The use of a moderator was also the trick at Oklo - an essential part of that natural phenomenon was that the vein of uranium was in a natural aquifer, and so t

The use of a moderator was also the trick at Oklo - an essential part of that natural phenomenon was that the vein of uranium was in a natural aquifer, and so the water served as a natural moderator. (And since it boiled off at higher powers, also served as a natural control mechanism.)