>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.
Also note that due to isotope decay in the ore, a natural reactor is no longer possible. From the wikipedia article:
"A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope 235U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. [...] the current abundance of 235U in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite."
Another fascinating detail from the article, due to our understanding of fission, we can get some incredible results:
"The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down"
Indeed. I said a bunch and I meant a bunch. Trace amounts is not a bunch.