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In nuclear physics, nuclear chemistry, and astrophysics, nuclear fusion is the process in which two or more atomic nuclei join together, or fuse, to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. Large scale thermonuclear fusion processes, involving many nuclei fusing at once, must occur in matter at very high densities and temperatures.
The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission.
In the simplest case of hydrogen fusion, two protons have to be brought close enough for the weak nuclear force to convert either of the identical protons into a neutron forming the hydrogen isotope deuterium. In more complex cases of heavy ion fusion involving two or more nucleons, the reaction mechanism is different, but the same result occurs - one of combining smaller nuclei into larger nuclei.
Nuclear fusion occurs naturally in all active stars. See astrophysics. Synthetic fusion as a result of human actions has also been achieved, although this has not yet been completely controlled as a source of nuclear power. In the laboratory, successful nuclear physics experiments have been carried out that involve the fusion of many different varieties of nuclei, but the energy output has been negligible in these studies. In fact, the amount of energy put into the process has always exceeded the energy output.
Uncontrolled nuclear fusion has been carried out many times in nuclear weapons testing, which always results in a deliberate explosion. These explosions have always used the heavy isotopes of hydrogen, deuterium (H-2) and tritium (H-3), and never the much more common isotope of hydrogen (H-1), sometimes called "protium".
Building upon the nuclear transmutation experiments by Ernest Rutherford, carried out several years earlier, the fusion of the light nuclei (hydrogen isotopes) was first accomplished by Mark Oliphant in 1932. Then, the steps of the main cycle of nuclear fusion in stars were first worked out by Hans Bethe throughout the remainder of that decade.
Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project, but this was not accomplished until 1951 (see the Greenhouse Item nuclear test), and nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the Ivy Mike hydrogen bomb test. Research into developing controlled thermonuclear fusion for civil purposes also began in the 1950s, and it continues to this day.
Nuclear Fission and Fusion
Fission and fusion are two processes that alter the nucleus of an atom.
Nuclear fission provides the energy in nuclear power plants and fusion is the source
of the sun's energy. The use of fission in power plants can help conserve fossil fuels.
Without the energy produced by the fusion of hydrogen in the sun, the Earth would
quickly change into a cold planet that could not support life as we know it.
• Compare and contrast nuclear fission and nuclear fusion.
• Understand the conservation laws that apply to nuclear reactions.
• Find missing reactants/products in nuclear equations.
• Identify nuclear reactions as examples of fission or fusion.
• Structure of atoms and isotopes.
• Knowledge of mass number and atomic number.
• Atomic symbols including mass number, atomic number, and charge.
Transmutation is the transformation of the nucleus of an atom so that the
atom is changed from one element into a different element. This can be
accomplished through many types of reactions, including fission and fusion.
Nuclear transformations always obey two fundamental conservation laws:
(1) mass number is conserved and (2) electrical charge is conserved.
Energy and mass are not conserved, but can be inter-converted according to
Einstein's equation, E = mc2.
Nuclear Fission and Fusion
© POGIL - 2005 2/5
Written by Michael Fusaro; Assessed by: Rohini Quackenbush, Erin Graham, and Lizabeth Tumminello
Edited by Linda Padwa and David Hanson, Stony Brook University
The process of fission occurs when a nucleus splits into smaller pieces. Fission can be
induced by a nucleus capturing slow moving neutrons, which results in the nucleus
becoming very unstable.
The following equations represent fission reactions, where n = neutron.
92U + 1
0n â†’ 141
56Ba + 92
36Kr + 3 1
92U + 1
0n â†’ 131
50Sn + 103
42Mo + 2 1
92U + 1
0n â†’ 137
52Te + 97
40Zr + 2 1
92U + 1
0n â†’ 138
54Xe + 95
38Sr + 3 1
92U + 1
0n â†’ 152
60Nd + 81
32Ge + 3 1
Fusion occurs when 2 nuclei join together to form a larger nucleus. Fusion is brought
about by bringing together two or more small nuclei under conditions of tremendous
pressure and heat.
(Phillips, Strozak, Wistrom, Glencoe Chemistry. 2002 p. 766)
The following equations represent fusion reactions, where p = proton.
1H + 2
1H â†’ 3
1H + 1
2He + 3
2He â†’ 4
2He + 21
1H + 3
1H â†’ 4
2He + 1
An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released. The energy is both heat and light energy. Einstein said that a very small amount of matter contains a very LARGE amount of energy. This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it can make a tremendous explosion in an atomic bomb.
A nuclear power plant (like Diablo Canyon Nuclear Plant shown on the right) uses uranium as a "fuel." Uranium is an element that is dug out of the ground many places around the world. It is processed into tiny pellets that are loaded into very long rods that are put into the power plant's reactor.
The word fission means to split apart. Inside the reactor of an atomic power plant, uranium atoms are split apart in a controlled chain reaction.
In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms splitting those. Those particles given off split still other atoms in a chain reaction. In nuclear power plants, control rods are used to keep the splitting regulated so it doesn't go too fast.
If the reaction is not controlled, you could have an atomic bomb. But in atomic bombs, almost pure pieces of the element Uranium-235 or Plutonium, of a precise mass and shape, must be brought together and held together, with great force. These conditions are not present in a nuclear reactor.
The reaction also creates radioactive material. This material could hurt people if released, so it is kept in a solid form. The very strong concrete dome in the picture is designed to keep this material inside if an accident happens.
This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy.
This water from around the nuclear core is sent to another section of the power plant. Here, in the heat exchanger, it heats another set of pipes filled with water to make steam. The steam in this second set of pipes turns a turbine to generate electricity. Below is a cross section of the inside of a typical nuclear power plant.
Power plant drawing courtesy Nuclear Institute
Another form of nuclear energy is called fusion. Fusion means joining smaller nuclei (the plural of nucleus) to make a larger nucleus. The sun uses nuclear fusion of hydrogen atoms into helium atoms. This gives off heat and light and other radiation.
In the picture to the right, two types of hydrogen atoms, deuterium and tritium, combine to make a helium atom and an extra particle called a neutron.
Also given off in this fusion reaction is energy! Thanks to the University of California, Berkeley for the picture.
Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity. But they have been having trouble learning how to control the reaction in a contained space.
What's better about nuclear fusion is that it creates less radioactive material than fission, and its supply of fuel can last longer than the sun.