Fusion Reaction

A fusion reaction is a nuclear reaction in which light atomic nuclei combine to form a heavier nucleus, releasing energy in the process. It is the energy source of the Sun and other stars.

Simple Definition (High School)

When two light nuclei such as hydrogen fuse into a heavier nucleus such as helium, the total mass decreases by a tiny amount between before and after the reaction. This lost mass is converted into an enormous amount of energy according to Einstein’s equation $E = mc^2$. This is fusion. It is the exact opposite of nuclear fission, in which a heavy nucleus such as uranium splits apart and releases energy.

Major Fusion Reactions (Undergraduate and Above)

The reaction most readily achievable for fusion on Earth is the one between deuterium (D) and tritium (T).

$$\text{D} + \text{T} \rightarrow {}^4\text{He} \ (3.5 \text{ MeV}) + n \ (14.1 \text{ MeV})$$

The neutron carries away 80 percent of the energy, while the helium nucleus (alpha particle) carries the remaining 20 percent. Other reactions such as the following are also under study.

$$\text{D} + \text{D} \rightarrow {}^3\text{He} + n + 3.27 \text{ MeV}$$ $$\text{D} + \text{D} \rightarrow \text{T} + p + 4.03 \text{ MeV}$$ $$\text{D} + {}^3\text{He} \rightarrow {}^4\text{He} + p + 18.3 \text{ MeV}$$

The D-T reaction is chosen because it delivers a reaction rate more than an order of magnitude larger than the others, and at a lower temperature.

Conditions for the Reaction to Occur

Because both nuclei carry a positive charge and repel each other, making them react requires a temperature above 100 million degrees, a sufficient plasma density, and a sufficient energy confinement time. These conditions are summarized as the Lawson criterion.

Learn More

• What Is Fusion
• Fusion Reactions
• Lawson Criterion

Related Terms

• Plasma - The medium in which fusion reactions occur
• Confinement - Maintaining conditions suitable for fusion
• Tokamak - A device for achieving fusion
• Stellarator - Another fusion device