That's a very unconventional use of the words "uncontrolled" and "simple". The reaction inside a nuclear bomb (fission or fusion) has to be incredibly tightly choreographed down to the microsecond in order to work as intended, whereas a fission reactor is (egregiously oversimplifying) put some fissile stuff next to some other fissile stuff for a while and let it get warm.

The Chicago Pile came way before Trinity. Rock formations have even been known to come together accidentally to form natural nuclear reactors, but I think it will be a very long while before the first natural nuclear bomb.