Density of U235, specifically… you could crystallize pure uranium in the Earth’s crust today, and it would not be enriched enough for a nuclear reactor.
So in the continental crust the abundance of uranium is more than one hundred times greater than the average abundance of uranium in the entire Earth or in the Solar System.
Otherwise uranium would have been almost the least abundant chemical element, as it is on average in the entire Solar System.
Tin is among the metals that are depleted in the crust of the Earth. Elsewhere tin is a few hundred times more abundant than uranium.
A continental crust like on Earth can appear only on planets that have been melted, so that the light and heavy minerals have combined and separated by density, with the continental crust floating over the molten magma and then solidifying. Most of the Earth has been melted by the collision that has formed the Moon, but possibly also by other smaller collisions.
In most other places in the Solar System it will be unlikely to find so easily accessible and abundant uranium as here on Earth.
The real world was worse than this fiction. It's easy to find pictures of old advertisements for radioactive products, in the years following the discovery of radioactivity. Radioactive pills "to boost your energy", radioactive cosmetics, radioactive false teeth... Now imagine what happened to the people that used these.
Even that is nothing close to the fiction I am talking about, having a class of slaves that operate a fully open nuclear reactor with hand tools.
I once read a horrifying fiction story about a pre-industrial culture that used nuclear reactions in open piles of uranium ore tended by slaves as a heat source, but cannot remember the name or author.