Answer:
Fusion is generally impossible because the strongly repellent electrostatic forces between the positively charged nuclei prevent them from colliding and fusing. One of the key reasons we haven't been able to realize the promise of fusion is because its energy requirements are outrageously large. High-energy neutrons are produced during fusion reactions, although they are not radioactive in and of themselves. They do, however, have such a strong influence on the reactor's walls.
