What should I read in the chapter on nuclei for the CBSE class 12 board exams?

Hmm, Nothing in that chapter can be said that it is not nice or not important. But yet, For exams I suggest you to prepare..

1. Binding Energy & Binding Energy Per Nucleon(With BE Curve)
2. Radioactive Decay Law
3. Alpha, Beta+, Beta- and Gamma Decay
4. Nuclear Reactions
5. Nuclear Fission and Fusion
6. Nuclear Chain Reaction
7. Nuclear Reactor