The primary type of nuclear reaction occurring in the Sun is nuclear fusion.

Here's a breakdown:

* Fusion: This process involves the merging of atomic nuclei to form heavier nuclei, releasing a tremendous amount of energy in the process.

* Sun's Fusion: The Sun primarily fuses hydrogen nuclei (protons) into helium nuclei. This process involves a series of steps known as the proton-proton chain reaction.

Let me know if you'd like to delve deeper into the details of the proton-proton chain reaction!