By Robert K. Kaufmann and Cutler J. ClevelandThe world's first web-based textbook on environmental science. Learn more


Nuclear Fission and Fusion

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Nuclear fission is the splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lower-mass elements), accompanied by the release of a relatively large amount of energy in the form of kinetic energy of the two parts and in the form of emission of neutrons and gamma rays. Nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. Fission and fusion are two of the most fundamental processes in the Universe.

Fission and fusion are best understood in terms of force that binds the nuclei of atoms together-known to physicists as the strong nuclear force, or just the strong force. It is by far strongest of the four fundamental forces of nature (the others being gravity, the electromagnetic force, and the so-called weak force). The discovery and understanding of the strong force transformed our understanding of the universe. It also led to development of nuclear technologies in revolutionary news applications in electricity generation, medicine, and in the manufacture of weapons of mass production.

To understand the awesome power of the atom, we must begin with a deeper understanding of the basic physics underlying the atom itself.

Einstein's famous equation

The mass of any nucleus is less than the sum of the separate masses of its protons and neutrons. How can this be? The reason, as Albert Einstein first demonstrated, is that that mass and energy are two different forms of the same thing. The "missing" mass of the protons and neutrons is converted to energy. Thus, energy is released when nuclei are built. Binding energy is the term that describes the amount of energy released when a nucleus is created, and it is calculated by finding the quantity of mass that "disappears" using Einstein's famous equation E=mc2. The higher the binding energy, the more tightly the protons and neutrons are held together in the nucleus. Binding energy is also the amount of energy you'd need to add to a nucleus to break it up into protons and neutrons again; the larger the binding energy, the more difficult that would be.

Here is an example. A hydrogen-2 nucleus, which has one proton and one neutron, can be completely separated by supplying 2.23 million electron volts (MeV) of energy. Electron-volts is a standard unit of energy in nuclear physics. Conversely, when a slowly moving neutron and proton combine to form a hydrogen-2 nucleus, 2.23 MeV are released in the form of gamma radiation.

The curve of binding energy is shown below. It shows the amount of binding energy per nucleon (a nucleon is either a neutron or a proton. The nucleon number is the sum of the number of neutrons plus protons in a nucleus; thus, it is equal to the atomic mass number) as a function of the atomic mass number A. This curve indicates how stable atomic nuclei are; a higher position on the indicates a more stable the nucleus. Notice the characteristic shape, with a peak near an atomic number of about 60. These nuclei (which are near iron in the periodic table and are called the iron peak nuclei) are the most stable in the Universe.

The curve of binding energy suggests two possibilities for converting significant amounts of mass into energy. The heaviest nuclei are less stable than the nuclei near A=60. This suggests that energy can be released if heavy nuclei split apart into smaller nuclei having masses nearer A=60. This process is called fission. It is the process that powers atomic bombs and nuclear power reactors. On the other hand, the lightest elements (like hydrogen and helium) have nuclei that are less stable than heavier elements up to A~60. Thus, combing two light nuclei together to form a heavier nucleus can release energy. This process is called fusion, and is the process that powers hydrogen (thermonuclear) bombs and (perhaps eventually) fusion energy reactors.


There are two types of fission. Heavy elements such as uranium, thorium, and plutonium, undergo both spontaneous fission, a form of radioactive decay. Radioactive decay occurs when and unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. Most elements undergo spontaneous fission very slowly, a process measured by half-life: the time required for one half of a radioactive sample to decay (Table below).

Induced fission occurs when an atom splits when struck by a neutron or other particle. For example, the fission of U235 releases two smaller "daughter atoms, three neutrons and energy:

Isotopes such as U-235 that undergo fission when struck by a neutron are called fissile isotopes.

If the neutrons released by the fission of one U-35 atom encounter other U-235 atoms, they initiate other fissions, producing even more neutrons. This continuing cascade of nuclear fissions called a chain reaction.

A few very fissile and readily obtainable isotopes (notably 235U and 239Pu) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be economically or technologically feasible.

A certain minimum amount of fissionable material is needed to support a chain reaction. If the sample is small, then the neutrons are less likely strike another a U-235 nucleus. In turn, if the neutrons don't hit a U-235 nucleus, no extra electrons and no energy are released. The reaction fizzles out. The minimum amount of fissionable material needed to produce a chain reaction occurs is called the critical mass. A quantity less than this amount is called subcritical.


Fusion reactions power the stars (including our Sun) and also produce all but the lightest elements. A substantial energy barrier must be overcome before the fusion of two nuclei occurs due to the repulsive force between their positively charged protons. If two nuclei can be brought close enough together, however, this repulsion can be overcome by the nuclear force which is very strong at close distances. In stars, that energy source is provided by very high temperatures and pressures. The core of the Sun is about 15,000,000° C (27,000,000° F), and its pressure is about340 billion times Earth's air pressure at sea level.

The most common fusion reaction in stars is thought to the proton-proton fusion chain. In this reaction, four hydrogen nuclei, each with the mass of one proton, fuse to form a single helium nucleus (two protons and two neutrons) that has a mass of 3.97 times the mass of one proton. An amount of mass equal to 0.03 times the mass of one proton is given up and converted to energy equal to 0.03 x (mass one proton) x c2 (remember that c is the speed of light). The energy released is in the form of gamma rays.

Energy Release from Nuclear Reactions

The energy released in most nuclear reactions is much larger than that in chemical reactions, such as the combustion of methane (natural gas), coal, or oil. The reason is that the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. As a result, the energy released from a kilogram of nuclear fuel releases nearly 64,000 times the energy than a kilogram of coal. The energy release from a kilogram of fuel derived from deuterium and lithium in a hypothetical fusion power plant is more than 10,000,000 times a kilogram of coal.

