Question

Definition for nuclear reaction in which nuclei combine to form more massive nuclei

Answer 1

nuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus. It is accompanied by the release or absorption of energy depending on the masses of the nuclei involved. ... It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen.

Answer 2

a nuclear reaction is a process in which two nuclei or nuclear particles collide, to produce different products than the initial particles. In principle a reaction can involve more than two particles colliding, but such an event is exceptionally rare.