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Figure 1 The fission process. When U-235 is bombarded by a neutron, it breaks into smaller fragments and 2 or 3 neutrons releasing a huge amount of energy.
Figure 1 The fission process. When U-235 is bombarded by a neutron, it breaks into smaller fragments and 2 or 3 neutrons releasing a huge amount of energy.

Fission is the splitting of an atomic nucleus. Very heavy nuclei, like isotopes of uranium and plutonium, are easiest to split. The process involves bombardment of these atoms by small sub-atomic particles like neutrons, which splits them into two fission products; two to three neutrons and excess energy in the form of gamma rays are also produced. These neutrons collide with more uranium atoms, thus initiating a selfsustaining chain reaction that results in more fissions and the continuous release of enormous amount of energy (Figure 1). Any substance capable of sustaining a fission chain reaction is known as a fissile material. The materials most suitable for fission are 235U and 239Pu. These radioactive isotopes are fissionable by what are termed as thermal neutrons. The isotopes 238U (and 232Th) are fissile if bombarded by fast neutrons.

The physics of nuclear energy is simple. When a neutron collides with an atom of the isotope uranium-235, a highly excited atom of uranium-236 is formed, which immediately fissions into two lighter nuclei. Fission reactions can produce any combination of lighter nuclei, as long as the number of protons and neutrons in the products add up to the fissioning nuclei. Among possible reactions are:

235U + n 236U 143Ba + 90Kr + 3 n + energy (g ray)

235U + n 236U 140Cs + 93Ru + 3 n + energy (g ray)

235U + n 236U 134Xe + 100Sr + 2 n + energy (g ray)

The profound feature of fission is that the mass of the products (fission fragments and neutrons) is less than the mass of a 236U atom. The mass difference (or the mass defect) appears as the kinetic energy of the fission fragments in an amount determined according to Einstein’s famous E = mc2 formula, where m is the mass defect and c is the speed of light in a vacuum (300,000 kilometers per second) (a).

Question: In the example of nuclear fission of uranium-235 given above, the mass appears to be the same on both sides of the reaction (235+1 = 143+90+3 = 134+100+2 = 236). Thus, it seems that no mass is converted into energy. In this case, where does the energy come from?

Answer: The statement is not entirely correct. Actually, the mass of a nucleus is more than the sum of the individual masses of its protons and neutrons, and contains the extra mass equal to the binding energy that holds the protons and neutrons of the nucleus together.

Question: Carbon has two stable isotopes, 12C and 13C. The isotope 14C is unstable and decays to 14N, with a neutron changing into a proton. Describe the reaction and how it can be used in dating fossils and other archeological artifacts.2 Answer: Both 14C and 14N have the same atomic mass. The difference is in the number of protons and neutrons in their nuclei. 14C with eight neutrons and six protons is less stable than tightly bound 14N which has seven protons and seven neutrons. The half life of the decay is 5730 years which is used in determining the age of fossils and other biological organisms (See box “Carbon Dating”).

Example: What is the energy equivalent of one gram of graphite, that is, the energy available if all its mass were annihilated?

Solution: The energy release is calculated as E = mc2 = (0.001 kg)(3x108 m/s)2 = 9x1013 J, or the energy-equivalent of 10,000 tons of TNT!



(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) Radioactive dating is not limited to biological systems. A similar technique called pottasium-argon dating can be used to determine the age of volcanic rocks that contain potassium. When volcanic liquids --called lava-- was solidified into a rock, it contained an isotope of potassium K-40 which decays into argon gas. The technique works by melting the rock and measuring the concentration of the trapped argon gas to estimate the rock’s age.

Further Reading

Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004.

Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005.

International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission.

Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion.

External Links

Federation of American Scientists (

International Atomic Energy Agency (

DoE Office of Nuclear Energy, Science & Technology (

American Nuclear Society, (

World Association of Nuclear Operator (WANO) (