Enrico Fermi built the first "Nuclear Pile"... called a "pile" because bricks of graphite were literally piled up one on another with nuggets of uranium inserted into some of them until they began to react spontaneously. (yeah, for those who like to pick nits, I have left a LOT of details out....so sue me.
Literally the first man made nuclear reactor. The first controlled nuclear fission reaction. His research led to much mathematics which led to even more research which led to what became the Manhattan Project.
Was he the father of the atomic bomb? Sorta. But the atomic bomb had MANY parents. But his research was groundbreaking.
Reading about this is interesting. I suggest you start with "The making of the Atomic Bomb" by Richard Rhodes. The chapter about Fermi and his "pile" is interesting.
Surprisingly, self sustaining reactors actually happen in nature. But no one knew what to look for until Fermi and his team (and others) made the first one happen.