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Introduction to Nuclear Energy

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Fast Facts About
Nuclear Energy

Principal Energy Use: Electricity

Nuclear energy is a carbon-free and extremely energy dense resource that produces no air pollution. Nuclear reactions produce large amounts of energy in the form of heat. That heat can be used to power a steam turbine and generate electricity. There are two types of nuclear reactions:

  1. Nuclear fission occurs when a large atom is split into smaller atoms, producing lots of heat and long-lived radioactive waste. See our Nuclear Fission page for more information.
  2. Nuclear fusion occurs when two nuclei combine to form a single nucleus, releasing massive amounts of heat with no long-lived radioactive waste (the sun is a nuclear fusion reactor). See our Nuclear Fusion page for more information.

All commercial nuclear power plants today use nuclear fission. The highly radioactive byproducts of nuclear fission must be secured away from people for hundreds of thousands of years, but we have no proven long term solutions for doing that. Nuclear fusion is still in the research phase.

Both nuclear fission and nuclear fusion have benefited from large amounts of government funding for basic science, technology, fuel-sourcing, and regulation; and both forms have origins in the defense industry (nuclear bombs - fission; hydrogen bombs - fission and fusion).

Fission and Fusion Characteristics

  Fission Fusion
Process A heavy atom is hit with a neutron which splits into smaller atoms and releases more neutrons causing a chain reaction and releasing lots of energy Two light nuclei are forced together (overcoming the Coulomb barrier) and fuse to form a single larger nucleus, releasing lots of energy
  • Reliable baseload power source since the 1960s
  • 10% of world electricity generation
  • 19% of US electricity generation
  • Not yet commercially viable
  • Potential to provide carbon-free electricity at a large scale
Fuel Used Enriched uranium 235 Deuterium and tritium (isotopes of hydrogen)
Energy Density Extremely energy dense:
1 uranium pellet
(~the size of a pencil eraser) = 
  • 17,000 cubic feet of natural gas
  • 120 gallons of oil
  • 1 ton of coal
4x more energy dense than fission fuels
  • Carbon-free
  • No air pollution
  • Produces long-lived highly radioactive waste
  • Carbon-free
  • No air pollution
  • No long-lived radioactive waste

Climate Impact:

  • Near-zero emissions (fission and fusion)

Environmental Impact:
Low to Medium

  • No air pollution (fission and fusion)


  • Radioactive waste is toxic for hundreds of thousands of years
  • Risk of radiation leaks from nuclear meltdowns
  • Large amounts of water used for cooling; thermal pollution of water
  • Mining of uranium can pollute water and degrade land and habitat
  • Nuclear waste can be used for bombs; high security required to reduce the risk of proliferation


  • No long-lived radioactive waste

Updated October 2023

Our Lecture on
Introduction to Nuclear Energy

This is our Stanford University Understand Energy course introduction to nuclear energy. We encourage you to watch this 5-minute video for important context before diving into the more in-depth content on our Nuclear Fission and Nuclear Fusion pages.

Diana Gragg

Presented by: Diana Gragg, PhD; Core Lecturer, Civil and Environmental Engineering, Stanford University; Explore Energy Managing Director, Precourt Institute for Energy
Recorded on: September 13, 2023   Duration: 5 minutes

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Lecture slides available upon request.

Fast Facts Sources
Overview: Breeze, P. Power generation technologies, 3rd. edition. Newnes, 2019; What is Nuclear Energy? The Science of Nuclear Power. IAEA, 2022; Nuclear Explained. EIA, 2022; Nuclear Fusion Power. WNA, 2022.
Chart of Characteristics: Fission vs Fusion: What's the Difference? DOE, 2019; Nuclear Power for Electrical Generation. NRC; Nuclear Explained. EIA, 2022; Nuclear Power in the World Today. WNA, 2023; Fusion Device Information System. n.d.
More details available on request.
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