Put simply, the “nuclear tone” is the tone on the focus word of the sentence. This tone bears a heavy load in communicating the intonation of the sentence, which in turn reveals the sentence's purporse (statement, command, yes/no question, polite speech, sarcasm, etc.), and also serves to clearly identify the focus word. If there is no reason to focus on a particular word, the nuclear tone will usually be on the last content word in the sentence. In theory, any word can be the focus word, and changing the focus will significantly change the meaning of the sentence. A typical sentence we use to demonstrate this is “I didn’t say she stole the painting.” Try focusing on different words and noticing how the meaning changes. Note also that in English, the nuclear tone will rarely be level — that usually sounds foreign.

Thanks so much @Phil

I would like to confirm if I understand correctly.

- Nuclear Tones are of British School of Intonation. They focus on the pitch level and there are 6 basic patterns, namely,

1.Fall

2.High Fall

3.Rise Fall

4.Fall Rise

5.Low Rise

6.High Rise

- American School of Intonation focuses on type of sentences and there are 3 basic patterns for,

1.Statement

2.Yes/No Questions

3.WH Questions

Emphatic Stress, Contrastive Stress and other kind of stresses are taught separately under American School of Intonation.

Thanks again for your help.