The fuel for a fusion reaction is light atomic nuclei, typically isotopes of hydrogen like deuterium (D) and tritium (T).

Here's a breakdown:

* Deuterium: An isotope of hydrogen with one proton and one neutron.

* Tritium: An isotope of hydrogen with one proton and two neutrons.

When these isotopes collide at extremely high temperatures and pressures, they can fuse together to form a heavier nucleus, releasing a tremendous amount of energy. This is the process that powers the sun and other stars.

Here's a common fusion reaction:

Deuterium + Tritium -> Helium + Neutron + Energy

While deuterium and tritium are the most common fuels used in research and development, other isotopes like helium-3 and lithium are also being explored.

It's important to note that fusion reactions require extremely high temperatures (millions of degrees Celsius) and pressures to overcome the electrostatic repulsion between the positively charged nuclei. This makes achieving controlled fusion on Earth a complex and challenging task.