>A bunch of radioactive stuff together is basically the definition of a nuclear reactor though.

It really isn't.

A pile of radioactive waste isn't a reactor. Marie Curie's notes are famously contaminated with radioactive materials but they aren't a reactor. This is about as close as the boy scout got.

The Oklo fossil reactor is unique because it happened to form in the right circumstances to produce a fission chain reaction, which does make it a reactor. Not every uranium mine is a reactor, in fact this is the only one known.