Well, that was 2 billion years ago. Since U-235 has a shorter half-life than U-238, the natural uranium enrichment was far higher back then. And indeed that was

Well, that was 2 billion years ago. Since U-235 has a shorter half-life than U-238, the natural uranium enrichment was far higher back then. And indeed that was a water-moderated reactor. [1] Some people have postulated that the moon itself was formed in a nuclear fission excursion [2], and/or that there is or was a nuclear reactor in the center of Earth [3]. These are both not commonly believed. [1] https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... [2] https://phys.org/news/2010-01-moon-nuclear-explosion.html https://phys.org/news/2010-01-moon-nuclear-explosion.html [3] https://arxiv.org/abs/1308.5934 https://arxiv.org/abs/1308.5934