The answer would be that in a nuclear reaction, the elements change identities while in a chemical reaction they do not.
Entropy is increases in both reactions, and although a nuclear reaction does emit radiation, it also emits heat. The nuclear reaction takes place in the nucleus, where protons and neutrons are converted or ripped away from the conglomerate in a forceful reaction; the chemical reaction takes place in the electron cloud, instead of the nucleus, with the exchange of electrons being the primary motive for chemical change. It’s this difference that makes nuclear reactions significantly more powerful than chemical reactions.
