VERNON — Primordial radionuclides heavier than lead end up at the stable lead endpoint. Lighter primordial nuclides like 40K have a different endpoint; likewise, fission products and activation products.
What really helps you by waiting is reduction in exposure from cobalt-60 (60Co). With a five-year half-life, you can get over a 99-percent reduction in exposure rate by waiting for 40 years.
Unless it is a low-leakage (neutrons) core, 60Co builds up in the reactor pressure vessel walls and liner as well as the core support structures.
So, yes, the physics does support a waiting period for radioactivity reduction.