noun, の adjective
The kanji and the word are the same. That means they share meanings as well. This word can also mean core because that's where the nucleus is in a cell, and nuclear because we like to blow ourselves up nowadays...
The reading is the same as the kanji reading you learned.
Why is the nucleus located at the center of the cell?
Okita-kun became interested in atomic nuclei when he was an elementary school student, and now he's working as the nucleus of this team.
The nuclear missile launch is three hours behind schedule now.