Natural nuclear fission can cause false alien alarms.

Today there is only one natural nuclear reactor on Earth. That place is Oklo in Gabon. The U-235 level is so low that a natural nuclear reactor could form when some magma eruption happens through the uranium ore. But on some young exoplanets. The natural nuclear reactors can be quite common. The natural nuclear reactors on exoplanets. Can give false evidence about the alien existence. The age of the exoplanet must be right that the natural nuclear reactions can begin at the level, that outsiders can see it. 

The molecular cloud that forms exoplanets must involve enough uranium. That it can form a natural nuclear reactor. And the second thing is that something must rise that uranium on the exoplanet's shell. The uranium is the heaviest natural element. But if the planet's core is very active the vulcanism can raise uranium to the surface. 

Because that happened near the geological fault. That formed those natural nuclear fission reactors. Those uranium reactors seem to be in the sandstone line. That tells us that maybe those natural nuclear fission reactors formed when two uranium deposits impacted. The other can come horizontally when the magma pushes uranium into the sandstone vein. The other can rise horizontally when magma pushes uranium up following the fault. The "surface" means that the uranium is closer to the shell than usual. But it's still very deep. 

The natural nuclear fission reactors of Oklo:

1) Nuclear reactor zones

2) Sandstone

3) Uranium ore layer

4) Granite (Wikipedia, Oklo) 

When we think about the Earth's geological history. The natural nuclear reactors could be quite common in Earth younghood. But then the mass of fissile material decreased and geological catastrophes buried those natural nuclear reactors. The same thing the volcanic eruption that formed the natural nuclear reactor can bury it into magma. If the bottom of that nuclear field falls to the magma, that uranium can fall to the planet's core. 

Can civilization form without fire? Or can civilization use volcanic temperatures to make tools? 

Another interesting thought that is connected with natural fire is this. Maybe the humans melted metals first time in volcanic lava. That means that ancient homo sapiens maybe saw that the volcanic eruption melted some black stones to iron metal. That thing can seem non-remarkable detail. But the thing. 

That we used to determine one of the most vital elements for civilization to advance or begin is the ability to make fire. The fire should close the sea planet's species away from the ability to form civilization. 

Fire doesn't burn in water. But underwater volcanoes can melt metals. The interesting question is what made humans make fire? And what was the first time when humans melted iron? How do they realize that stones can be melted? The interesting theory is that humans saw that volcanic temperatures melted metal ore into another form. 

https://bigthink.com/starts-with-a-bang/earth-natural-nuclear-reactor/

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

https://en.wikipedia.org/wiki/Oklo