The levels of radioactivity are just educated guesses.
Recent studies of antineutrinos from within the Earth indicate that the previous estimate of the contribution of radioactive decay to the Earth’s overall heat-loss needs to be revised slightly.
Previous estimates, based on the bulk silicate Earth model, proposed a total of 20 TW, of which 8 TW was from the uranium-238 decay chain; 8 TW from the thorium-232 decay chain and the final 4 TW from potassium-40.
The more recent figures, based on the study of antineutrinos, indicate that the combined uranium-238 decay chain and thorium-232 decay chain produce 20 TW. This experiment cannot detect the lower-energy antineutrinos from potassium-40 decay, so educated guessing is not entirely eliminated. However, if this applies to little more than 16% of the heat from radioactive decay, it is probably no longer justifiable to claim that it is all an educated guess.
