K, not Na
Similar elements, of course, but different.
The idea is that given an Fe-S core, then other light elements can substitute in for the S, and it is clear that SOME light elements sre in the core - it is too low density for pure Fe-Ni.
As for the "natural Uranium reactor", that was a loony idea based on a sphere of pure uranium at the earth's core. Instead, the U (and Th) are dispersed through the whole core and decay slowly without accelerated fission (if there WERE a reactor, then all the U would be gone after 4.6 Ga)