What is Nuclear Fusion?

Hi, and welcome to this video on nuclear fusion.

Let’s begin with a quick review of the atom, elements, and standard chemical reactions, as this will prove useful for understanding the details of nuclear fusion.

An atom has a positive nucleus surrounded by a cloud of electrons. The nucleus is made up of nucleons – neutrons and protons. An element is defined by the number of protons in the nucleus. If an atom loses or gains a proton, it becomes a new element. For example, if a sodium atom loses a proton, it would become a negative ion of neon. However, when an atom loses or gains electrons, it retains its elemental identity. For example, when a sodium atom loses an electron, it becomes a positive ion of sodium.

In chemical reactions you carry out in the lab or kitchen, electrons do most of the work. They redistribute themselves amongst the atoms, to create new bonds and new compounds, but the nuclei never change. In nuclear reactions, like nuclear fusion, the electrons play minor roles, and it is the nucleons that interact to create new elements.

Now let’s turn to fusion. The basic concept is pretty simple. The nucleons of multiple atoms recombine to form new elements.

While the idea of nuclear fusion is relatively easy to grasp, it’s quite difficult to achieve at standard conditions. Perhaps it occurred to you when we described the process that it would be quite difficult to merge two positively charged nuclei – after all, like-charges repel each other. This raises the interesting question of what holds nuclei together in the first place?

In 1932, shortly after the discovery of neutrons, Eugene Wigner suggested the existence of a force other than gravity and electromagnetism that holds the nucleons together. This turned out to be correct. The strong nuclear force binds nucleons together and, as the name suggests, it is very strong. However, this then leads to another question- if this force is so strong, why aren’t all nucleons pulled together into one giant nucleus?

This doesn’t happen because the strong nuclear force is only strong at very short distances. So when nucleons are far apart, the electrostatic force dominates. But once nucleons are close together (about 1 femtometer or 10^-15 meters!), the strong nuclear force is much larger than the repulsion of the electrostatic force.

Thus, for nuclear fusion to occur, the nuclei need a huge amount of energy to overcome the repulsive force felt between protons at large distances. This is typically achieved with kinetic energy. In other words, the nuclei are speeding at each other so fast that they can overcome the electrostatic force.

Once the electrostatic force is overcome, an enormous amount of energy (much more than a chemical reaction) can be released, if the nuclei involved are small. In other words, the new nuclei formed are more stable than the reactant nuclei and there is a net release of energy, so the reaction is exothermic. The cutoff for nuclei that qualify as ‘small’ is around iron and nickel. Fusion of atoms larger than iron and nickel are typically endothermic, meaning there is no net release of energy.

Nuclear fusion of smaller atoms is the process responsible for powering stars, including our own sun, which generates energy through the fusion of hydrogen atoms to form helium. As stars age, they begin to fuse helium into larger atoms and then those into even larger atoms.

Scientists are currently attempting to harness the energy from nuclear fusion to provide an alternative source of energy for the world. Unfortunately, it is challenging to generate the conditions necessary for fusion. As we mentioned earlier, the atoms need lots of energy to overcome the repulsive electrostatic force. For example, the core of the sun is a plasma with a temperature of ~10⁷ K. In these conditions, electrons are stripped from their atoms and the bare nuclei move so fast that they readily achieve fusion.

To replicate these conditions on earth, scientists often use specially designed magnetic fields to contain the plasma because no physical material could hold something so hot.

Lastly, let’s wrap up with a brief discussion of Einstein’s famous E = mc2 equation and how it relates to nuclear fusion.

Einstein’s equation was a revolutionary insight into the relationship between mass and energy; specifically that mass can be converted directly into energy via certain processes. Often the occurrences of this conversion are very minor and almost undetectable, but in nuclear processes, like fusion, the energy released is so large a change in mass is easily detected. So, when nucleons combine to form a nucleus and energy is released, the mass of the resulting nucleus is always less than the sum of the separate nucleons. In other words, mass has been converted into energy.

Nuclear Fusion Example

You can test this yourself with the following example. Calculate the difference in mass between the reactants deuterium and tritium (both isotopes of hydrogen) and the products helium and a neutron. Then use this mass difference to calculate the energy released from their reaction. (Use the equivalence provided for atomic mass units and mega-electron volts to calculate the energy difference).

\(_{2}^{1}\textrm{H}\text{ (deuterium) }+_{3}^{1}\textrm{H} \text{ (tritium) } \rightarrow _{4}^{2}\textrm{He}+_{1}^{0}\textrm{n}\)
Deuterium: 2.0140 amu (atomic mass units)
Tritium: 3.01605 amu
Helium: 4.00260 amu
Neutron: 1.008665 amu

\(2.0140 + 3.01605 – (4.00260 + 1.008665) = 0.018785\text{ amu}\)
1 amu = 931.5 MeV (mega-electron volts)

0.018785 amu times 931.5 MeV over 1 amu equals 17.5 MeV

\(0.018785\text{ amu }× \frac{931.5\text{ MeV}}{1\text{ amu}}=17.5\text{ MeV}\)

So, in this reaction, 0.018785 amu is converted to 17.5 MeV. This is an order of magnitude with more energy than what is produced from nuclear fission of uranium! Plus, there are no toxic pollutants made in the process!

To conclude, nuclear fusion of small atoms is a highly exothermic process in which nucleons recombine to form new elements. Scientists are currently working to harness the energy from nuclear fusion to provide an alternative to fossil fuel energy. However, the conditions to achieve fusion in the first place are so difficult to produce, let alone maintain, that it may never become a viable source of energy.

Thanks for watching, and happy studying!