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Basics of Nuclear Fuels

  • A large amount of energy is required to hold the nucleus (or center) of atoms together

  • The nuclei of unstable atoms can be split apart, releasing vast amounts of energy in the process (fission)

  • Smaller atoms can theoretically be smashed together, releasing even more energy (fusion). This is how energy is produced in the sun, but human technology for this process is still in development

  • Waste products from nuclear fission can spontaneously emit harmful radiation and must be properly contained

Atoms are particles that make up all the material in the world. They are composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons. It takes a large amount of energy to keep the protons and neutrons of the nucleus held together. When the nucleus of an atom undergoes a change, either by splitting apart (fission) or merging together with another nucleus (fusion), a large amount of nuclear energy is released.


The scale of energy from nuclear fuels

It takes a lot of energy to hold protons and neutrons together in a nucleus, so nuclear fuels are much more energy dense than a chemical fuel like coal or oil. The energy released from splitting one uranium atom is about 10 million times that of forming one carbon dioxide molecule from a carbon fuel and oxygen.


Source. XKCD

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Figure is modified from University Physics Volume 3.

Types of nuclear fuels and processes

Nuclear fission: Some heavy atoms that exist in an unstable state can undergo fission (nucleus splitting) with very little initial energy input. A type of uranium called U-235 is the most commonly used atom in nuclear power plants because it is unstable and easy to split when hit with a low energy neutron. Thorium and plutonium are also possible candidates for nuclear fission fuels. U-235 is relatively rare among other types of uranium, so it must be mined and processed before it can be used as a fuel. Enriched uranium is processed to contain 3-5% of U-235 before it is used in a power plant.


Source. US Energy Information Administration (publicly sourced from Pennsylvania State University Radiation Science and Engineering Center)

Nuclear fusion: All of the energy that is produced in the sun comes from nuclear fusion rather than fission. In the sun, two hydrogen atoms combine to form helium, releasing large amounts of energy. Humans are still working on the technology to use nuclear fusion as an energy source, but if it is developed, we could theoretically use smaller, more abundant atoms as nuclear fuels.


Waste and Safety

While nuclear fuels generate significantly less relative waste than chemical fuels (because of their energy density), these waste products are generally still radioactive and pose a safety hazard. The danger of these materials can be rated from very low level to very high level risk, and the waste is treated accordingly. In nuclear fission, uranium splits apart into smaller atoms, which include strontium-90, cesium-137, iodine-131 among others. These are radioactive and, if not properly contained, can contaminate groundwater and pose health problems. This is most often done by encasing the radioactive waste in cement or other containers and burying it deep in the ground. However, the dangers of nuclear waste are often overestimated, since nuclear fallout (significant amounts of radioactive material traveling through the air) only occurs in the wake of an explosion from a bomb, test site, or a serious accident.



Radioactivity is the spontaneous decay of an unstable nucleus into a more stable nucleus, leading to the emission of high energy rays and particles. This is different from nuclear fusion or fission, since it is a much smaller change to the nucleus and doesn’t release as much energy. Radiation causes damage at the cellular and molecular level, which can lead to detrimental health effects. While our bodies are resilient to certain amounts of radiation that we encounter every day, large amounts of exposure to radioactivity causes acute radiation sickness. Likewise, long-term exposure to moderate amounts of radiation can damage DNA and lead to cancer.


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Questions for deeper thinking

  • What are some of the pros and cons of using nuclear fuels compared to using chemical fuels (coal, methane, etc.)?

  • What are some of the impacts and consequences of uranium mining?

  • Once the uranium fuel is used, what are the waste products and how do we use/dispose of them?

  • How has the history of using nuclear technology for bombs affected the general public’s opinion of nuclear energy?

  • How has your own perception of nuclear energy changed over time?

Sources and further reading

  • Wolfson, Richard. Energy, Environment, and Climate, 2nd ed. 2012

  • Energy Information Agency (EIA): Nuclear Energy Explained

  • US Department of Energy: Nuclear Energy Basics  

  • Energy Education: Nuclear Fuels 

  • How Stuff Works: Why is Uranium-235 ideal for nuclear power? 

  • University Physics: Nuclear Binding Energy 

  • US Department of Energy: How does fusion energy work? 

  • Chemistry LibreText: Types of Radioactivity

Page last updated:  November 20, 2022​

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