Why are hydrogen isotopes used in fusion reactions?

Hydrogen isotopes are used in fusion reactions because they have low atomic masses and high fusion cross-sections.

In more detail, fusion reactions involve combining lighter atomic nuclei to form heavier ones, a process that releases a significant amount of energy. The isotopes of hydrogen, namely deuterium and tritium, are particularly suitable for this process due to their low atomic masses. The lower the atomic mass, the easier it is to overcome the electrostatic repulsion between the nuclei and bring them close enough for the strong nuclear force to bind them together.

The fusion cross-section, which is a measure of the probability of a fusion reaction occurring, is also high for hydrogen isotopes. This means that when these isotopes collide, there is a high chance of a fusion reaction taking place. This is particularly true for deuterium-tritium reactions, which have the highest cross-sections of any potential fusion fuel.

Furthermore, deuterium is abundant in seawater, making it a readily available source of fuel. Tritium, on the other hand, can be bred from lithium in a fusion reactor, making it a sustainable source of fuel. This availability and sustainability of fuel are crucial factors in the practical application of fusion energy.

The energy released in a fusion reaction is also directly related to the mass of the nuclei involved. The mass of the helium nucleus produced in a deuterium-tritium fusion reaction is slightly less than the combined masses of the original deuterium and tritium nuclei. This mass difference is converted into energy according to Einstein's mass-energy equivalence principle (E=mc^2), resulting in a large energy yield.

In summary, hydrogen isotopes are used in fusion reactions due to their low atomic masses, high fusion cross-sections, and the availability and sustainability of deuterium and tritium as fuel sources. These factors, combined with the large energy yield of deuterium-tritium fusion reactions, make hydrogen isotopes the most practical choice for fusion energy production.