> Here's what I got: the radiation level just 1 meter away from the "elephant foot" (the solidified molten core), at the time of the accident was about 1000 times lower than the solar irradiation. At 100 meters it was 10 million times lower (because of the inverse square law).

No, the elephants foot isn’t a point source at its surface.

To use an extreme example going from 1m away from the sun to 100m away from the sun doesn’t result in a 10,000x drop off in energy density. Instead the exponential drop-off occurs relative to the center of the sun because energy is coming from any point on the surface visible to that location. A similar principle applies with the elephants foot, though the geometry is more complicated.