Atomic Bomb
noun
原 is often tied to atomic things, as atoms are the origin of all things. If an atom explodes, you have yourself an atomic bomb.
げんばく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
原爆のことを言ってるんじゃないよ。
I’m not talking about atomic bombs.