Beta rays, a product of I131 decay,have a very short range. If concentrated in the thyroid, it will for all practical puirposes, not be biologically eliminated before it decays. This will ionize a lot of the surrounding tissue.
On the other hand, gamma rays have a low absorbtion cross-section in the body and so do very little damage. Finally, Xenon is inert and should cause no damage at all.
Accordingly, the trick is for people to minimize the uptake of I131. There is some atmospheric in dust I131 near the reactor. There is contaminated ground and sea water. There could also be uptake in marine life and vegetation.
It would seem to me, that except for people who cannot avoid being close to the plant and its environs, Iodide tablets are not called for. Avoidance would be much more practical.