Applications of fission and fusion

The U-235 chain reaction is the basis of nuclear power, which is defined as the controlled use of nuclear reactions to generate energy for propulsion, heat, and the generation of electricity. In a nuclear power plant, the heat released from the fission events converts water into steam, and the stem spins a turbine that in turn runs a generator that produces electricity. Nuclear electricity accounts for about 20% of global electricity consumption.

Nuclear propulsion refers to propulsion technologies that use some form of nuclear reaction as a principal power source. Nuclear power is used in most large submarines. A growing number of large surface ships such as icebreakers use nuclear reactors as their power plants. NASA is also exploring nuclear propulsion as a possible means of power spacecraft, including a manned mission to Mars.

Nuclear medicine uses the nuclear properties of materials in diagnosis and therapy, particularly in body imaging and the treatment of disease. Diagnostic techniques in nuclear medicine use radioactive tracers that release emit gamma rays from within the body. These tracers are generally short-lived isotopes linked to chemical compounds that permit specific physiological processes to be studied.

Nuclear medicine imaging techniques such as Positron emission tomography (PET ) provide heath care providers with a non-invasive methods to look inside the body. The techniques combine the use of computers, detectors, and radioactive substances. Iodine-131 is commonly used to treat thyroid cancer, and ranks among the most successful form of cancer treatment. Strontium-89 and (increasingly) samarium 153 are used for the relief of cancer-induced bone pain. Five Nobel Laureates have been closely involved with the use of radioactive tracers in medicine.

Radiometric dating (radioactive dating) is used by scientists from many disciplines to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. It is the primary source of information about the age of the Earth, and it significant source of information about rates of evolutionary change.

The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:

Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values
Uranium-238 Lead-206 4.5 billion years
Uranium-235 Lead-207 704 million years
Thorium-232 Lead-208 14.0 billion years
Rubidium-87 Strontium-87 48.8 billion years
Potassium-40 Argon-40 1.25 billion years
Samarium-147 Neodymium-143 106 billion years

Uranium-lead is one of the oldest and most refined radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range.

The generation of electricity from a fusion reactor has been the subject of intense study and debate for nearly half a century. Most proposals for fusion power plants involve using a fusion reaction to generate heat, which is then used to operate a steam turbine, similar to fission-driven nuclear power plants as well as fossil fuel-driven power plants.

The most promising technology is based on fusing deuterium and tritium. Deuterium is an abundant, naturally occurring isotope of hydrogen. The tritium is produced from the breeding of lithium. The deuterium-tritium mixture is placed in a reactor chamber and there ionized and heated to thermonuclear temperatures. The fuel is held away from the chamber walls by magnetic forces, and the fusion process generates neutrons. The neutrons' energy of motion is given up through many collisions with lithium nuclei, thus creating heat that is removed by a heat exchanger which conveys it to a conventional steam electric plant.

There have been enormous strides in understanding the basic physics and engineering behind a fusion power plant. However, the enormous challenge of producing and maintaining temperatures of 10 million degrees C in the reactor has not been fully overcome. As a result, it is unlikely that fusion will be a commercial source of electricity in for the foreseeable future.

Fission and fusion also are the basis for nuclear weapons, which are the most powerful weapons that humans have every developed and used. There are two types of nuclear weapons. In fission weapons, a mass of enriched uranium or plutonium is configured into a supercritical mass that releases enormous energy when detonated. These are known as atomic bombs, A-bombs, or fission bombs, and were the type of weapon that the U.S. used against Japan in World War II.

The second type of nuclear weapon produces a tremendous amount of its energy through nuclear fusion reactions. These are known as hydrogen bombs, H-bombs, thermonuclear bombs, or fusion bombs. Hydrogen bombs work by using the energy released by a fission bomb to compress and heat fusion fuel. Such weapons can be up to a thousand times more powerful than fission bombs Six nations countries have detonated hydrogen bombs: United States, Russia, United Kingdom, People's Republic of China, France, and India.

Early History

The basic discovery and chemical proof of Otto Hahn and Fritz Strassmann that an isotope of barium was produced by neutron bombardment of uranium was published in 1939 in a paper in Germany in the Journal Naturwissenschaften, and earned Hahn a Nobel Prize. Shortly after, their Austrian co-worker Lise Meitner (a political refugee in Sweden at the time) and her nephew Otto Robert Frisch correctly interpreted the results as the splitting of the uranium nucleus after the absorption of a neutron-nuclear fission-that released a large amount of energy and additional neutrons. A direct experimental evidence of the nuclear fission was performed by Frisch.

The idea that nuclear fusion might explain stellar energy release goes back at least seventy years, to the writings of the American physicist William Draper Harkins (1873-1951), the Dutch physicists R. d'E. Atkinson and F. G. Houtermans, and the French physicist Jean Baptiste Perrin. Atkinson and Houtermans used the measured masses of low mass elements and applied Einstein's discovery that E=mc2 to predict that large amounts of energy could be released by fusing small nuclei together.

German physicist Hans Bethe presented the clearest development of this idea in about 1938. Bethe worked out a series of reactions by which six hydrogen nuclei might combine to form a single helium nucleus, with the release of two extra hydrogen nuclei. The production of two hydrogen nuclei in the reaction make it possible for a second fusion reaction to begin. The Bethe cycle, then, is a chain reaction that, once initiated, can proceed on its own as long as sufficient raw material (hydrogen) is available. Bethe will win the 1967 Nobel Prize in physics "for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars."




Cleveland, C. (2013). Nuclear Fission and Fusion. Retrieved from