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Nuclear Fusion

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Fast Facts About
Nuclear Fusion

Principal Energy Use: Electricity
Form of Energy: Nuclear

Fusion reactions power the sun and the stars. Nuclear fusion occurs when nuclei from two or more atoms are forced together (overcoming the Coulomb barrier*) and fuse to form a single larger nucleus, releasing lots of energy (by E = mc2), usually in the form of fast moving neutrons. The energy of the neutrons can then be captured (usually by converting to heat) and used to generate electricity.

Nuclear fusion has the potential to be an extremely energy dense and carbon-free energy resource that does not produce air pollution or radioactive waste. However, while nuclear fusion happens continuously in (and even powers) the sun, making nuclear fusion happen on earth is extremely challenging (think about putting the sun in a box).

The most commonly used fuels for nuclear fusion are deuterium and tritium (isotopes of hydrogen), which combine to form helium. Currently, fusion is in the research phase and is not currently commercially viable, though billions of dollars from both the public and private sectors are being invested in the fusion space.

*The Coulomb barrier is the amount of energy needed to overcome the electrostatic forces between nuclei so they can get close enough to fuse.

Fusion Fuels


  • Abundant resource (33 mg of deuterium in every m3 of seawater)
  • Obtained through hydrolysis of heavy water (water with deuterium instead of hydrogen) which splits water molecules into oxygen and deuterium gas


  • Naturally occurring tritium is rare (global inventory is around 20 kg)
  • Can be bred from lithium, an abundant resource (the ability to do this within the fusion reaction is important for large scale fusion power)

Nuclear Fusion Fuel is Extremely Energy Dense

more energy dense than coal

more energy dense than natural gas

more energy dense than nuclear fission

Key Terms

Scientific Breakeven
(Fusion “Ignition”)

Fusion reaction produces at least as much energy as is being lost to the environment.

Scientific breakeven was recently achieved at the National Ignition Facility at the Lawrence Livermore National Laboratory.

Engineering Breakeven

Fusion power output is at least equal to the input power needed to assemble, heat, and confine the plasma, either using lasers or magnets.

Facility Breakeven

Fusion power output is at least equal to the input power plus the power needed to run ancillary systems such as tritium breeding, cooling, and gas handling.

Requirements and Challenges for Deuterium-Tritium Fusion Reactions


Fusion reactions require extremely high temperatures
>100 million degrees Celsius
to put the reactants into a plasma* state and overcome electrostatic forces between the nuclei to force the nuclei to fuse.

This is over 6x the temperature of the core of the sun. The sun is massive, which allows the center (where fusion occurs) to have high pressures that we cannot replicate. That means we must compensate by going even higher in temperature.

Beyond the challenge of achieving such high temperatures, these temperatures are too hot to use any materials to confine the plasma because they would melt anything on Earth.

*Plasma is an electrically charged gas where the electrons have been stripped from the atoms

Reaction Time

Commercial fusion would require a continuous, self-sustaining reaction where fuel is continually added.

Current time record for fusion reactions is 17 minutes 36 seconds, achieved by Experimental Advanced Superconducting Tokamak (EAST) on December 30, 2022.

Net Energy Production
(Q* > 1)

Releasing more energy from the fusion reaction than is put in to make the reaction happen (breakeven) has only recently been achieved.

It’s happened on two occasions, both at the National Ignition Facility in Livermore, CA:

  • December 2022 (Q = 1.5)
  • July 2023 (Final results still being calculated)

In reality, Q of 10 – 100 would be the minimum required for engineering breakeven thereby allowing for commercialization.

*Q = fusion power/injected power

Fusion Energy Confinement

Gravitational confinement (like the sun uses) is not an option on Earth. Instead, we use other reactor configurations to confine the plasma.

Magnetic Confinement

Plasma is confined in a reactor with a magnetic field created by very strong magnets such as high temperature superconductors.

Requires relatively:

  • Low plasma density
  • Low temperature plasma
  • Long confinement times (seconds)

Inertial Confinement

Fusion fuel is compressed and heated to a plasma via a quick burst of energy imploding a fuel-filled target with a huge amount of energy, typically using high powered lasers.

Requires relatively:

  • High plasma density
  • High temperature plasma
  • Short confinement times (nanoseconds)

Leading Fusion Endeavors


50+ Countries

involved in research on plasma physics and nuclear energy technology development


35 countries collaborating to build the world's largest fusion reactor

$6.2 Billion

invested in fusion companies in 2023


9 National Labs

engaged in fusion research

50 Universities

conducting fusion research

40+ Private Companies

in the fusion space

$50 Million

public-private partnership investment from US government


  • The fuel is abundant (nearly inexhaustible); deuterium is common in seawater, and tritium can be created during fusion
  • No radioactive waste; the product of fusion reactions is helium
  • No air emissions like GHGs, particles, etc.
  • Super energy dense; net energy production is about 4 times that of fission
  • Safety: a large-scale nuclear accident akin to what can occur in a fission reactor is not possible in a fusion reactor; fusion is difficult to start up and keep running so failure modes involve shutting down as opposed to runaway reactions as in fission


  • Technology is in the research phase
  • Tritium scarcity; tritium is expensive and must be bred from lithium during the fusion reaction
  • Very energy intensive to get the fusion reaction going; reactor needs to produce more energy than what is put into it
  • Fusion reactions are not yet self-sustaining
  • Containment: new materials needed to contain and harness energy and heat from fusion reactions
  • Regulatory approval
  • Cost: fusion research is very expensive

Climate Impact: Low

Low gradient
  • Near-zero emissions

Environmental Impact: Low

Low gradient
  • Two main sources of fuel, hydrogen and lithium, are widely available in many parts of the Earth
  • No radioactive waste

Updated October 2023

Before You Watch Our Lecture on
Fusion Energy

We assign videos and readings to our Stanford students as pre-work for each lecture to help contextualize the lecture content. We strongly encourage you to review the Essential videos and readings before watching our lecture on Fusion Energy. Include selections from the Optional and Useful list based on your interests and available time. 


Optional and Useful

Our Lecture on
Fusion Energy

This is our Stanford University Understand Energy course lecture on nuclear fusion. We strongly encourage you to watch the full lecture to understand the potential role of nuclear fusion as a energy system and to be able to put this complex topic into context. For a complete learning experience, we also encourage you to watch / read the Essential videos and readings we assign to our students before watching the lecture.

Clea Kolster

Presented by: Clea Kolster, PhD; Partner and the Head of Science, Lowercarbon Capital
Recorded on: May 26, 2023   Duration: 26 minutes

Table of Contents

(Clicking on a timestamp will take you to YouTube.)
00:00 Introduction
02:51 What is Fusion?
07:54 Who and How to Harness Fusion Energy
18:11 Recent Breakthroughs and Reactor Highlights
20:31 The Next Frontiers in Fusion

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Additional Resources About
Nuclear Fusion

Government and International Organizations