What is nuclear fusion?

How it works

Nuclear Fusion is the process of joining two light atomic nuclei into a single heavier nucleus, releasing an enormous amount of energy. These reactions power the Sun, as well as all other stars in the universe.

Conditions for reaction

Nuclear Fusion reactions occur in a specific state called Plasma. The Sun's core, composed mostly of hydrogen atoms, reaches temperatures of around 15 million °C and pressure 340 billion times greater than Earth's atmospheric pressure at sea level. These conditions allow the formation of Plasma: the fourth state of matter, a hot and ionized gas, composed of positive and negative particles (electrons), with a density about one million times lower than the air we breathe.

At these extreme temperatures, hydrogen particles in the solar core vibrate intensely, reaching high levels of agitation and velocity, causing them to collide with each other. Due to these extreme conditions, these particles receive enough energy to approach each other, overcome their electrical repulsion (the Coulomb barrier), and finally fuse. To increase the success rate of these collisions, the process must occur in a small, confined space — which is provided by the star's gravitational force.

To illustrate this process, we will use the fusion between Deuterium and Tritium, two isotopes of Hydrogen.

From the reaction between Deuterium and Tritium, a Helium atom and a neutron are released. This is because Deuterium is composed of 1 neutron and Tritium of 2 neutrons, which would result in a new nucleus of 2 protons and 3 neutrons. However, this 2:3 ratio is unstable. Therefore, the "extra" neutron is released and a stable Helium-4 atom is formed (2 protons and 2 neutrons).

Where does the energy come from?

As Lavoisier said, "In Nature, nothing is created, nothing is lost, everything is transformed" — and the same applies to fusion. In a fusion reaction, the heavier nucleus that forms has a slightly smaller mass than the sum of the masses of the two original lighter nuclei. This difference in mass is called mass defect.

Δm = mass defect; Z = number of protons; m_p = proton mass; N = number of neutrons; m_n = neutron mass; M_nuc = mass of the atomic nucleus formed. The difference between the sum of individual masses and the actual mass of the nucleus corresponds to the energy released in the reaction.

This quantity of mass that "disappears" is converted into energy, following Einstein's formula E = mc².

ΔE = variation in energy released; Δm = mass defect; c = speed of light in vacuum (≈ 3 × 10⁸ m/s). A small loss of mass converts into an enormous amount of energy due to the large value of c².

Since c is a gigantic number (approximately 3 × 10⁸ m/s), a small quantity of mass converts into a massive quantity of energy. The energy generated is called Binding Energy — the amount of energy released when forming (or breaking) a new nucleus from its subatomic particles (protons and neutrons).

Mass defect in the D-T reaction

Masses of the reactants: Deuterium (²₁H = 2.014 101 u) and Tritium (³₁H = 3.016 049 u), totaling 5.030 150 u. Masses of the products: Helium-4 (⁴₂He = 4.002 602 u) and neutron (¹₀n = 1.008 664 u), totaling 5.011 266 u. The mass defect Δm = 0.018 884 u is converted into 17.6 MeV of energy via E = mc².

From a single reaction between one Deuterium atom and one Tritium atom, 17.6 MeV are obtained — to understand what this number means, see the section "But how much energy would Fusion really generate?" below.

The Binding Energy released in the Nuclear Fusion reaction manifests as Kinetic Energy. In the D-T reaction, the 17.6 MeV are distributed as follows: 3.5 MeV go to the alpha particle (the Helium atom) and 14.1 MeV go to the neutron.

The Deuterium-Tritium (D-T) reaction

The D-T reaction was not chosen at random. Currently, the fusion between Deuterium and Tritium is the most widely studied by researchers. Among all possible fusion reactions, it has the largest cross section — meaning the highest probability of occurring — and also the largest Q-value, representing the amount of energy released per reaction.

Unlike the Sun, whose gravitational force naturally provides the conditions necessary for fusion to occur, Earth has a gravitational constant of approximately 9.8 m/s², about 28 times smaller than the star's (~274 m/s²). To compensate for this, on Earth we need to work with very high temperatures to provide the conditions for fusion.

D-T fuel reaches its fusion conditions at much lower temperatures than other elements, while releasing more energy than other fusion reactions. The Deuterium-Tritium reaction requires a minimum temperature of about 100 million °C, while other reagents require minimum temperatures of 300 million °C.

Furthermore, Deuterium and Tritium are isotopes of Hydrogen, the most abundant element on the planet. Deuterium is very common and can be found in seawater: 1 in every 6,500 hydrogen atoms in that body of water is Deuterium. Tritium, on the other hand, is a radioactive isotope rarely found in nature, with a half-life of only 12.32 years. However, Tritium can be produced through a process called breeding, which consists of fusing Lithium with a neutron. A fusion power plant could therefore become self-sufficient — something of extreme relevance for the economic viability of the process.

But how much energy would Fusion really generate?

Nuclear Fusion is widely studied for its great potential to produce a clean and inexhaustible source of energy. Global energy demand is continuously growing. Currently, the main sources of the world's energy matrix are extremely polluting, releasing CO₂ into the atmosphere and causing potentially irreversible damage to the environment — read more in the Environment section.

In addition to being a renewable and non-polluting energy source, Nuclear Fusion presents an extraordinary energy gain:

Now, what research is involved in producing Fusion on Earth?