Hello there.
Sorry if did duplicant but my idea is:

• Use same system like light system in minecraft.

• All source of radiation would radiate invisible light and blocks around source act like glass.

• Through blocks with less blast resistance, the radiation would decrease less than with more blast resistance. Calculations
  based on blast restance also make sense because the density of the material depends on the blast resistance.

• Eventually, the irradiated blocks could have a separate level of radiation instead of the entire chunk.

If you want i can whole idea fully document and send.
Maybe dis idea will big impact to game performance but even if the system were to be without storing radiation in blocks, it would honestly be better for me than the current system. I consider this mod to be one of the better mods.
Sorry for my english, when anything be incomprehensible i will try to explain.

Thanks you for your time.

The radiation system will be redone to a reasonable extent, and although the exact nature of the mechanics will likely not be quite like what you describe, it should give rise to some notions of radiation shielding, as well as a distinction between fallout and radiation :)