@framemaker- Here is an overly simplified explanation of a fusion reaction. In one type of nuclear fusion, two isotopes of hydrogen (the most abundant element in the universe) are combined to form helium isotopes. These helium isotopes are then combined to form an unstable beryllium isotope. The unstable beryllium isotope then decays back into a new helium 4 isotope creating energized particles and radiation as a by-product.
Another type of fusion reaction only involves hydrogen isotopes that form a helium isotope and a high-energy neutron particle.
The difficulty with fusion reactions is the conditions required for the reaction to take place. Fusion requires high heat and pressure, on the magnitude of what you would find in the sun, to create the reaction. This makes fusion destructive by nature (to the reactor). The nuclear reactor diagrams I have seen for fusion involve strong electromagnets to stabilize the core.