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What is Nuclear Energy?

Nicholas Amendolare, John Simmons, Christianlly Cena
  • Author
    Nicholas Amendolare

    Nicholas Amendolare is a high school and middle school science teacher from Plymouth, Massachusetts. He has a bachelor's degree in environmental science from Worcester Polytechnic Institute and a master's degree in education from Harvard University. He has been a teacher for nine years, has written for TED-Ed, and is the founder of

  • Instructor
    John Simmons

    John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master's degree in Physics and is currently pursuing his doctorate degree.

Learn about what is nuclear energy with some nuclear energy examples. Understand the types of nuclear energy and learn about the nuclear power plant. Updated: 08/31/2021

Table of Contents


Nuclear Energy Definition

Nuclear energy has impacted lives globally during the 20th and 21st centuries. It ended the second world war, raised tensions during the cold war, and now powers more than four hundred reactors in thirty-two countries across the world, bringing electricity to millions.

Nuclear energy is a form of energy released from the nucleus of atoms. This energy can be released either through fission (when a large nucleus is split apart) or fusion (when small nuclei are fused together).

Energy is the power necessary to do work or create force. Energy can be produced through both chemical and nuclear processes. Chemical energy is stored in the bonds of chemical compounds and is released during chemical reactions. Individual atoms remain intact and the same, but join with other atoms in a chemical reaction. By contrast, nuclear energy is created when the nuclei of many individual atoms split or fuse. This fundamentally changes the atom, transmuting it into a different kind of element.

People use chemical energy when using alkaline batteries or burning gasoline. In these cases, molecules are broken apart, atoms are rearranged, and energy is released. But with nuclear energy, the very nature of individual atoms is changed. For example, uranium is broken into smaller, radioactive pieces, and massive amounts of energy capable of leveling entire cities are released.

Einstein and Nuclear Energy

In 1905, Albert Einstein published four papers that revolutionized the modern understanding of physical science. One of these papers dealt with mass-energy equivalence, the idea that mass and energy are the same and a concept famously represented by Einstein's equation: {eq}e=mc^2 {/eq}.

In the equation, e stands for energy, m stands for mass, and c stands for the speed of light. This equation enables the calculation of the amount of energy per unit of mass. It also shows how mass can be manipulated to produce large amounts of (nuclear) energy.

If a mass of atoms (such as uranium) undergoes a nuclear reaction with the product having a slightly reduced mass, the reaction is said to have produced a mass defect, which is released as energy. This explains how a relatively small mass of uranium can power a nuclear reactor. In fact, one fuel pellet of uranium (about the size of a pencil eraser) creates as much energy as 2,000 pounds of coal or 149 gallons of oil.

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Types Of Nuclear Energy

The two basic types of nuclear reactions are fission and fusion. Fission occurs when the nucleus of an atom is split apart. Fusion occurs when the nuclei of atoms are fused together. Both nuclear fission and fusion are examples of matter (anything that has mass and takes up space) being converted into energy.


The atom with the least mass is hydrogen. This element contains one proton in its nucleus along with zero to two neutrons. If two hydrogen atoms are heated up and combined, their nuclei can fuse together producing a larger atom that contains two protons and two neutrons.

Two hydrogen atoms fusing into helium.

A diagram of fusion

Adding two hydrogen atoms together creates one helium atom. Because a helium nucleus has slightly less mass than the two hydrogen nuclei, that mass defect results in a massive energy release. Nuclear fusion is quite common in the natural world, and humans frequently benefit from the energy created by nuclear fusion. For example, the Sun is powered by fusion, and everything from solar power to wind power to the flow of rivers is simply stored fusion energy from the Sun.


Fission is the opposite of fusion. It is the process of splitting atoms. Typically, this will involve uranium, the atom with the most mass found in nature. Uranium is a mined, heavy, gray metal. Its nucleus contains ninety-two protons along with 141 to 146 neutrons.

When uranium is bombarded by extra protons or neutrons, its nucleus grows, becomes unstable, and splits into smaller pieces. Because these pieces have slightly less mass than the original nucleus, a mass defect occurs. The difference of mass is released as massive amounts of energy. This is the process that occurs in nuclear weapons and nuclear power plants: uranium atoms are manipulated to split and create energy.

The fission of Uranium-235.

A diagram of fission

In nuclear reactors, this release of energy is tightly monitored and controlled. The reaction happens a few atoms at a time, and the excess energy is extracted and stored. In nuclear weapons, the reaction is deliberately out of control. In an atomic bomb, a chain reaction occurs as the uranium splits, releasing energy and extra particles. These particles bump into new uranium atoms, splitting more atoms and so on.

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  • Activities
  • FAQs

Nuclear Energy True or False Activity

In this activity, you will check your knowledge regarding the definition and examples of nuclear energy, as presented in the lesson.


Determine whether the following statements are true or false. To do this, print or copy this page on a blank paper and underline or encircle the answer.

1. An atom is a substance that has inertia and occupies physical space and consists of various types of particles.

True | False

2. Nuclear fission generates radioactive waste that is harmful to life and has a huge impact on the environment.

True | False

3. In terms of energy output, fusion generates more energy than fission.

True | False

4. A physical reaction is a reaction that changes the nucleus of an atom.

True | False

5. Work is the measure of energy transfer that occurs when an object is moved over a distance by an external force or effort.

True | False

6. All nuclear power plants use nuclear fission, and most nuclear power plants use uranium atoms as fuel.

True | False

7. Nuclear energy is the energy in the core of an atom that can be used to create electricity via nuclear reactions.

True | False

8. Particles, such as protons, are employed to split uranium nuclei into fragments.

True | False

9. Nuclear fusion has no radioactive waste and uses helium particles as an energy source.

True | False

10. A nucleus is the massive, positively charged central part of an atom, made up of electrons and neutrons.

True | False

Answer Key

  1. False, because the correct statement is, Matter is a substance that has inertia and occupies physical space and consists of various types of particles.
  2. True
  3. True
  4. False, because the correct statement is, A nuclear reaction is a reaction that changes the nucleus of an atom.
  5. True
  6. True
  7. True
  8. False, because the correct statement is, Particles, such as neutrons, are employed to split uranium nuclei into fragments.
  9. False, because the correct statement is, Nuclear fusion has no radioactive waste and uses hydrogen particles as an energy source.
  10. False, because the correct statement is, A nucleus is the massive, positively charged central part of an atom, made up of protons and neutrons.

What is a simple definition of nuclear energy?

The International Atomic Energy Agency defines nuclear energy as "a form of energy released from the nucleus, the core of atoms, made up of protons and neutrons."

What are common uses of nuclear energy?

The most common use of nuclear energy is a controlled fission reaction of uranium in nuclear power plants. The mass deficit of this reaction produces energy that is converted into electricity.

What are the 2 types of nuclear energy?

The two types of nuclear energy are fission (when a large nucleus is split apart) and fusion (when nuclei are fused together).

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