Nucleus
Core, Nuclear
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.
どうして核は細胞の中心部に位置するんですか？
Why is the nucleus located at the center of the cell?
沖田君は小学生の時に原子核について興味を持ち、今ではこのチームの核として働いてくれています。
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.
核ミサイルの発射が、もう予定より三時間も遅れています。
The nuclear missile launch is three hours behind schedule now.