Nuclear Fusion: Combining of Two Nuclei


By Robert Hazen, Ph.D.George Mason University

In nuclear fusion, two nuclei combine to form a single, larger nucleus. If the mass of the resultant nucleus is less than the original two particles, then energy is released in the process. By far, the most common fusion reaction in nature combines two hydrogen atoms to make a helium atom.

An illustration of a star with light and heat emitting from it.
The Sun is a huge nuclear fusion reactor. (Image: xello/Shutterstock)

How Nuclear Fusion Occurs in the Stars

The combination of two hydrogen atoms is the primary energy-producing process in the Sun and in most other stars. The Sun converts about 700 tons of hydrogen into helium every second, and roughly half a percent of that mass disappears and becomes energy. That’s the radiant energy that comes to us and radiates the entire solar system with light. 

Under most circumstances, hydrogen atoms aren’t going to combine this way. The positive charges of the one proton in one hydrogen repel the positive charge of the other proton in the other hydrogen, so the two nuclei prevent that close approach. Fusion occurs inside stars, where there is tremendous temperature and tremendous pressure, and that provides the push to bring these two atoms together. 

You can imagine the pressure that makes the atoms get closer and closer together, temperature causing them to vibrate violently; and once in a while, the two nuclei get so close together that they actually can fuse together. You can think of this as like Velcro; if you get two objects close enough together, they stick, and two hydrogens become the helium atom.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Developing Nuclear Fusion Reactors

Scientists and engineers have been engaged in several attempts to develop nuclear fusion reactors. These would provide a wonderful, sustained source of energy because hydrogen is a very cheap fuel. It’s also a very clean reaction as only helium gets produced.  

An image of the Tokamak facility
The Tokamak facility is used for confining the plasma. (Image: Efman/Shutterstock)

One strategy is to use hydrogen plasma. A plasma is this hot gas with positive ions surrounded by a negative sea of electrons. It can be confined and compressed in a magnetic bottle, and then it can be heated with very high electrical currents, or it can be hit with a bunch of lasers, and then you have to try to sustain nuclear-fusion reactions.

There’s been a long-time commitment to trying this. For example, at Princeton University, there is the Tokamak facility. It is a large, donut-shaped magnet ring in which you can confine the plasma and try to heat it up. 

Another strategy is to form tiny pellets of a compound that’s very rich in hydrogen and then simultaneously hit that pellet with hundreds of lasers; high-power lasers that heat the pellet up to incredibly high temperatures. There’s now a facility called the National Ignition Facility at California’s Lawrence Livermore Laboratory that has 192 of the highest power lasers focusing in on a single BB-sized pellet.

Learn more about what makes some types of atoms particularly unstable and reactive.

How Hydrogen Bombs Work

The hydrogen bomb was developed shortly after World War II by both the United States and the Soviet Union. A hydrogen bomb works by converting hydrogen atoms into helium plus energy.

In a hydrogen bomb, there is a core of a dense, hydrogen-rich compound, such as lithium hydride. Rather than using normal hydrogen, the radioactive isotope of hydrogen, hydrogen-3, or tritium is used. The material with a  hydrogen-rich core is then surrounded by atomic bombs. The atomic bombs themselves are surrounded by conventional explosives. 

An image of B53 hydrogen bomb
Hydrogen bombs can be of any size. (Image: U.S. Air Force/Public domain)

You set off the conventional explosives that trigger the atomic bombs; the atomic bombs then create the temperature and the pressure needed to begin the fusion reactions of the central fuel. Because the hydrogen compound is inert, you can make a hydrogen bomb as large as you want; there is no practical limit to a hydrogen bomb’s size.

Learn more about the life cycle of stars.

Cold Fusion: Does It Really Work?

There is one other fusion-research effort called cold fusion. Two scientists from Utah claimed that they were able to initiate a hydrogen-fusion reaction in a bench-top experiment. All they did, they said, was to pass an electric current through two platinum-like rods, rods of a material called palladium. They bathed those palladium rods in heavy water, that is, deuterium-rich water. They claim that when the electric current flowed, they had an excess of heat being produced and that heat came from the nuclear fusion of the hydrogen into helium. 

If true, this result promised unlimited cheap energy for the world. The two scientists announced their results at a press conference, and eventually, after quite a bit of prodding, they released the details of their experiment on the Internet. 

Thousands of scientists around the world dropped everything in order to try to duplicate these important results. However, after many efforts, they generally found out that it didn’t work.

Common Questions about Nuclear Fusion

Q: How does a nuclear fusion occur? 

Extreme pressure causes the atoms to come closer to each other, while extremely high temperatures cause atoms to vibrate. In this case, the nuclei of the atoms fuse to form a larger nucleus. This is how a nuclear fusion occurs.

Q: How can nuclear fusion reactors be developed using plasma? 

Hydrogen plasma is a hot gas that can be used to develop a nuclear fusion reactor. In this process, the plasma is first enclosed and compressed into a magnetic bottle; then, its temperature is increased by laser or electric current. With this strategy, a stable nuclear fusion reactor can be developed.

Q: What other strategy besides plasma is there for developing a nuclear fusion reactor? 

Another strategy for developing a nuclear fusion reactor is to use small pellets that are very rich in hydrogen. Hundreds of lasers should operate simultaneously to hit those pellets so that they reach a tremendous temperature, enough to force the nucleus of two atoms to fuse.

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