What Makes Nuclear Fusion Work?

combining smaller atoms into bigger ones has life-changing potential

By Jonny Lupsha, Wondrium Staff Writer

Nuclear fusion is an astronomically powerful source of energy. If harnessed, it could prove unimaginably useful for humanity’s purposes. How does nuclear fusion make energy?

Sun emitting energy
The nuclear fusion process of the Sun results in hydrogen converting into helium. Photo by oneinchpunch / Shutterstock

Nuclear fusion and nuclear fission are two opposite methods of attaining energy. With nuclear fission, a larger atom is split into two or more smaller atoms. This occurs when a neutron collides with the larger atom, causing it to excite and split. Nuclear fission powers nuclear reactors. Nuclear fusion, on the other hand, occurs when smaller atoms are combined into larger ones in a process that scientists are still trying to crack.

Recently, scientists caused a fusion reaction which, for the first time, produced more energy than it took to start the reaction. In other words, it was the first net gain of energy caused by nuclear fusion. So how does nuclear fusion happen? In his video series Nuclear Physics Explained, Dr. Lawrence Weinstein, Professor of Physics at Old Dominion University, walks viewers through this complicated process.

How Does Nuclear Fusion Work?

The best way to explain nuclear fusion is by looking at the Sun—not literally. Nuclear fusion provides the Sun with its energy, and the Sun is mostly made of hydrogen and some helium.

“The curve of binding energy shows that if we fuse lighter elements to form heavier ones, it releases energy,” Dr. Weinstein said. “The biggest single gain comes from fusing four protons into helium-4, and the energy gained is the difference in the mass times the speed of light squared. So, four times the mass of the proton, minus the mass of the helium-4 nucleus, is about 28 million electron volts.”

Twenty-eight million electron volts is written as 28 MeV. How big is 28 MeV? Dividing 28 MeV by four times the proton mass is 0.7%, meaning that almost 1% of the mass of the hydrogen gets converted to energy when it fuses to helium. This figure will be more important later, but for now, which conditions must be met for this to happen?

“We need a high enough temperature so the protons can fuse, and we need enough density so there are enough protons so that they can fuse,” Dr. Weinstein said. “Nuclear fusion then only happens in the core of the star, where the temperatures and densities are high enough.”

How Much Mass Does the Sun Convert to Energy?

In order to measure how much mass that nuclear fusion in the Sun converts to energy, we start with Einstein: E=mc2. The mass consumed, then, is the energy output divided by c2.

“The Sun puts out 4 x 1026 joules per second, [and] the speed of light is 3 x 108 meters per second, so squared, that’s 1017,” Dr. Weinstein said. “When I divide the two, we find the Sun has to consume 4 x 109 kilograms a second, which is 4 megatons of mass converted to energy every second.”

When it comes to the amount of hydrogen the Sun needs to convert to energy, our 0.7% figure from earlier returns. Since 0.7% is the same as 0.007, we take that 4 megatons (or 4 billion kilograms) per second, divide by 0.007 and get a total of 500 megatons per second of hydrogen converted to helium.

The Sun puts out 4 x 1026 watts of power, but its power density is just 2 x 10-4 watts per kilogram of its mass. Humans put out about one watt for every kilogram of our mass. In other words, we put out 104 times more power per kilogram than the Sun, because the Sun consumes its energy much more slowly.

However, the Sun gets the last laugh here. Our released energy is a chemical reaction of about 1 eV, meaning the Sun puts out 107 times more energy per reaction and we’re burning ourselves out 100 billion times faster than the Sun does. Luckily, we refuel three times a day.

Nuclear Physics Explained is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily