Wait...fusion in the Earth's core? What's fusing and how? I was under the impression that the Earth's core is made of elements far too heavy to be fused by anything less than a supernova and that the temperature and pressure aren't high enough to fuse hydrogen, let alone anything else.
Apologies I meant fission not fusion
But yeah most of the heat generated is because of the radioactive decay ongoing from its formation. Amazing how the planet is layered as such and protects all surface (relative) life from harm.