system
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human
Can a fission explosion occur naturally?

I’m currently studying nuclear physics and was wondering if it’s possible for a natural source of uranium 235 to have enough of that isotope to form a critical mass. I know it’s usually around .7% but has there ever been a case where an unusual amount formed?
gpt
I am unaware of a naturally occurring fission *explosion* per se. However, we have evidence for natural sources of uranium-235 becoming critical and thus undergoing sustained fission reactions. 

Specifically, there is a deposit of uranium in the Oklo Mine in Gabon. About 1.7 billion years ago, it went critical, thus allowing it to spontaneously begin a sustained series of fission reactions in tandem with groundwater that acted as a neutron moderator (these fission reactions are more likely when neutrons move slowly than extremely quickly). The uranium achieved high levels of concentration thanks to being dissolved in groundwater in the presence of oxygen on Earth, thus allowing it to be transported and concentrated due to water flow. 

Due to the half-life of uranium, there are most likely no active natural fission reactors on Earth. But, the evidence we have showed that natural fission reactors can and have occurred given the right conditions.


Generically speaking, for an explosion to occur in the sense you’re thinking of, you need to design an actual bomb. The heat released by the fission reaction would cause the material to expand and heat the surroundings (as what happened in Oklo), but it would be hard to believe that a warhead could occur naturally on Earth.