Areaction in which small nuclei combine to form a heavier nucleus is

The answer to your question is: Nuclear fussion

Explanation:

In Chemistry there are 2 nuclear processes:

Nuclear fission: is the process in which the nucleus of a radiactive element splits into two smaller nucleus, or two elements woth lower atomic number.

Nuclear fussion, is a reaction is which to small nucleus combine to form a heavier element.

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .