enriched uranium rods heating a boiler

Because of fission.

It’s a free source of heat, no fuel required

Enriched uranium is fuel.

The nuclear bomb they say requires fission and to “split an atom”.

This is exactly what happening in enriched uranium rods in nuclear power plants. Unstable uranium atoms split and produce heat, gamma rays and neutrons. Neutrons hit another uranium atoms provoking them to split and produce more heat and neutrons. Rate of reaction adjusted by how many neutrons hit another uranium atoms. Rate could vary from natural uranium decay when only tiny part of neutrons hit uranium atoms to nuclear blast.when nearly all neuthrons hit other uranium atoms. Nuclear power plants work in the middle of that range - faster than natural decay but slower than nuclear bomb.

It is like burning natural gas. You could burn it slowly in your gas stove, or you could fill a room and burn it all in once destroying building. Nuclear fission just produce much more energy than chemical process of burning.