USSR produced the energy equivalent of 50 million tons of explosives.
Fusion Reactors
The possibility of controlled fusion for the purposes of power generation has remained an attractive prospect for more than half a century. The fusion reactions that offer the greatest potential for power generation involve the deuterium and tritium isotopes of hydrogen.
The possibility of controlled fusion for the purposes of power generation has remained an attractive prospect for more than half a century. The fusion reactions that offer the greatest potential for power generation involve the deuterium and tritium isotopes of hydrogen.
The fuel for nuclear fusion is extremely abundant. Hydrogen is the most plentiful element in the universe, and its isotopes can be harvested from ordinary water. The deuterium supply within the world’s oceans represents more available energy than the world’s supply of fossil fuels or uranium. Thirty liters of seawater contain 1 gram of deuterium, from which the energy equivalent of 10,000 liters of gasoline can be extracted through fusion. Although tritium has a very low natural abundance, it can be artificially produced.
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On paper, fusion offers a number of advantages over fission for the purposes of power generation. We have already discussed how fusion reactions have a higher energy density than fission reactions, meaning more energy can be extracted per unit mass of fuel. Furthermore, unlike fission reactors, fusion reactors have no risk of meltdown. Nuclear fusion also produces no air pollution (the only byproduct is helium) and produces no radioactive waste.
Unfortunately, there are a number of challenges to overcome before fusion can become a reliable power source. The greatest challenge to controlled nuclear fusion is the extremely high temperature necessary to ensure that nuclei are energetic enough to fuse. As we have seen, hydrogen fusion in the Sun requires temperatures of at least ten million degrees. Temperatures of this magnitude are difficult to achieve and maintain in a laboratory or power plant.
Fusion reactors require not only extremely high temperatures, but also a sufficiently high density of nuclei to ensure that fusion reactions will occur with a great enough frequency. When atoms are heated past their ionization energy, their electrons are no longer bound to the nucleus. A collection of superheated atoms forms a cloud of positively charged nuclei and negatively charged electrons called a plasma. Containment of high-temperature plasma poses a challenge because ordinary structural materials vaporize at temperatures above several thousand degrees. Therefore, scientists must utilize nonmaterial containment methods to generate and store plasmas.
The two primary techniques for containing plasmas are known as magnetic confinement and inertial confinement. Magnetic confinement relies on the fact that the pathways of charged particles, such as the ions within a plasma, can be bent using strong magnetic fields. A “magnetic bottle” is a magnetic field configured in such a way that moving particles will be reflected backward, and thus contained, when they encounter areas of greater field strength. A tokamak is a doughnut-shaped device that confines a plasma using a combination of two magnetic fields (Figure 12). The tokamak was developed in the former Soviet Union following World War II.
Inertial confinement fusion (ICF) relies on high-intensity laser beams to heat and compress a tiny pellet that contains a mixture of deuterium and tritium (Figure 13). Whereas magnetic confinement sustains fusion by extending the time that ions interact at high speeds, the goal of inertial confinement is to cause fusion fast enough that there is not enough time for nuclei to move apart. The deuterium-tritium pellet is quickly ionized into a plasma and heated to temperatures above 10^8 K. Fusion is achieved in under 10^-9s, or less than one billionth of a second.
The largest and highest-energy ICF device in the world is the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory in California (Figure 14). The NIF uses the most powerful laser ever constructed to ignite fusion in a hydrogen pellet 2 mm in diameter. In 2012, the NIF conducted a laser pulse with a power of 500 trillion watts—equal to 1,000 times the power used by the entire United States at any instant in time.
Today, the goal of fusion power has united several nations toward constructing the world’s largest experimental fusion reactor. ITER (formerly an acronym for International Thermonuclear Experimental Reactor) is a collaboration between seven members (the European Union, India, Japan, China, Russia, and the United States). The purpose of ITER is to demonstrate magnetic confinement fusion as a feasible energy source. Construction is currently underway in France, with an anticipated 2027 start date for first deuterium-tritium experiments.
Many challenges remain before fusion can become a viable power source, but ongoing advances in science and engineering bring it continuously closer to fruition. It is entirely conceivable that within your lifetime, fusion power will become a reality and have a transformative impact on our future global energy plan.