How are nuclear reactions used to meet the world's energy needs? Both fusion and fission are used to generate electricity, but more fission reactors than fusion reactors are in use. Fission reactors are used to generate electricity, but fusion is not yet practical enough to be used. Both fusion and fission are used to generate electricity, but more fusion reactors than fission reactors are in use. Fusion reactors are used to generate electricity, but fission is not yet practical enough to be used.