Sorry, it does not just magically decrease at a much earlier rate than predicted, even though that would be nice


The types of isotopes and their half lives are known.

Maybe you have successfully partially decontaminated your area, but it is preposterous fantasy to think (and state, as you did in your title) that this is happening on a scale as large as Tokyo.

Your thread title is very misleading. Tokyo will be heavily contaminated for a long time.