2 billion years ago there was a water-moderated nuclear reactor in Gabon:
http://physics.isu.edu/radinf/Files/Okloreactor.pdfAt the time that the Gabon reactor went critical, the abundance of 235U was 3%, similar to that in current commercial nuclear reactors. The approximate shape of the reactor zones is that of a compact mass of uranium oxide surrounded by porous rocks, which were presumably hydraulically connected to surface or ground water, allowing moderation and reflection of the neutrons produced by spontaneous fission or cosmic ray induced fission.
The relatively large size and spherical shape of the uranium bearing region reduced buckling. When the surrounding porous rocks were saturated with water, the subsequent moderation and reflection allowed the reactor to achieve criticality. It is likely that criticality was not continuous.
As the reactor power increased, the water moderator would heat, reducing its density and its effectiveness as a moderator and reflector. This process, known as a negative temperature coefficient, helps to control power during transient conditions in manmade nuclear reactors.